Suggestions for an isolation transformer

[Spork Schivago]

Hi!   I am looking for an isolation transformer.   I figured with the devices we're repairing, 1kva would be just about perfect.

I was hoping to find one that had output for the 120VAC devices (or 115VAC, etc) and 240 (220VAC, etc).  I wanted one in an enclosure, with something like a GFCI receptacle or a breaker.

Could someone please help me try to find one?  Thanks.

[Jwillis]

Get a 1kVA 230v - 115v step down transformer wire a GFCI receptacle or breaker to it  and put it in an enclosure .

[Ian.M]

A step-down transformer may be an autotransformer which wont provide *ANY* isolation. 

A GFCI wont function on the secondary side of a true isolation transformer, as there can never be an imbalance no matter what happens downstream of it as the current into one end of any single winding secondary is the same as the current out of the other end of the same winding.  Similarly, the isolation transformer prevents primary side GFCIs tripping due to secondary side faults.

If you want a secondary side GFCI to function you need to bond secondary neutral to ground before the GFCI which means the secondary circuit is no longer floating, which in many applications defeats the purpose of having an isolation transformer in the first place.

[Gregg]

Toroiad makes nice dual output medical grade isolation transformers like this one: https://www.ebay.com/itm/Toroid-Medical-Grade-Isolation-Transformer-ISB-060M-SN-89520-3684/392263234379

[soldar]

Hi!   I am looking for an isolation transformer.   [...]  I wanted one in an enclosure, with something like a GFCI receptacle or a breaker.

Isolation transformer (floating output) and GFCI are contradictory. If the output is floating then the GFCI is pointless and if you want the GFCI to have any function then the output cannot be floating from earth. It is either one or the other.

[Cliff Matthews]

1kVA and enclosed - That's likely a 45lb chunky box, there will be serious $hipping charges..

[Gregg]

Here is a 1KVA dual voltage in and out for $175 or make an offer with free shipping: https://www.ebay.com/itm/Toroid-ISB-100W-Medical-Grade-Isolation-Transformer/233170056596  You have to look at the label carefully to determine the rating

[Zero999]

Hi!   I am looking for an isolation transformer.   I figured with the devices we're repairing, 1kva would be just about perfect.

I was hoping to find one that had output for the 120VAC devices (or 115VAC, etc) and 240 (220VAC, etc).  I wanted one in an enclosure, with something like a GFCI receptacle or a breaker.

Could someone please help me try to find one?  Thanks.

As you want both 120VAC as well as 240VAC, you need one with two 120V secondary windings which can be either connected in series or parallel to give either 120VAC or 240VAC.

Unfortunately I don't know of any that come in a case but the bare transformers can be purchased from various places. I just searched Digikey and found the following. It's fairly easy to put one in a box with a socket and a switch to select between 120V and 240V.
https://www.digikey.com/product-detail/en/hammond-manufacturing/1182R117/HM2485-ND/4809367
https://www.digikey.com/product-detail/en/triad-magnetics/VPT230-4350/237-1360-ND/2090098

A circuit breaker is a good idea, but as Soldar said, a GFCI won't function with an isolation transformer because it relies on a ground path to work and an isolation transformer breaks the path to earth.

[Cliff Matthews]

As suggested, DPDT switches can be used on both sides..

[Zero999]

As suggested, DPDT switches can be used on both sides..

Yes, that's what I had in mind.

The user needs to be careful though to avoid operating it or anything connected to it, with the switches in the wrong positions.

The secondary switch could be 3PDT with the third pair of contacts selecting between two different sockets: the standard one for 120V and 240V in your area.

The primary switch could be replaced with a five pin connector: one pin for earth and the remaining four for the two primary coils. Rather than switching between 120V and 240V input, a separate cable could be made for each voltage rating, with the connector wired so the primary windings are either connected in series or parallel.

[Spork Schivago]

Here is a 1KVA dual voltage in and out for $175 or make an offer with free shipping: https://www.ebay.com/itm/Toroid-ISB-100W-Medical-Grade-Isolation-Transformer/233170056596  You have to look at the label carefully to determine the rating

I think that is the one I will get.   And I don't have to make any modifications?   That's perfect, especially with the on-off switch.   I should have been more clear about wanting a breaker / GFCI.   I just wanted a way to power it off, if needed.    And the power switch is perfect.

I will buy that one.

[Spork Schivago]

I ended up ordering the eBay one.   I submitted an offer for 150$ and it was accepted.   From reading the manual a little (I haven't had time to fully read it yet), it appears that there's selectable input and selectable output.   For example, if I plug the transformer into a 115V source, I select 115V on the input side.   If I want 230V output, I select the 230V on the output side.

It's not clear yet if I can select 115V for input and 230V for output.   It would be nice if I could.   I do not run across 230V that require repairing often.   I do have a bunch of server related stuff that is running on 230VAC in my datacenter.   Hopefully it doesn't ever need repairing though!

I have submitted a quote for some C14 to NEMA 5-15r adapters.   That should allow me to plug these TVs into the isolation transformer, when it arrives.   I can hardly wait!

[Wolfgang]

... and dont forget  to include an inrush current limiting circuit. When you switch on a 1kVA transformer at the wrong moment in time (zero crossing) the magnetizing surge current could easily blow your fuse. I did it with a resistor of a few ohms that is bridged out after a few seconds.

[Cliff Matthews]

... and dont forget  to include an inrush current limiting circuit. When you switch on a 1kVA transformer at the wrong moment in time (zero crossing) the magnetizing surge current could easily blow your fuse. I did it with a resistor of a few ohms that is bridged out after a few seconds.

I recall TPAI's video on the subject. At 17:44 he demonstrates residual domains. Is this effect somewhat less in toroidal trafo's?
https://youtu.be/_pEmpvcNmXg?t=1064

I think that is the one I will get.   And I don't have to make any modifications?

Modifications? I think many "medical grade" units still have the mains earth ground passing through to the output, so you can disconnect and test for high M-ohm's leakage, or use a SPST switch in to open it (be careful not to flip-on by accident..)

FWIW, in case you consider Wolfgang's turn-on surge suggestion, you could consider TPAI's circuit attached..

[Wolfgang]

The inrush current of toroidal transformers is *larger* than classic types. Reason is type of core material and less stray flux.

[edgelog]

Here is a 1KVA dual voltage in and out for $175 or make an offer with free shipping: https://www.ebay.com/itm/Toroid-ISB-100W-Medical-Grade-Isolation-Transformer/233170056596  You have to look at the label carefully to determine the rating

I think that is the one I will get.   And I don't have to make any modifications?   That's perfect, especially with the on-off switch.   I should have been more clear about wanting a breaker / GFCI.   I just wanted a way to power it off, if needed.    And the power switch is perfect.

I will buy that one.

Is it only me or did you ask for a 1kVA transformer, then buy a 100W one?

[Cliff Matthews]

He ordered a 600.

[Gregg]

A careful look at the image shows input of 9.7A/4.35A; the output is harder to read but looks like 8.34A/4.1A at 115V and 230V respectively.
I have the 600 VA version that I bought on the bay for $58 delivered and it does have the mains ground connected to the output, but it is very easy to modify.  I'm thinking of installing two green binding posts so that I can bridge them whenever I want, put a resistor to increase the ground impedance or leave it float as desired.  There is a lot of room in the case for modifications.  I have some standard duplex L5-20R-IG isolated ground spec grade receptacles and am going to install one in the enclosure.  I have several of these if anyone wants one for the cost of shipping, they are bright orange in color. 
(edit-added a picture of the eBay item)

[Cliff Matthews]

Take it apart    We need pics!  IMO $58 for 1kVA is a good deal members should know about (while it lasts..)

[Gregg]

Take it apart    We need pics!  IMO $58 for 1kVA is a good deal members should know about (while it lasts..)

I guess I didn't make myself clear.  The isolation transformer that Spork bought is a 1KV as shown on the picture I posted above.  Maybe Spork will get around to modifying his before I have time to modify mine.
The one I bought about 6 months ago for $58 was during one of the eBay 15% off deals and is a 600VA model (plenty big for my purposes).  In true EEV fashion, the first thing I did was take it apart but didn't take pictures. (my bad  ) It is very nicely packaged as one would expect for medical gear.  I think someone on the forum did post some pictures of the same unit, I just didn't easily find the post. 

[Spork Schivago]

... and dont forget  to include an inrush current limiting circuit. When you switch on a 1kVA transformer at the wrong moment in time (zero crossing) the magnetizing surge current could easily blow your fuse. I did it with a resistor of a few ohms that is bridged out after a few seconds.

How would I go about building the inrush current limiting circuit and where would I put it?   Would a NTC thermistor do the trick instead of the resistor?   Thanks!!!!

[Spork Schivago]

Here is a 1KVA dual voltage in and out for $175 or make an offer with free shipping: https://www.ebay.com/itm/Toroid-ISB-100W-Medical-Grade-Isolation-Transformer/233170056596  You have to look at the label carefully to determine the rating

I think that is the one I will get.   And I don't have to make any modifications?   That's perfect, especially with the on-off switch.   I should have been more clear about wanting a breaker / GFCI.   I just wanted a way to power it off, if needed.    And the power switch is perfect.

I will buy that one.

Is it only me or did you ask for a 1kVA transformer, then buy a 100W one?

I am not sure what you mean by 100W.   Are you referring to the model number of the 1KVA transformer I purchased?   The ISB-100W?   I guess we could say it's an IsoBox ISB-100W- 1000VA isolation transformer.   Here's a link to their site:

https://toroid.com/Home/Product-Details/ProductID/16

I hope I understand the confusion.   If not, perhaps you could go a bit more in detail with what you mean by 100W.

[Spork Schivago]

He ordered a 600.

No, that's the ISB-060W, mine is the ISB-100W, which is 1kVA.   Thanks!

[Spork Schivago]

Take it apart    We need pics!  IMO $58 for 1kVA is a good deal members should know about (while it lasts..)

I guess I didn't make myself clear.  The isolation transformer that Spork bought is a 1KV as shown on the picture I posted above.  Maybe Spork will get around to modifying his before I have time to modify mine.
The one I bought about 6 months ago for $58 was during one of the eBay 15% off deals and is a 600VA model (plenty big for my purposes).  In true EEV fashion, the first thing I did was take it apart but didn't take pictures. (my bad  ) It is very nicely packaged as one would expect for medical gear.  I think someone on the forum did post some pictures of the same unit, I just didn't easily find the post.

I would like help with the modifications, just to make sure I do not do anything horribly wrong.   There are a few reasons I need one of these transformers, but one of the reasons is to increase safety a litle.   And I do not want to modify this in the wrong way, were it defeats that purpose.

Thank you guys!!!!!!!

[Spork Schivago]

I think 150$, free shipping, was a very good price for this unit.   I'm ordering some C14 to NEMA 5-15r adapters.   I figure there's no since in going for the 20r ones.   I gave a reason as to why I wanted this and was submitting a bid for 25$ less, and they said because I am trying to make a safer environment for my daughter, they are upgrading the shipping for free (it was free shipping, but now it's faster shipping for free).   I think, new, after taxes and shipping, this would be around 800$ or so.

[Ian.M]

You'll regret not getting the NEMA 5-20R adapters first time you need to work on a device with that plug on.   In many cases you can disable heating elements etc. possibly patching in a 60W incandescent bulb as a temporary load instead so you can power the device to test its control board, even though the isolating transformer cant handle its max operating current.

You may also want to treat yourself to a Cliff QuickTest mains connector if you deal with much mains stuff that doesn't have fitted plugs. http://www.cliffuk.co.uk/products/tools/quicktest.htm

You'll see one in use in Big Clive's videos looking at cheap chinese crap  when it comes without a mains plug.

[Cliff Matthews]

Happy you got a good deal. Coincidentally, this week a scary isolation trafo project was on a popular South American YT channel  I follow. Wow! Most wouldn't trust what these call tools for a day.. they'd end-up doing something safe like knitting

[edgelog]

I am not sure what you mean by 100W.   Are you referring to the model number of the 1KVA transformer I purchased?   The ISB-100W?   I guess we could say it's an IsoBox ISB-100W- 1000VA isolation transformer.   Here's a link to their site:

https://toroid.com/Home/Product-Details/ProductID/16

I hope I understand the confusion.   If not, perhaps you could go a bit more in detail with what you mean by 100W.

Yes, that was exactly the reason for my confusion. Seems not very smart to call a 1kW transformer "100W", but there you go.

[schmitt trigger]

You got yourself an excellent deal!

I would also recommend a soft start circuit. An NTC thermistor works, but I am unsure whether you can get one rated for these power levels. I use a similar resistor-limiting circuit to the one posted above for my 1kVA unit. Although I use an old 60 watt incandescent bulb in a small candelabra base. Because I had one already, and because if you are connecting a load which you are unsure about its integrity, like a very old radio, the lamp will remain lit up if a short exists.

[Spork Schivago]

You'll regret not getting the NEMA 5-20R adapters first time you need to work on a device with that plug on.   In many cases you can disable heating elements etc. possibly patching in a 60W incandescent bulb as a temporary load instead so you can power the device to test its control board, even though the isolating transformer cant handle its max operating current.

You may also want to treat yourself to a Cliff QuickTest mains connector if you deal with much mains stuff that doesn't have fitted plugs. http://www.cliffuk.co.uk/products/tools/quicktest.htm

You'll see one in use in Big Clive's videos looking at cheap chinese crap  when it comes without a mains plug.

I think I will order one of those Cliff QuickTest connectors.   I do not understand something Ian.M.   Are you talking about that T shaped input some receptacles have?   Are those the NEMA 5-20Rs?   I never ran across something that used that yet, but that is a great idea.   I have not placed the order for the adapters yet and will look for the NEMA 5-20r's.   Thanks!

[Spork Schivago]

I am not sure what you mean by 100W.   Are you referring to the model number of the 1KVA transformer I purchased?   The ISB-100W?   I guess we could say it's an IsoBox ISB-100W- 1000VA isolation transformer.   Here's a link to their site:

https://toroid.com/Home/Product-Details/ProductID/16

I hope I understand the confusion.   If not, perhaps you could go a bit more in detail with what you mean by 100W.

Yes, that was exactly the reason for my confusion. Seems not very smart to call a 1kW transformer "100W", but there you go.

Yeah!   I saw the 100W as well and had to double check before placing the order, so I definitely understand the confusion there!    I thought maybe you saw something else I didn't or something.

Thanks!

[Spork Schivago]

You got yourself an excellent deal!

I would also recommend a soft start circuit. An NTC thermistor works, but I am unsure whether you can get one rated for these power levels. I use a similar resistor-limiting circuit to the one posted above for my 1kVA unit. Although I use an old 60 watt incandescent bulb in a small candelabra base. Because I had one already, and because if you are connecting a load which you are unsure about its integrity, like a very old radio, the lamp will remain lit up if a short exists.

When I receive my isolation transformer, I will open it up and take pictures.   I am not sure what you mean by a soft start circuit, I am still learning, and there is a lot to learn.   I want to be safe though and really appreciate all the suggestions.   I am still a layman in many ways.   I also have some memory problems from when I was in the service and that makes things a bit harder sometimes.   So do I just install a resistor in series somewheres for the in rush current protection?  Not sure what the soft start circuit is but I will try researching it.   Thanks again for the info guys!!!!

[Spork Schivago]

Hrmm.   I cannot seem to find any C14 to NEMA 5-20R adapters.   I can find the cable ones, but not just a straight adapter.   I really didn't want a cable one.   I wanted a small, one piece adapter.   Does anyone know where I can find some?   I guess worse case, I could just purchase the cables, but I'd really rather not doing that....

[Cliff Matthews]

I'd put a beefy 12v relay across 2 NTC's (10 ohm 5 amp) in parallel on the line side of the transformer. Then take a tiny clock radio transformer on the isolated side to run a 2 second relay delay 555 circuit to shunt the NTC's for full power to the primary.

[Ian.M]

Here's a chart of the common NEMA plugs (suffix P) and receptacles (suffix R) likely to be found in North American domestic and small commercial usage.

Note the NEMA 5-20R 125V 20A receptacle accepts NEMA 5-20P 20A and NEMA 5-15P 15A plugs and also the ungrounded NEMA 1-15P 15A plug.  Its the most universal receptacle.

Similarly the NEMA 6-20R 250V recepticle also accepts NEMA 6-15P plugs, but N.A. 240V plugs and sockets are a hell of a mess to start with due to the common practice of appliances needing split phase 120V/240V because they often run control circuits from the lower voltage. 

[Spork Schivago]

I'd put a beefy 12v relay across 2 NTC's (10 ohm 5 amp) in parallel on the line side of the transformer. Then take a tiny clock radio transformer on the isolated side to run a 2 second relay delay 555 circuit to shunt the NTC's for full power to the primary.

Hrmm.

I am lost here Cliff.   What's the purpose of those relays?  And what do you mean with the 555 circuit to shunt the NTC's?

Now why 10 ohm 5 amp NTCs and why two of them?   I see the current for the ISB-100W is 8.3 amp / 4.17 amp.   So why not use one 10 amp NTC?   How does the 10 ohm resistance play a role here?

I'm looking for now for 12vdc relays.   I could make a circuit board.   I like surface mount.   I don't mean to sound stupid, but if I don't know something, I figure if I don't ask, I won't ever understand.

[Cliff Matthews]

Asking is precisely what this forum is for.  The purpose of the relay, is to relieve the NTC (here, also called an inrush limiter) of the temporary soft-start task, that's all. I chose two (10 ohm/ 5 amp) in parallel because they're a common find when harvesting parts and 5 ohms should be sufficient for a breaker not to trip. PC power supplies use them to soft charge the input bulk capacitors and often they look like large dark-green or black ceramic capacitors, with a sandy hi-temp exterior. See pg. 29 here http://www.farnell.com/datasheets/33552.pdf

A 555-timer (2 second monostable circuit) drives a transistor and relay, which powers the relay shunting/closing it across the NTC inrush limiter(s). The small 9-12v transformer powers the 555 circuit and relay, and its primary could be could be connected either on the input or output side of the isolation transformer. I would use the output side, since any shorted DUT on your bench will open the relay making the line-side surge a little softer for other devices using that same distribution circuit. I'll have to assume for the moment, your 1kVA unit will have a fuse or breaker integrated (??)

[schmitt trigger]

You got yourself an excellent deal!

I would also recommend a soft start circuit. An NTC thermistor works, but I am unsure whether you can get one rated for these power levels. I use a similar resistor-limiting circuit to the one posted above for my 1kVA unit. Although I use an old 60 watt incandescent bulb in a small candelabra base. Because I had one already, and because if you are connecting a load which you are unsure about its integrity, like a very old radio, the lamp will remain lit up if a short exists.

When I receive my isolation transformer, I will open it up and take pictures.   I am not sure what you mean by a soft start circuit, I am still learning, and there is a lot to learn.   I want to be safe though and really appreciate all the suggestions.   I am still a layman in many ways.   I also have some memory problems from when I was in the service and that makes things a bit harder sometimes.   So do I just install a resistor in series somewheres for the in rush current protection?  Not sure what the soft start circuit is but I will try researching it.   Thanks again for the info guys!!!!

Sorry, I should’ve said inrush limited circuit.

[Wolfgang]

... and dont forget  to include an inrush current limiting circuit. When you switch on a 1kVA transformer at the wrong moment in time (zero crossing) the magnetizing surge current could easily blow your fuse. I did it with a resistor of a few ohms that is bridged out after a few seconds.

I recall TPAI's video on the subject. At 17:44 he demonstrates residual domains. Is this effect somewhat less in toroidal trafo's?
https://youtu.be/_pEmpvcNmXg?t=1064

I think that is the one I will get.   And I don't have to make any modifications?

Modifications? I think many "medical grade" units still have the mains earth ground passing through to the output, so you can disconnect and test for high M-ohm's leakage, or use a SPST switch in to open it (be careful not to flip-on by accident..)

FWIW, in case you consider Wolfgang's turn-on surge suggestion, you could consider TPAI's circuit attached..

The turn-on solution proposed definitively works, no doubt. You could save some parts by creating the relay coil voltage by a capacitive power supply, or by using a relay with a HV (115/220V) coil.

[Spork Schivago]

Asking is precisely what this forum is for.  The purpose of the relay, is to relieve the NTC (here, also called an inrush limiter) of the temporary soft-start task, that's all. I chose two (10 ohm/ 5 amp) in parallel because they're a common find when harvesting parts and 5 ohms should be sufficient for a breaker not to trip. PC power supplies use them to soft charge the input bulk capacitors and often they look like large dark-green or black ceramic capacitors, with a sandy hi-temp exterior. See pg. 29 here http://www.farnell.com/datasheets/33552.pdf

I have seen them before and I have done some research in the past on them.   Are they always used as an inrush limiter?   My understanding is the resistance changes with temperature.   The higher the temp, the less resistance.   soft-start task....I know there's a huge in-rush current with the startup of an AC motor, but there's also one with transformers?   I can purchase this one:   https://www.mouser.com/ProductDetail/Ametherm/SG336?qs=sGAEpiMZZMtPe6tiLMgnfDIkRBydcYq9ZpRZtoOVAxM3vGe5MWY2Wg%3D%3D

if I just wanted one, instead of using two in parallel, right?

A 555-timer (2 second monostable circuit) drives a transistor and relay, which powers the relay shunting/closing it across the NTC inrush limiter(s). The small 9-12v transformer powers the 555 circuit and relay, and its primary could be could be connected either on the input or output side of the isolation transformer. I would use the output side, since any shorted DUT on your bench will open the relay making the line-side surge a little softer for other devices using that same distribution circuit. I'll have to assume for the moment, your 1kVA unit will have a fuse or breaker integrated (??)

I have some 555 timers, which I could use.   I received the isolation transfomer today, damaged, of course.   That's my luck.   Mail lady entered the damage in the system and said if the actual unit was damaged, to contact the seller, who would start a claims.   I did that.   We figure one of the postal people dropped the box.   The box was damaged, the isolation transformer is damaged.   It's the mounting brackets.

So I contacted the company who made the transformer to see if they could get me a price on replacement mounts, and they said they'd contact me by the end of the day with an answer.   Right now, I don't really need the mounts, but eventually, I'd love to mount this to my work bench.   That's where I'd be using it once my daughter gets a bit older.   I cannot use the current mounts.   If I unbent them, the metal wouldn't be structurally sound anymore, and the thing could end up falling and hurting someone.

[Cliff Matthews]

Pretty good size.. If you're OK with ~4-5 volt output sag between 0 and 20% load, you could skip the relay. Bear in mind at this power level these get toasty. I've seen them installed with clearance for air-flow high off the PCB using copper rivet's with 1-inch rigid tube supports. Startup surge may not blow a new breaker (that hasn't been tripped too many times) especially if the mains cct isn't already heavily loaded, so that could be reason enough to delay an upgrade.

[Spork Schivago]

Pretty good size.. If you're OK with ~4-5 volt output sag between 0 and 20% load, you could skip the relay. Bear in mind at this power level these get toasty. I've seen them installed with clearance for air-flow high off the PCB using copper rivet's with 1-inch rigid tube supports. Startup surge may not blow a new breaker (that hasn't been tripped too many times) especially if the mains cct isn't already heavily loaded, so that could be reason enough to delay an upgrade.

I think I will use the relay.   I just need help with the schematic.   Should I try to keep the circuit board I install cool with a heatsink or a heatsink / fan?   Thanks!

[Spork Schivago]

So I called Toroid, and they're absolutely wonderful!   I talked to a Mr. AJ.   He is sending me the new brackets (both of them) and it's only costing me 11$ plus some cents.   He didn't say nothing about shipping charges, and I gave him my CC number, but I'm sure there will probably be something for shipping.

I know how to select the output voltage for this transformer, but I cannot figure out how to set the input voltage.   I'm including a picture of where the input cord goes.   It doesn't appear to slide like the output one.   Also, being a transformer, if the input voltage is 120VAC and I have the output select to 240VAC, would I get that 240VAC?   Or is this one of those 1:1 transformers, where the output is dictated by the input voltage?

Thanks!!!!!

[Cliff Matthews]

Put a slot head screw driver under the protruding plastic tab just above the red 230v indicator. The fuses and likely the selection switch is under that.

[Gregg]

As per Cliff's post; a small flat screwdriver will allow access to a red plug that changes the voltage.  See picture

[Spork Schivago]

Put a slot heat screw driver under the protruding plastic tab just above the red 230v indicator. The fuses and likely the selection switch is under that.

Thank you Cliff!   I believe I found a problem!   I was looking at the two fuses and both of them are 10A 250VAC fuses, but the silk screen printing says 115VAC should be 5AT and the 230VAC should be 10AT for the fuses.   I do not know what the T means, however, I think the 115V selector size has the wrong size fuse in there.   Don't you?

I believe I should replace the 115VAC 10 amp fuse with a 5 amp fuse, is that correct?

[Cliff Matthews]

5a is for 230v and 10a for 120v - Hey, do we get any pics of the belly of this beast?

[Psi]

A cheap isolating transformer can usually be made from the transformer found in most UPS's
I would stay away from the really cheap UPS, some of the transformers in them are not suitable.
Look for one that does voltage stabilization/Boost.
Voltage stabilization/boost allows the UPS to step up/down mains a little. eg if mains in your area is a bit high like 245V the UPS with adjust itself to output ~235V and bring it back into spec. The super cheap UPS's don't do this but mid range ones usually do.
Look for something over 1000W, those usually have the feature. The manual should say.

The transformer in the UPS usually has 3 windings. For a 230V UPS it might go something like this.
1- Battery winding (UPS turns battery voltage into AC and feeds this winding when on battery. This winding is usually 12/24/36V to match battery cell count)
2- Mains output winding (230V)
3- Mains input winding with multiple taps. etc, it might have 3 taps for 220/230/240.   or 5 taps for 210/220/230/240/250
The UPS switches the mains input between those tabs (using relays) to control the output voltage between step up - normal - step down.

So you can easily repurpose these transformers to be an isolation transformer. Since they already are.
It's much easier to find a dead UPS for cheap/free than an isolating transform.
99% of the time dead UPS's are dead because the electronics have blown up or battery leaked, the transformer is almost always totally fine.

If you get a dead UPS the cool thing is the transformers already mounted in a case with input and output mains plugs on the back!!
Ya just need to rip out the electronics and rewire it.

[L_Euler]

I know you went ahead and bought one, but I've made a couple really simple, but effective isolation transformers by just hooking a pair of identical standard transformers back to back. For instance, 120VAC to primary of transformer 1 with secondarys of both transformers connected to each other gets you 120vac isolated at the primary of transformer 2 as long as you don't use the ground or neutral wire.

[Cliff Matthews]

I know you went ahead and bought one, but I've made a couple really simple, but effective isolation transformers by just hooking a pair of identical standard transformers back to back. For instance, 120VAC to primary of transformer 1 with secondarys of both transformers connected to each other gets you 120vac isolated at the primary of transformer 2 as long as you don't use the ground or neutral wire.

That's what a lot of us do in a pinch, but No-Load to Full-Load regulation is about 10% for one transformer. AFAIK, that's 120v noload and 108v full load, so double that for back-to-back and full load output could drop below 100v.. not to mention Isolation transformers have a lower capacitive coupling for better/safer DUT isolation.

[L_Euler]

I know you went ahead and bought one, but I've made a couple really simple, but effective isolation transformers by just hooking a pair of identical standard transformers back to back. For instance, 120VAC to primary of transformer 1 with secondarys of both transformers connected to each other gets you 120vac isolated at the primary of transformer 2 as long as you don't use the ground or neutral wire.

That's what a lot of us do in a pinch, but No-Load to Full-Load regulation is about 10% for one transformer. AFAIK, that's 120v noload and 108v full load, so double that for back-to-back and full load output could drop below 100v.. not to mention Isolation transformers have a lower capacitive coupling for better/safer DUT isolation.

Agreed.  However, if you know enough to understand you need an isolation transformer, you probably know enough to cope with the limitations of this method.

[SoundTech-LG]

Not sure I would suggest this, but the ebay used price, and free shipping were hard to resist. Suggestions on the primary 240V to 208V drop are welcome. My solution is to use another transformer with a 30 amp 24 volt winding in series with the primary. It will still be a bit high. I had it apart. Forgot to snap photos of the huge toroid it contains. Hopefully I'll do that soon.

https://www.amazon.com/Tripp-Lite-SU5000XFMRT2U-Rackmount-Transformer/dp/B0007DGFEU

[Spork Schivago]

5a is for 230v and 10a for 120v - Hey, do we get any pics of the belly of this beast?

Yes sir, I had them backwards, I've been real tired.   I will open it up very soon and take pictures.   Since it was dropped in shipping, I ran some basic tests and am a little confused.   I have it set to 115VAC input, 115VAC output.   I hook up a C13 / C14 cord to the output and put my multimeter, in volts mode, across the two terminals of the plug.   I read roughly 125VAC.    I take one of the probes from the DMM and hook it to the neutral hole on the plug and put the red probe on the big metal bolt on the top and read roughly 7VAC.

I take the red probe and put it on the side nuts of the isolation transformer and read roughly 47VAC.   Is this normal and safe?    Or is there a chance something is making that chassis hot?

Thanks!

[Spork Schivago]

I can only upload 2MB of pictures at one time, so this will be in multiple posts.

[Spork Schivago]

Here's some more.more.

[Spork Schivago]

The final two.

[Wolfgang]

... looks very similar to the transformer I used. I Just added the inrush current limiter, fuses, an output voltage switch (115/1230), and two analog AC panel meters, plus a few hefty sockets - job done. Never let me down so far.

[Cliff Matthews]

Hmm, those SG-44 inrush limiters are rated at 8 amps.. which is 960 watts. That's pushing it a bit, I'd think. Still, how often will anyone push these to the limit? If it ever stops working and the fuse is not to blame, one of these will be cracked open and the other one will be your spare if you never use the 240v input.
The 125v you measured is unloaded but still OK (1v under USA limit of +/- 5%). It might be educational, for those following along, to know what you measure for various loads up to a hair-dryer on low heat setting (medium and high may blow it's fuse). I think the 7v is just a floating average from minimal capacitive coupling. To be sure it's prepared for isolation use, either L or N on any of the output terminals should measure high resistance to any contacts on the primary side of the transformer. It's time to unplug and do some resistance checks.. Nice purchase!

*edit Varistor SG-44 PDF attached

[bson]

I can only upload 2MB of pictures at one time, so this will be in multiple posts.

The input and output mains earths both look connected to the chassis.  I wouldn't trust this transformer to isolate any 3-prong device without carefully determining what it does with its earth lead.  If the goal is noise isolation it's perfect the way it is - you want a shared earth.  But if it's to float you should consider removing the output side earth wiring since it prevents effectively floating the outputs.  Or maybe leave it on one terminal and clearly label it to give you a choice.

[Cliff Matthews]

Yup, that was mentioned with an optional GND-lift switch in #13 (or he has the option to modify his DUT bench power cords with ground clipped). Regarding picture uploads to the forum, Dave has mentioned a free utility called "IrfanView" before and I can confirm this is not only 10x faster than the Windows viewer, but it also allows a lot of sharpness detailing and compression as well. It's got to be one of the best freeware offerings from Europe I've ever used, hands down.

[Spork Schivago]

Would the SG-26 or SG-32 be better choices?

Code: [Select]

SG-26
-----
R@25°C: 5.0Ω
R Tolerance (±%): 15
Imax: 12 Amps
RImax: 0.060Ω
Surge Rating: 100 Joules

SG-32
-----
R@25°C: 4.0Ω
R Tolerance (±%): 20
Imax: 14 Amps
RImax: 0.050Ω
Surge Rating: 100 Joules

SG-44
-----
R@25°C: 5.0Ω
R Tolerance (±%): 20
Imax: 8 Amps
RImax: 0.050Ω
Surge Rating: 40 Joules


[Spork Schivago]

I got memory problems, and totally forgot about post #13, but yeah, I think I am going to remove the grounds that go to the outputs.

For the resistance check, just measure from L or N on the output to all the wires that don't go to output or to the grounds connected on the chassis?   Thanks!

**EDIT:

My Craftsman 82357 is an older meter, but it's been fairly good to me and when I used to have the manual, had good specs (although, I haven't had the manual in years and cannot locate one).   Regardless, it shows OL (overload), or infinite resistance, when measuring from the L and N on the output side to any of the other wires coming out of the transformer, including the ground wires.

So now, I just need to remove the grounds that go from the chassis to the outputs, maybe upgrade the thermistors, replace the one fuse with a 5A fuse, and then build the soft-start circuitry thinga-ma-jig.   Can someone help me find parts for that circuit and show me where I wire everything up?   The relays and NTCs?   Thank you!

[Spork Schivago]

If I used that 12-volt transformer, what would be the purpose of that?   I am not sure I understand enough to build the soft-start stuff myself yet.   I do not want to attempt to do something I don't fully understand, for fear of getting hurt, or hurting someone.

[Cliff Matthews]

I had a look for a manual for a Craftsman 82357, and aside from usual Russian bait-link-and-switch for virus, found nothing. I'll just attach a photo so if anyone here has the PDF, they can help out.

As for mods, hold off until you take 5 minutes to check the output voltage with both a 60 or 100 watt lamp and a hair dryer (on low heat) over a period of 1 minute each. This way you can observe any voltage change as the limiter heats up.

Why? It's unwise to change perceived faults in a design before you truly observe the original. I'm thinking that with a relay engaged, this transformer may output >126v to light loads (in other words, it may depend on the limiter a bit for light loads).

*edit - while your at it, you should measure the mains too (input line voltage), it may be the reason you're getting 125v out. 

[Cliff Matthews]

If I used that 12-volt transformer, what would be the purpose of that?   I am not sure I understand enough to build the soft-start stuff myself yet.   I do not want to attempt to do something I don't fully understand, for fear of getting hurt, or hurting someone.

You are correct, if you're not up to that, don't do it. The 12v transformer's only function was to power the 555 and hence the relay after a 2 second delay. But after reading your 'stock' output voltages, it appears the limiter(s) do have a useful function at low currents, so I think you should hold off any mods (with the exception of removing the previously mentioned output grounding).

As for changing the limiters, the SG-26 may be OK but the SG-32 is likely not OK (just stay with the stock, or ask the MFG about why they used an under-rated part.. after you get your mounting brackets..). Once again, you got a good deal.

[Spork Schivago]

Sears used to have the manual for that DMM, but they no longer have it.   I even tried the WayBack Machine, but no luck, unfortunetly.

I will hold off on the mods, minus the ground lifting.   I was thinking of purchasing something like what is included in the attachments here, one of those ground lugs, if I could find the correct size.   I would use epoxy or something to attach it to the case, and isolate those output grounds.   Then, if I ever wanted the output grounds hooked up, I could just run a jumper wire from the new ground lug things to the old posts.

I cannot figure out another safer way to lift those ground wires.   I could wrap them in electrical tape, but I would worry the tape might eventually fall off...

I have a special light socket that has a hot and a neutral wire.   I can use that to test the 60 watt and 100 watt bulbs, assuming I can find 60 watt and 100 watt bulbs.   How do I test the voltage with the hair dryer hooked up though?   Where could I put my probes without damaging the hair dryer?   I don't think I can put them inside the transformer.   My wife would kill me if I tore apart her hair dryer...


**EDIT:

The lifting of the grounds was easier than I originally thought.   I removed the nut for each one of them, but where they hooked up to the output receptacles, they were just slid on, they weren't soldered like the hots and neutrals.   Last night, I had just taken a quick glance and seen the hots and neutrals were soldered and must have assumed the grounds were as well.   So now I removed all the grounds going to the output recepatcles.   I left the ground hooked up that goes to the input power receptacle, and I left the primary side ground hooked up.   Now, no grounds go to the output receptacles and they are floating!

Thanks!

[Spork Schivago]

If I used that 12-volt transformer, what would be the purpose of that?   I am not sure I understand enough to build the soft-start stuff myself yet.   I do not want to attempt to do something I don't fully understand, for fear of getting hurt, or hurting someone.

...
As for changing the limiters, the SG-26 may be OK but the SG-32 is likely not OK (just stay with the stock, or ask the MFG about why they used an under-rated part.. after you get your mounting brackets..). Once again, you got a good deal.

Now, why would the SG-32 not be okay?   I will stay with stock for now and ask the manufacturer, but I want to learn, and to me, the SG-32, I thought would have been better, because it could handle up to 14 amps.   So I'm missing something with these NTCs still...

[Gregg]

I cannot figure out another safer way to lift those ground wires.   I could wrap them in electrical tape, but I would worry the tape might eventually fall off...

For the ground wires, put the ones for your isolated outlets to a binder post.  Leave the input ground bonded to the case and run a wire to another binder post adjacent to the isolated ground,  Then you can easily jump between the binder posts for non-isolated ground or make a plug in jumper with banana plugs if you switch the grounding scheme often.  Same idea for meter connection, just install a pair of the flush type sleeve capable meter jacks in the case and bob's your uncle. 

[Spork Schivago]

I cannot figure out another safer way to lift those ground wires.   I could wrap them in electrical tape, but I would worry the tape might eventually fall off...

For the ground wires, put the ones for your isolated outlets to a binder post.  Leave the input ground bonded to the case and run a wire to another binder post adjacent to the isolated ground,  Then you can easily jump between the binder posts for non-isolated ground or make a plug in jumper with banana plugs if you switch the grounding scheme often.  Same idea for meter connection, just install a pair of the flush type sleeve capable meter jacks in the case and bob's your uncle. 

What do you mean by the meter connection?   Are you talking about modifying this isolation transformer so it's a bit like my Rigol Programmable Power Supply, where I can take probes, plug them into the banana jacks, and use them to power some circuit?

For what it's worth, I found a 5-amp 250V fuse down in my basement.   It's physically larger than the LittleFuse that was in there, but it fits.   I don't have any of those LittleFuse's to my knowledge, but it would be nice to get a 5-amp 250V LittleFuse one of these days.   The fuse door / input selector switch door closes a little harder because of the larger fuse.   The fuse holder appears to be designed for either type (the LittleFuse or the larger ones).

[Cliff Matthews]

NTC resistance curves are in the data sheet. The original is a 5 ohm (@25c) 0.6 inch disc that will drop to <1 ohm >80c (see RT/R25 curve A) while passing ~1amp.. so with transfo losses, that's roughly a 100 watt lamp on the output. The SG-32 NTC is 4 ohms so when cold, the secondary side may produce >126v (over US limit). The SG-32 is also larger 0.9 inches so it can dissipate more power before it reaches its "sub-ohmic" operating point while possibly melting that nylon barrier strip. I think only 1 NTC is used at 120v-in while both are used at 240v-in - that's why I said you'll have a spare. Hope that helps!

[Spork Schivago]

Code: [Select]

Mains Input Voltage: 122.5VAC
60 Watt Incandescent Bulb for Load: 124.1VAC - 124.5VAC
No Load Output: 126.5VAC

[Multi-Watt Bulb]
30 Watt: 124.8VAC - 125.2VAC
70 Watt: 123.7VAC - 124.2VAC
100 Watt: 122.7VAC - 123.1VAC

I had trouble finding the incandescent bulbs and need to hook up a three-way touch switch between the three-way bulb and the isolation transformer first before I can get the power output of the 100 watt bulb.

**EDIT:  I added the measurements for the 30 / 70 / 100 watt bulb.   Here are the measurements, in VAC, for those powers.

[Gregg]

Spork,
Just put a pair of the banana jacks like these https://www.ebay.com/itm/10-Pcs-4mm-Safety-Protection-Plug-Binding-Post-Banana-Jack-Black-Red-W5H1/173105429056 somewhere in the transformer case connected to output so that you can easily connect your meter.  You could put fuses and/or a double pole switch if you want.

[Spork Schivago]

I don't see how I could connect the binding posts though.   Anything connected to the case will be connected to ground.   Unless you guys mean drill holes through what I call the front, where the output receptacles are, and hook them to the transformer.   That could come in handy.

I have a programmable DC power supply, but no AC, and that could be really handy, even though it isn't programmable.   Having a ~120VAC / ~240VAC transformer case that accepts that common bannan plug would come in handy...and I think I know what you mean now.   I can hook the DMM up real quick like while I have a device plugged in to measure the load easy like.

[Spork Schivago]

Now I got some questions about this isolation transformer.   If I have a DUT hooked to the isolation transformer, and someone where to touch something hot, like a charged capactitor on the anode side, they wouldn't get shocked, because their is no path to a common ground through the person, right?   But if they where to touch the anode and cathode at the same time, then they would get shocked?   The current would flow through the capacitor, through them, and back through the capacitor, correct?

**EDIT:

Hairdryer on low -- Hot showed 119.7VAC - 120.9VAC.   High, as preditected, broke the transformer.   I will see if I blew a fuse or if time off will fix it.   Those NTCs could act like a soft fuse, a resettable one, right?   Or no?

[Spork Schivago]

I don't understand why the fuse blew when I switched the hair dryer to the higher current mode of operation.   Why did the NTC not open up?   The higher current should have caused a higher temp, and I thought that was the purpose, to prevent too high of a current from going out the isolation transformer outputs.

[Gregg]

Now I got some questions about this isolation transformer.   If I have a DUT hooked to the isolation transformer, and someone where to touch something hot, like a charged capactitor on the anode side, they wouldn't get shocked, because their is no path to a common ground through the person, right?   But if they where to touch the anode and cathode at the same time, then they would get shocked?   The current would flow through the capacitor, through them, and back through the capacitor, correct?

The best way to wrap your head around what is happening with your isolation transformer is to think in terms of complete circuits; mainly that YOU don’t want to become part of any circuit.
The originating circuit is the primary of the isolation transformer, but if it is truly isolated, it is completely separate from the secondary.  The source of the primary, here, is the transformer somewhere outside your house that takes high voltage and converts it to 115/230 volts that in turn goes to your house via the meter and main disconnect.  The primary has the neutral grounded and therefore there is potential from the line to anything grounded like your body if you are in contact with ground.  (This is a little over simplified, but it will suffice for this example.)  The line and neutral connected the primary of your isolation transformer forms a complete circuit back to the transformer outside your house.  The key is to think in terms of “complete circuit”.

The secondary of your isolation transformer can be considered a source that is separated from the primary and has virtually no voltage in reference to ground or the primary; it is forming a separate circuit or you could think of it as the source for a separate circuit. 
(There may be some induced or capacitive voltage between this isolated secondary and ground that you can read with a high impedance digital meter but if you put something like a 1K resistor across where you are probing it will drop to an insignificant figure because there is virtually no current available.)
The secondary of the isolation transformer has no connection with the primary unless you ground one leg, then the purpose of isolation kind of goes away. 

When you power your DUT with the isolated source but it has ground planes or a chassis that should be grounded, all sorts of weird things can happen.  If you visualize every possible circuit that can be associated with your DUT and your test equipment and take steps to mitigate disaster, you’ll be off to a good start.  No guarantees, you are on your own, think three times before probing and never touch two parts of a mains circuit at the same time.  At least gloves and a face shield are highly recommended for beginners.  Check with your multimeter before sticking your probe on something may save lots of grief and smoke.

[Gregg]

I don't understand why the fuse blew when I switched the hair dryer to the higher current mode of operation.   Why did the NTC not open up?   The higher current should have caused a higher temp, and I thought that was the purpose, to prevent too high of a current from going out the isolation transformer outputs.

Fuses are NOT current limiters; they are there to prevent the wiring form over heating from too much current.  The hair dryer on high drew more current than your fuse would allow and thereby saved your transformer.

(Edit for clarity): The hair dryer on high tried to pull more current than your fuse would allow and the fuse blew saving your transformer.

[ArthurDent]

Here is the description on line that should clear things up.

"With an NTC thermistor, when the temperature increases, resistance decreases. ... A PTC thermistor works a little differently. When temperature increases, the resistance increases, and when temperature decreases, resistance decreases. This type of thermistor is generally used as a fuse."

So the NTC is used to limit inrush current the first few seconds after power is supplied to a transformer by being high resistance when cold and low resistance when hot. They only need to be used for a few seconds at power up so this is why a relay circuit is often used to bypass them once the power has been on for a few seconds. The NTC can then cool down and be ready to do its job again.

The PTC is just the opposite, when the current gets too great, the resistance increases quite a bit and reduces or disconnects power to the load. Most return to low resistance when they cool down.

[Cliff Matthews]

I think you have the idea. If someone (I wouldn't know why..) was touching an earth ground while touching something inside the DUT you are right, there is no low resistance path.

Q- At 30-100w each time you observed some rise in output voltage over time (this is where NTC got closer to ideal temp/resistance and had less voltage drop). Did you see this with the hair-dryer on low heat 500w a bit quicker?

What you observed so far, is this model boosts mains by 4v at no load and is quite reliant on the 4-5v drop of the NTC. Do you see why clamping a relay across the NTC might have been a bad idea? Then output for light loads could be over 130v

*edit - For those that have not looked at the data sheet, these NTC's are part of the design to regulate the transformers output at light loads..  At heavier loads these NTC's will drop to a negligible resistance (under 0.1 ohm).

Those NTCs could act like a soft fuse, a resettable one, right?

NTC's are also called Varistor's - They act like a variable resistor transitioning into a low resistance wire at ~105c

[Spork Schivago]

Now I got some questions about this isolation transformer.   If I have a DUT hooked to the isolation transformer, and someone where to touch something hot, like a charged capactitor on the anode side, they wouldn't get shocked, because their is no path to a common ground through the person, right?   But if they where to touch the anode and cathode at the same time, then they would get shocked?   The current would flow through the capacitor, through them, and back through the capacitor, correct?

The best way to wrap your head around what is happening with your isolation transformer is to think in terms of complete circuits; mainly that YOU don’t want to become part of any circuit.
The originating circuit is the primary of the isolation transformer, but if it is truly isolated, it is completely separate from the secondary.  The source of the primary, here, is the transformer somewhere outside your house that takes high voltage and converts it to 115/230 volts that in turn goes to your house via the meter and main disconnect.  The primary has the neutral grounded and therefore there is potential from the line to anything grounded like your body if you are in contact with ground.  (This is a little over simplified, but it will suffice for this example.)  The line and neutral connected the primary of your isolation transformer forms a complete circuit back to the transformer outside your house.  The key is to think in terms of “complete circuit”.

The secondary of your isolation transformer can be considered a source that is separated from the primary and has virtually no voltage in reference to ground or the primary; it is forming a separate circuit or you could think of it as the source for a separate circuit. 
(There may be some induced or capacitive voltage between this isolated secondary and ground that you can read with a high impedance digital meter but if you put something like a 1K resistor across where you are probing it will drop to an insignificant figure because there is virtually no current available.)
The secondary of the isolation transformer has no connection with the primary unless you ground one leg, then the purpose of isolation kind of goes away. 

When you power your DUT with the isolated source but it has ground planes or a chassis that should be grounded, all sorts of weird things can happen.  If you visualize every possible circuit that can be associated with your DUT and your test equipment and take steps to mitigate disaster, you’ll be off to a good start.  No guarantees, you are on your own, think three times before probing and never touch two parts of a mains circuit at the same time.  At least gloves and a face shield are highly recommended for beginners.  Check with your multimeter before sticking your probe on something may save lots of grief and smoke.

I am not so much worried about me.   I am careful and do safe stuff now, like check capacitors, even if there's bleeder resistors, stuff like that.   I was hoping I could use this isolation transformer to slowly introduce my young daughter into electronics, when she gets a bit older, with it being a little safer.   I realize that even if there was no potential for shock, it would be a horrible idea to not teach her safe practices.   Sometimes, it's really hard to repair electronics now because I spend a lot of time with my daughter.   It would be nice to be able to work on some of this stuff with her around, but I would never do anything to put her in danger.

I want to start learning more about how to use my oscilloscope and power supplies.   I think this is where the isolation transformer will come in handy for the time being.   I don't want to accidently blow up my scope because I tried measuring something on the primary side of a SMPS on accident, or something dumb like that.

[Spork Schivago]

I don't understand why the fuse blew when I switched the hair dryer to the higher current mode of operation.   Why did the NTC not open up?   The higher current should have caused a higher temp, and I thought that was the purpose, to prevent too high of a current from going out the isolation transformer outputs.

Fuses are NOT current limiters; they are there to prevent the wiring form over heating from too much current.  The hair dryer on high drew more current than your fuse would allow and thereby saved your transformer.

(Edit for clarity): The hair dryer on high tried to pull more current than your fuse would allow and the fuse blew saving your transformer.

So the transformer would have tried providing that current and overheated and died?   Wouldn't the NTC though limit the amount of current the DUT was trying to draw and protect the transformer?   

[Spork Schivago]

Here is the description on line that should clear things up.

"With an NTC thermistor, when the temperature increases, resistance decreases. ... A PTC thermistor works a little differently. When temperature increases, the resistance increases, and when temperature decreases, resistance decreases. This type of thermistor is generally used as a fuse."

So the NTC is used to limit inrush current the first few seconds after power is supplied to a transformer by being high resistance when cold and low resistance when hot. They only need to be used for a few seconds at power up so this is why a relay circuit is often used to bypass them once the power has been on for a few seconds. The NTC can then cool down and be ready to do its job again.

The PTC is just the opposite, when the current gets too great, the resistance increases quite a bit and reduces or disconnects power to the load. Most return to low resistance when they cool down.

I confused the two again, the NTC and the PTC.   I was thinking the NTC where a PolyFuse.   I see where I went wrong now.   Thank you!!!!!!!

[Spork Schivago]

I think you have the idea. If someone (I wouldn't know why..) was touching an earth ground while touching something inside the DUT you are right, there is no low resistance path.

Q- At 30-100w each time you observed some rise in output voltage over time (this is where NTC got closer to ideal temp/resistance and had less voltage drop). Did you see this with the hair-dryer on low heat 500w a bit quicker?

What you observed so far, is this model boosts mains by 4v at no load and is quite reliant on the 4-5v drop of the NTC. Do you see why clamping a relay across the NTC might have been a bad idea? Then output for light loads could be over 130v

*edit - For those that have not looked at the data sheet, these NTC's are part of the design to regulate the transformers output at light loads..  At heavier loads these NTC's will drop to a negligible resistance (under 0.1 ohm).

Those NTCs could act like a soft fuse, a resettable one, right?

NTC's are also called Varistor's - They act like a variable resistor transitioning into a low resistance wire at ~105c

Yes!   I did, and I figured the voltage change was from the NTC heating up.   I did notice that it wasn't so noticable with the hair dryer though.   In my mind, I thought it was just reaching optimal temperature after running for a while, and providing a more stable resistance, I did not realize the DUT was affecting the flucuations in voltage.   Thank you!!!!   And now I understand why the relay would be a bad idea.  That's pretty smart, how they designed this to work with those NTCs.   Pretty simple, but effective.

[Spork Schivago]

Is there a way to modify the isolation transformer so instead of blowing fuses if a DUT tried to draw too much current, it would temporarily shut down until the load was removed or the short / whatever was fixed?

I thought maybe a PolyFuse, but I think they would be a really bad idea here because I don't think they make one with the proper paremeters, such as the Vmax or Imax being acceptable to just replace the fast acting fuses with a PTC.   I also worry that maybe they wouldn't open fast enough.   I was thinking maybe with some additional parts, I could use a PTC or something.   I feel I might blow a fuse more times than I'd like to replace them, using this to help repair electronics.

Why would a ground plane or a grounded chassis on the DUT cause problems with an isolation transformer?   To the isolation transformer, doesn't it just appear that the secondary side of the transformer is the primary power source?   The begining and end of the circuit?

**EDIT:  I think I made a mistake, thinking I was done with the modifications of this isolation transformer.   I unhooked the grounds from the outlet side and from the chassis side, however, I really think I should have the grounds connected to each other, but still floating.   This way, let's say two people are working on two seperate devices, and both devices have a short.   One to line, one to ground.   If they don't realize it and touch each other, while touching their DUT, they could be shocked, perhaps fatally.   So I think I should daisy chain the output grounds together, and leave them floating (just connected to each other).

[Cliff Matthews]

IMO, grounds from each DUT should float. An 8-amp breaker on the output should work. https://www.amazon.com/AMP-32VDC-250VAC-CIRCUIT-BREAKER/dp/B00OGL7GMO/ref=sr_1_3?keywords=8+Amp+Circuit+Breaker

Now "if 2 people touch each other, while touching their shorted DUT's" I don't know what say.. (sounds kinky for starter's and second, one DUT would require neutral to touch the chassis, while the other would have to have the line touching the chassis..)

OK then, It sounds rare but you can put up a warning sign that says "DANGER - NO TOUCHING IN MY SHOP!!" 

[Spork Schivago]

IMO, grounds from each DUT should float. An 8-amp breaker on the output should work. https://www.amazon.com/AMP-32VDC-250VAC-CIRCUIT-BREAKER/dp/B00OGL7GMO/ref=sr_1_3?keywords=8+Amp+Circuit+Breaker

Now "if 2 people touch each other, while touching their shorted DUT's" I don't know what say.. (sounds kinky for starter's and second, one DUT would require neutral to touch the chassis, while the other would have to have the line touching the chassis..)

OK then, It sounds rare but you can put up a warning sign that says "DANGER - NO TOUCHING IN MY SHOP!!" 

Yes, it would be rare, but would there be any downfalls for having the output grounds connected?   The various devices would then have a ground at the same potential.   Is there any reason I wouldn't want to tie them together?   If there's no downfalls, even though it'd be a very rare circumstance that would need to occur for someone to get hurt, I'd rather take the extra precaution.

Wouldn't I need two circuit breakers?   I was thinking one like this:
https://www.amazon.com/dp/B004QMYXME/ref=sspa_dk_detail_0?psc=1&pd_rd_i=B004QMYXME&pd_rd_w=Sp2ZL&pf_rd_p=733540df-430d-45cd-9525-21bc15b0e6cc&pd_rd_wg=YAzN3&pf_rd_r=K9E4PA5W4FF9PPEPE7SZ&pd_rd_r=2a9f89bd-571c-11e9-8fee-2b0771c23e42

One rated for 5 amp, the other rated for the 10 amp?   Instead of having the lines go to the fuses, I just run them to the breakers instead?

[Cliff Matthews]

I see where you're going. You want to put the breaker on the primary side? (replacing the fuses?).

Not a good idea physically, since they are part of the IEC input assembly, and not a good idea electrically to eliminate them since they still should be used as a fail-safe. FWIW, the 5 amp fuse will only be used if red the primary 230v is selected as input, and it's worth noting the fuses are unrelated to what output voltage you select with the switch on the right side.

The breaker I mentioned was for the secondary (output) and would have been 9 amps, but I'm not sure those are easy to get.

[ArthurDent]

"Wouldn't I need two circuit breakers?   I was thinking one like this:

Make sure if you use breakers they are rated for the proper voltage. The rating on the one you showed is 32VDC for auto/RV use.

Also think about how long an overload will take to trip the breaker. Thermal type breakers like the one shown in the link or on most power strips can take some time with a moderate overload to trip. If you've ever used an electric space heater on a power strip, you may have had it trip after several minutes, then you had to wait a few minutes for it to cool down so it can be reset.

A fast acting fuse or a thermo magnetic trip breaker might be better.

https://thegrid.rexel.com/en-us/knowledge/product-faqs/w/wiki/323/how-circuit-breakers-combine-thermal-and-magnetic-protection

[soldar]

I can only upload 2MB of pictures at one time, so this will be in multiple posts.

You could reduce the pixel size and file size as a consideration to those of us with slow connections. There is no need to post such huge sizes.

[Gregg]

Fuses have a wonderful property of taking time to replace; time that one can reflect on why the fuse went open circuit.  Breakers are too easy to quickly reset without thinking; I’ve see bad results of just resetting a breaker without first carefully thinking of all the ramifications.
For circuit breaker over-current protection I suggest making up some 2 pole low current breakers to put on the output.  There are many DIN rail mount ones available like this: https://www.ebay.com/itm/MERLIN-GERIN-C60N-2-POLE-6-AMP-CIRCUIT-BREAKER/173373376160  Two pole breakers are preferred for the output since there is no common reference.  Many of the pop-out breakers like Cliff mentioned will not trip against finger pressure (if you continue to hold the button in after it tripped, it will stay connected). 
I have made up several boxes with three pole breakers like the one mentioned above for when I used to attach test equipment direct to 480 volt 3 phase bus bars.  I have them rated for ½ amp through 4 amp; they are very versatile and probably saved my butt more than once.
I wouldn’t worry too much about the breaker opening fast to protect the transformer, the fuse on the input should do that job quite well.  Transformers are very robust and will take fairly large overloads for a short time, heat being a major factor in insulation breakdown; that is why things like welders have a short interval duty cycle to get the ratings they advertise. As long as you don’t overheat your transformer and let it cool down after an overload, it will be OK.

[Cliff Matthews]

I can only upload 2MB of pictures at one time, so this will be in multiple posts.

You could reduce the pixel size and file size as a consideration to those of us with slow connections. There is no need to post such huge sizes.

Yes, IrfanView image compression was already suggested in #59
( https://www.eevblog.com/forum/beginners/suggestions-for-an-isolation-transformer/msg2315613/#msg2315613 )

[soldar]

Yes, IrfanView image compression was already suggested in #59

Sorry I missed that. I heartily support the recommendation for Irfanview which is what I use. I wish there were a native Linux version although it does run well under (or is it "over"?) WINE.

[Red Squirrel]

I've had good luck finding transformers in the 1kva and even higher on ebay.  I just put in actual numbers like "1000va transformer" (or 1kva etc) in the search then sort by "price + shipping".

I ended up impulse buying a 3000va variac (not isolated) and a 1500va transformer (isolated) a while back.  I have not gotten around to it yet but my goal is to build a basic AC power supply using both together.   The transformer is a 120v to 480v so I feed it with the variac and can get any voltage between around 1-2v to around 500v.  Typically will use in 120v mode as isolation transformer but having options for lower/higher voltages is nice too.

If I feel like getting fancy I might drive the variac with a stepper motor and arduino and add some basic programming features.

[Cliff Matthews]

Hmm.. rectumfried and filterated that's 700v @ ~3 amps!   Test that Arduino code, it could be your last compile!

[jerryk]

I just purchased a Triplite IS500HG  with the intent on using it as an isolation transformer.  This thread popped up right the same day UPS dropped it at my door so I have been following it and trying to make sure I do things right when it comes to removing grounds on the output side.

I am going to disconnect both output grounds the feed the outlets on the secondary side and leave the case grounded back to mains.  There is one more ground wire that runs from the main case grounding post to the secondary side of the transformer.  I don't understand what it does.  It goes inside the winding wrap but as far as I can tell it does not pass any sort of ground reference to the neutral or the transformer core.  Do I leave it connected to the case ground that goes back to mains ground?  What is it's purpose?

I hope I'm not intruding on this thread with my question and appreciate any help.

Thanks - Jerry

[Cliff Matthews]

Mains ground only needs to connect to the case and the transformers electrostatic shield (mentioned in #49). Have you measured any resistance from the primary and mains ground to the secondary outputs?

Interesting PCB just below the breaker with 3 caps only, I see silk-screen text pointing to 6 unpopulated NTC's..

[jerryk]

Thanks Cliff for the info on the electrostatic shield.  I definitely missed that in my following of this thread.

Between mains ground and line and neutral on the output sockets reads open circuit.  In it's original condition as shipped the ground is passed straight to the ground on the output side so that read .2 ohms (no resistance).  I have disconnected both of those grounds and left the case and electrostatic shield grounded to the case which carries back to mains ground.

I assume that the overload circuit protection is in the switch at 4.5amps. I'm not sure if that is an NTC (for inrush current limiting discussed earlier?) in the close up photo of the primary windings. I have no idea what the unpopulated area of the filter cap board would be for but am all ears for ideas.

Here are better views of the circuit board and the rest of the unit.

Thanks for the Help.

Jerry   

[Cliff Matthews]

You're good to go if you lifted the output ground. No need for mods since turn-on surge will not be the same as the OP's bigger 1kVA unit. The PCB seems to be on the output side and has jumpers (likely for another model) it's also not too clear which symbols would have been either NTC or MOV on the PCB.. no matter, those caps will do an OK job of shunting out HF crap.

FWIW, the 4.5a protector is not an NTC, they don't work like that. An NTC never goes open-circuit unless you've blown it apart by exceeding its rated current. As more current is passed, it's resistance goes down until it's negligible (and has almost no effect on the circuit passing current through it. 

[jerryk]

FWIW, the 4.5a protector is not an NTC, they don't work like that. An NTC never goes open-circuit unless you've blown it apart by exceeding its rated current. As more current is passed, it's resistance goes down until it's negligible (and has almost no effect on the circuit passing current through it.

The photo below shows what I thought might be an NTC.  It's stuffed between the windings and the core and I was curious if not an NTC what is it?

With regards to 4.5a overload protection, since there is no separate circuit breaker in the unit that the on/off switch pictured above was a switch/breaker combo.  I could not find any datasheet on that switch.

Thanks for great thread - Jerry

[Bratster]

FWIW, the 4.5a protector is not an NTC, they don't work like that. An NTC never goes open-circuit unless you've blown it apart by exceeding its rated current. As more current is passed, it's resistance goes down until it's negligible (and has almost no effect on the circuit passing current through it.

The photo below shows what I thought might be an NTC.  It's stuffed between the windings and the core and I was curious if not an NTC what is it?

With regards to 4.5a overload protection, since there is no separate circuit breaker in the unit that the on/off switch pictured above was a switch/breaker combo.  I could not find any datasheet on that switch.

Thanks for great thread - Jerry

Looks kind of like it might be a thermal fuse?

Sent from my Pixel 2 XL using Tapatalk

[Cliff Matthews]

Yup that's a thermal fuse (commonly known as a TCO or 'thermal cut-off). It will be wired on the mains side, since the 4.5 amp overload breaker seems to be on the secondary side.

[Spork Schivago]

"Wouldn't I need two circuit breakers?   I was thinking one like this:

Make sure if you use breakers they are rated for the proper voltage. The rating on the one you showed is 32VDC for auto/RV use.

Also think about how long an overload will take to trip the breaker. Thermal type breakers like the one shown in the link or on most power strips can take some time with a moderate overload to trip. If you've ever used an electric space heater on a power strip, you may have had it trip after several minutes, then you had to wait a few minutes for it to cool down so it can be reset.

A fast acting fuse or a thermo magnetic trip breaker might be better.

https://thegrid.rexel.com/en-us/knowledge/product-faqs/w/wiki/323/how-circuit-breakers-combine-thermal-and-magnetic-protection

Thanks for that info!   Yes, I was worried about how long it'd take to trip.   I like the idea of a thermo magnetic trip breaker.   There's currently fast acting fuses in there on the primary side.   The ones I linked to I believe are rated for 32VDC or 125VAC, aren't they?   That's the way I read them.

[Spork Schivago]

I can only upload 2MB of pictures at one time, so this will be in multiple posts.

You could reduce the pixel size and file size as a consideration to those of us with slow connections. There is no need to post such huge sizes.

Yeah, I was using some free on-line jpeg compression utility, but it was only cutting the size in half, or a little bit more, enough to allow me to upload them.   Then I downloaded IrfanView and I'm just using that now.   I think it's much better anyways.   I had forgotten that IrfanView could reduce sizes until a member mentioned it.   Otherwise I would have used that from the beginning.   I will try to keep them small, especially for people with slower connections.

[Spork Schivago]

Here's the specs for my Craftsman 82357 DMM, just in case anyone wants them:

Code: [Select]

Basic Accuracy DC voltage 400mV - 1000V ±(0.1% of reading)
AC voltage 4V - 700V ±(1.0% of reading)
DC current 400µA - 20A (fused) ±(1.2% of reading)
AC current 400µA - 20A (fused) ±(1.5% of reading)
Resistance 400½ - 40M½ ±(0.5% of reading)
Capacitance 100nF - 100µF ±(2.0% of reading)
Temperature 0 to 1999°F / -20°C to 1370°C ±(3.0% of reading)
Frequency 500mHz - 10MHz ±(0.1% of reading)
Duty Cycle 0.1 to 99.9%


[Cliff Matthews]

Another as the site says, "rescued from the dustbin of history" if it works, don't fix it..    Site says the meter was made for Sears by Wens Precision Company and it seems their DMMs are all portable data loggers now. Interesting, never heard of them.. but it seems 5-years ago they were sold in AU. 

[SoundTech-LG]

Ok, finally got around to popping the top off of this beast again. The other photo is the Buck wired autoformer to drop my 240 Volt service down to roughly 208 Volts for this 5KVA server room step down isolation Xformer. It has a surge suppressing delay ckt. in it, and 2 fans to keep it cool. Overkill.

[Spork Schivago]

Should I add a fan to keep my isolation transformer cool?

[Cliff Matthews]

If not used 24/7, I'd vote no. It's UL approved, has large aluminum case, full-load is rare, it's ~90% efficient, fans irritate, the list goes on..

[Spork Schivago]

I think we're done with this thread then.   Thanks for all the help!   Gonna save some cash and order those C14 to NEMA 5-20R adapters.   They where a bit hard to find.   I could find the NEMA 5-15Rs all over the place, but the NEMA 5-20Rs are harder to find.   I only found one company who makes them.

https://www.stayonline.com/power-cords/iec-320-c14-plug-adapters-9958.asp

10$ each, and I want 11 of them.   9 for the transformer, 2 for my PDU to plug in my ONT and MoCA adapter.   I have business class internet for my datacenter, however, I feel the hardware my ISP has provided is more residential than business.   I do have the option of having a dedicated fibre line ran directly to my router but that is extremely expensive.   Hopefully, if my business takes off, I'll be able to afford that sometime.   That's not related to this thread through.   Thanks everyone for all the help and suggestions!!!!!   I understand a lot more now than I did when I started.

[Ian.M]

An IEC C14 plug is only rated for 10A (15A intermittent duty in North America), so you only need a couple of those C14 to NEMA 5-20R adapters for bench use so you don't have to change out NEMA 5-20P plugs on equipment under test.  Everywhere else you need an IEC C14 to NEMA 6-??R adapter for  permanant use the equipment  should *NOT* have a  NEMA 5-20P plug (if it has, its probably going to overload the C14 plug), so you can use the cheap and readily available the IEC C14 to NEMA 5-15R adapters.

[GTA]

I'm bringing back this old post to say that I have three available isolation transformers for sale to anyone who may be interested. They're Toroid Corp's ISB-060M, 600VA medical grade iso-transformers and I've listed them in the Buy/Sell/Wanted section: https://www.eevblog.com/forum/buysellwanted/fs-(ca)-isb-060m-600va-medical-grade-isolation-transformer/

[Manny Sparks]

If you want a secondary side GFCI to function you need to bond secondary neutral to ground before the GFCI which means the secondary circuit is no longer floating, which in many applications defeats the purpose of having an isolation transformer in the first place.

What if the ground is a floating ground i.e. a wire that connects the chassis of the appliances together and then to neutral before the GFCI, but is not connected to earth? My country's electrical code specifically requires that the ground not be connected to neutral, but it doesn't say why. I suppose the reason is a path to earth could appear if, say, a washing machine leaks water, bridging the gap between its chassis and the ground.

[Zero999]

If you want a secondary side GFCI to function you need to bond secondary neutral to ground before the GFCI which means the secondary circuit is no longer floating, which in many applications defeats the purpose of having an isolation transformer in the first place.

What if the ground is a floating ground i.e. a wire that connects the chassis of the appliances together and then to neutral before the GFCI, but is not connected to earth? My country's electrical code specifically requires that the ground not be connected to neutral, but it doesn't say why. I suppose the reason is a path to earth could appear if, say, a washing machine leaks water, bridging the gap between its chassis and the ground.

The neutral of an isolation transformer is not the same as the mains neutral. Your isolation transformer is floating, whilst the mains neutral is already connected to earth, normally either at the building entrance, or substation. Your country's electrical code will not be violated by connecting the neutral on the secondary side of an isolation transformer to earth. Indeed, there will be some situations when it's required or recommended, but those won't apply to a bench top transformer for test purposes.

[p.larner]

can i homebrew one of these?,i have two big 220-240v transfos from old farnell psu's,can i not just wire them back to back?,if so what happens wrt to earth/ground?.

[watchmaker]

I just went through this.  Then I asked about it on the TEA subgroup.  The relevant posts are bookmarked below.  The bottom line is that isolation transformers are no longer recommended.  It turns out that adds complication to the setup and the safest procedure is to use a differential probe. 

And since DUT is on your bench because it is defective, what if that defect is related to a shock hazard that a GFCI would detect?

https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg5267169/#msg5267169

I spent hours trying to digest the conflicting info on the internet. I became totally befuddled  because my experience with iso xformers in the shop class during the 60s and 70s predisposed me to believe in them.   Much has been learned in the last 50 years!  THANKS for those on TEA who got me straight.

I found that differential probes are like DMMs, as tech requirements changed, industry had to upgrade and dump perfectly good obsolete probes.  I bought a Yokogawa produced until 2017 for $75.  You do not need to spend $200 for a new but questionable probe!

This is cheaper than the Jameco Iso xformer I returned.

After that discussion, I found this Youtube which provided visual and empirical demonstrations that echoed what was said in those posts.

[madires]

I just went through this.  Then I asked about it on the TEA subgroup.  The relevant posts are bookmarked below.  The bottom line is that isolation transformers are no longer recommended.  It turns out that adds complication to the setup and the safest procedure is to use a differential probe. 

Be aware that in some specific situations an isolation transformer is still very useful, e.g. when repairing old TVs or radios with a hot chassis.

[watchmaker]

I think I learned the differential probe takes care of this.  But what you say is what I did in fact learn 50 years ago.

If taking VI measurements, use a battery DMM.  The issue as I came to understand it has very little to do with personal safety other than having an earthed probe come into contact with the wrong wire and exploding in your hand.  You do always need to follow the one hand in pocket rule.

The diff probe and battery DMM avoid this.  At least this is how I changed my behavior.

Regards,

Dewey

[madires]

Hot chassis means that the chassis is connected to mains. With a polarized mains plug it should be connected to neutral, but with a non-polarized plug there is a 50% chance it's the line/phase. You could check that before plugging in the DUT and turn the plug accordingly. Or you could use an isolation transformer. The problem with a hot chasis is that one can easily touch it by accident, even with one hand in the pocket (-> rubber gloves).

[Ian.M]

That was true in the hot chassis tube radio era as a half wave rectifier tube could supply enough current direct from mains, and the cost of implementing a bridge rectifier was prohibitive but transistorised and later integrated circuit hot chassis TVs typically had a bridge rectifier feeding the DC bus, making the chassis hot with half-wave rectified mains no matter which way round you connected the power plug.

There were also oddities like Salora's Ipsalo II  which omitted the usual switching transformer, and instead had a line output stage with an isolated primary, so the line output stage and line scan coils were 'hot' but most of the rest of the circuit wasn't

Working on such equipment was a specialised trade requiring extensive training, usually formally taught at a trade school or by one-to-one mentoring. The bench setup was quite particular as well, mostly in terms of removing as many grounds as possible from the bench and nearby area, and minimising the risk of contact with those that couldn't safely be removed or insulated (e.g. the CRO).  The problems with learning it from the internet are: finding reliable information in the first place that isn't either dangerously out of date or dangerously wrong; and also you've got nobody watching your back and correcting you if you start to develop dangerous habits.