Suggestions for an isolation transformer

[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.