Author Topic: Suggestions for an isolation transformer  (Read 5859 times)

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Offline Spork Schivago

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Suggestions for an isolation transformer
« on: March 27, 2019, 01:19:04 am »
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.
 

Offline Jwillis

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Re: Suggestions for an isolation transformer
« Reply #1 on: March 27, 2019, 02:58:29 am »
Get a 1kVA 230v - 115v step down transformer wire a GFCI receptacle or breaker to it  and put it in an enclosure .
 
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Online Ian.M

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Re: Suggestions for an isolation transformer
« Reply #2 on: March 27, 2019, 06:19:20 am »
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.
 
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Offline Gregg

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Re: Suggestions for an isolation transformer
« Reply #3 on: March 27, 2019, 05:29:07 pm »
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
 

Offline soldar

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Re: Suggestions for an isolation transformer
« Reply #4 on: March 27, 2019, 07:20:07 pm »
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.
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Offline Cliff Matthews

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Re: Suggestions for an isolation transformer
« Reply #5 on: March 27, 2019, 08:04:16 pm »
1kVA and enclosed - That's likely a 45lb chunky box, there will be serious $hipping charges..
 

Offline Gregg

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Re: Suggestions for an isolation transformer
« Reply #6 on: March 27, 2019, 08:18:09 pm »
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
 
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Offline Zero999

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Re: Suggestions for an isolation transformer
« Reply #7 on: March 28, 2019, 01:35:19 pm »
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.
 
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Offline Cliff Matthews

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Re: Suggestions for an isolation transformer
« Reply #8 on: March 28, 2019, 02:26:07 pm »
As suggested, DPDT switches can be used on both sides..
 
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Offline Zero999

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Re: Suggestions for an isolation transformer
« Reply #9 on: March 28, 2019, 03:01:11 pm »
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.
 

Offline Spork Schivago

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Re: Suggestions for an isolation transformer
« Reply #10 on: March 28, 2019, 03:22:49 pm »
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.
 

Offline Spork Schivago

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Re: Suggestions for an isolation transformer
« Reply #11 on: March 28, 2019, 03:27:36 pm »
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!
 

Offline Wolfgang

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Re: Suggestions for an isolation transformer
« Reply #12 on: March 28, 2019, 03:37:56 pm »
... 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.
 
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Offline Cliff Matthews

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Re: Suggestions for an isolation transformer
« Reply #13 on: March 28, 2019, 04:58:01 pm »
... 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

Quote
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..
« Last Edit: March 28, 2019, 05:49:06 pm by Cliff Matthews »
 
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Offline Wolfgang

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Re: Suggestions for an isolation transformer
« Reply #14 on: March 28, 2019, 05:44:01 pm »
The inrush current of toroidal transformers is *larger* than classic types. Reason is type of core material and less stray flux.
« Last Edit: March 31, 2019, 10:58:05 pm by Wolfgang »
 
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Offline edgelog

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Re: Suggestions for an isolation transformer
« Reply #15 on: March 28, 2019, 05:57:03 pm »
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?
 
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Offline Cliff Matthews

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Re: Suggestions for an isolation transformer
« Reply #16 on: March 28, 2019, 06:10:07 pm »
He ordered a 600.
 

Offline Gregg

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Re: Suggestions for an isolation transformer
« Reply #17 on: March 28, 2019, 08:57:19 pm »
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)
 

Offline Cliff Matthews

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Re: Suggestions for an isolation transformer
« Reply #18 on: March 28, 2019, 10:13:26 pm »
Take it apart  >:D  We need pics!  IMO $58 for 1kVA is a good deal members should know about (while it lasts..)
 

Offline Gregg

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Re: Suggestions for an isolation transformer
« Reply #19 on: March 29, 2019, 02:24:04 am »
Take it apart  >:D  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. 
 

Offline Spork Schivago

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Re: Suggestions for an isolation transformer
« Reply #20 on: March 29, 2019, 04:03:13 pm »
... 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!!!!
 

Offline Spork Schivago

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Re: Suggestions for an isolation transformer
« Reply #21 on: March 29, 2019, 04:06:34 pm »
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.
 

Offline Spork Schivago

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Re: Suggestions for an isolation transformer
« Reply #22 on: March 29, 2019, 04:07:44 pm »
He ordered a 600.

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

Offline Spork Schivago

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Re: Suggestions for an isolation transformer
« Reply #23 on: March 29, 2019, 04:10:37 pm »
Take it apart  >:D  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!!!!!!!
 

Offline Spork Schivago

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Re: Suggestions for an isolation transformer
« Reply #24 on: March 29, 2019, 04:15:33 pm »
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.
 


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