Where can I get an isolation transformer?

Thread Starter

strantor

Joined Oct 3, 2010
5,696
I've built a small batch of devices, which it turns out need isolated power. If I had known, I would have used isolated supplies. But they're already built and it's too late for that. So I'm looking for a small/cheap isolation transformer in an enclosed package that has a 120V input plug and output receptacle. Things like this are the closest I've found, but that's overkill and not exactly cheap. This "wall wart" device is more like what I had in mind, but instead of a receptacle it has screw terminals, which is not safe for 120V. If that wall wart had a receptacle on the back side, it would be perfect. Something like this could work if there were more room around the receptacle area.

Power requirements are low, about 80VA.
 

BillB3857

Joined Feb 28, 2009
2,549
I have used dual voltage primary filament transformers for low current requirements of some shop built test fixtures. Usually, the two primaries are either connected in series for 220V or parallel for 110V operation. I my application, I used one of the primary windings to the line, and used the other as a secondary, providing 110V ISOLATED to supply the unit I was working on.
 

peterdeco

Joined Oct 8, 2019
415
I think this might be what Bill was describing. 2 transformers 120VAC to 12VAC. Number one to 120V to supply 12V, the 12V connects to number two 12V secondary side. Number two primary now has 120VAC.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,696
I have used dual voltage primary filament transformers for low current requirements of some shop built test fixtures. Usually, the two primaries are either connected in series for 220V or parallel for 110V operation. I my application, I used one of the primary windings to the line, and used the other as a secondary, providing 110V ISOLATED to supply the unit I was working on.
I think this might be what Bill was describing. 2 transformers 120VAC to 12VAC. Number one to 120V to supply 12V, the 12V connects to number two 12V secondary side. Number two primary now has 120VAC.
Not a bad solution for a home brew application but I have 20 of these devices needing isolation and I don't have 40 filament transformers laying around, nor would I like to build 20 boxes to put them in.
 

SamR

Joined Mar 19, 2019
3,617
Check out Jameco. They have an inhouse value brand that was cheaper than any others last time I checked.
 

Tonyr1084

Joined Sep 24, 2015
6,466
I can get 230V isolating transformers easily here
https://www.louthtx.com/control-panel-range
https://airlinktransformers.com/cat...ange?input_voltage=230&output_voltage=230&va=
so wherever you, are a local transformer manufacturer will make ones for your local voltage.
It may be worth noting that a transformer rated for 230 volts will have no problems with handling 115 volts. You can always go down but you can't go up. Well, actually you can to a degree, but it's not recommended. I wouldn't use a 115 volt transformer on a 230 volt circuit. But if all I had was a 230 and all I needed was 115 then I'd have no qualms about using it. The voltage rating of a transformer is similar to the voltage rating on a capacitor. You don't NEED to have an exact volt match but you can't exceed the design capabilities.
 

Ian0

Joined Aug 7, 2020
3,785
It may be worth noting that a transformer rated for 230 volts will have no problems with handling 115 volts. You can always go down but you can't go up. Well, actually you can to a degree, but it's not recommended. I wouldn't use a 115 volt transformer on a 230 volt circuit. But if all I had was a 230 and all I needed was 115 then I'd have no qualms about using it. The voltage rating of a transformer is similar to the voltage rating on a capacitor. You don't NEED to have an exact volt match but you can't exceed the design capabilities.
Running a 230V transformer on 115V does wonders for the inrush current! You can't get the same power, but you can probably get a little more than the original rated current, as core losses will be so much less, that you can allow a little extra copper loss.

I think the TS is located in USA, so carriage from UK would probably be silly money, especially when there must surely be similar companies in USA. Airlink also does a 230V in 115V out with split primaries, so could be wired as 115V:115V.
55-0-55 is also a popular voltage here for the ubiquitous "site safety" transformer (an explanation for those based elsewhere: Health & Safety law requires 110V tools on building sites, run from a transformer with the centre-tap earthed, so that no part of the tool could be more than 55V above earth potential)
 

Thread Starter

strantor

Joined Oct 3, 2010
5,696
I've ordered the Hammond 171A transformers found by @MrChips , from Mouser. If anyone happens to find something better, please keep it to yourself so as to preserve my confidence that I've made the best possible purchase.
 

crutschow

Joined Mar 14, 2008
28,227
Isn't it strange how uncommon this is?
It would seem you found quite a few of them.
Why do you say they are uncommon?

The best option may be to use the Jameco device and deal with the Euro type output connector.
I would think you would not want a standard connector on the output anyway, so that someone doesn't inadvertently plug your device into an outlet without the transformer.
That would be a big liability issue.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,696
It would seem you found quite a few of them.
Why do you say they are uncommon?
  • I found a meager 3 examples, none of them met my needs, and I used them as examples of what I wasn't looking for, to supplement my explanation of what I was looking for.
  • I can't go to any local electrical supply and pick one up; I called around, everyone has control transformers that I could mount inside an enclosure, and potted power transformers, but not self-contained isolation transformers, plug-&-play with standard NEMA5-15 plugs & receptacles installed, requiring no enclosure.
  • Apart from ordering online, the only place I found to get them locally was Grainger, at their customary insane markup, and even then, they didn't have them in stock, had to order them in.
  • There was precisely one device uncovered that met my needs in all of my searching, and I wasn't even the one who found it. Even in the category of the ones that didn't meet my needs, the choices were very few.
The best option may be to use the Jameco device and deal with the Euro type output connector.
I quote myself:
I've ordered the Hammond 171A transformers found by @MrChips , from Mouser.
I would think you would not want a standard connector on the output anyway, so that someone doesn't inadvertently plug your device into an outlet without the transformer.
That would be a big liability issue.
It won't be a liability issue.
 
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