# Transformer Question

Started by February 2, 2017
```On Saturday, February 4, 2017 at 12:34:34 AM UTC-5, Phil Allison wrote:
> dca...@krl.org wrote:
>
> >
> > If you really  want to spend very little, get a couple of microwave ovens
> > and use two transformers to make one isolation transformer.
> >
> >
>
> ** Completely crazy idea that cannot work.
>
> Too many reasons to bother going into here, but it is both stupid and dangerous.
>
>
>
> .....   Phil

My experience is that it is not dangerous.  If you do a little searching on the internet, you will find numerous articles on making arc and spot welders using microwave transformers.  So apparently others have  had the same experience as -I have had.

Dan
```
```On Thu, 02 Feb 2017 13:43:39 -0600, boomer#6877250@none.com wrote:

>I was looking at the commercial isolation transformers and they are very
>costly, so I decided to build my own. After all, all they are is a
>transformer with a power cord on the primary and an outlet (and fuse) on
>the secondary. And I already have an enclosure to put it in.
>
>I'm looking at a bare transformer to use as an 120v isolation
>Transformer. (120v in, 120v out). The transformer primary is 480 / 240.
>The secondary is 240 / 120.
>
>This is for single phase 60 cycle AC. (U.S. power).
>
>Will it work if I connect the 240 lugs on the primary to 120 volts, and
>use the 240 lugs on the secondary to obtain 120 volts.
>Electrically, this makes sense, but I am not 100% sure, so I thought I'd
>ask.
>
>Also, this Transformer is rated at 750 va.
>
>Using the calculator chart on
>http://www.rapidtables.com/calc/electric/va-to-amps-calculator.htm
>750 va should give me 6.25 A output. (Which should be enough amperage
>for anything I need to test on my bench).
>
>However, since this transformer was intended to be used at 240 / 480 on
>the primary, will it still give me 750 va (6.25 A) on the secondary if I
>run it on 120v?
>
>Thanks
>

A dial-primary (120:240 to whatever) transformer makes a good
isolation transformer. Use the two primaries and ignore the secondary,
unless you have some use for it too.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On Saturday, 4 February 2017 17:16:45 UTC, John Larkin  wrote:

> A dial-primary (120:240 to whatever) transformer makes a good
> isolation transformer. Use the two primaries and ignore the secondary,
> unless you have some use for it too.

as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other?

NT
```
```On Sat, 4 Feb 2017 09:32:13 -0800 (PST), tabbypurr@gmail.com wrote:

>On Saturday, 4 February 2017 17:16:45 UTC, John Larkin  wrote:
>
>> A dial-primary (120:240 to whatever) transformer makes a good
>> isolation transformer. Use the two primaries and ignore the secondary,
>> unless you have some use for it too.
>
>as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other?
>
>
>NT

Kilovolts, probably. Test it for margin if you plan to float the load
way off ground.

It will have more inter-winding capacitance than one of those
extreme-expense isolation things.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On Saturday, 4 February 2017 18:30:34 UTC, John Larkin  wrote:
> On Sat, 4 Feb 2017 09:32:13 -0800 (PST), tabbypurr wrote:
> >On Saturday, 4 February 2017 17:16:45 UTC, John Larkin  wrote:

> >> A dial-primary (120:240 to whatever) transformer makes a good
> >> isolation transformer. Use the two primaries and ignore the secondary,
> >> unless you have some use for it too.
> >
> >as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other?

> Kilovolts, probably. Test it for margin if you plan to float the load
> way off ground.

that's at odds with my experience of hipot testing transformers. Enamel on enamel failed below 1kV. Unfortunately I don't remember what voltage it managed. Probably over 500v, but that's not a comfortable margin.

> It will have more inter-winding capacitance than one of those
> extreme-expense isolation things.

Yes. I don't know how much shock current that lets through.

NT
```
```On Sat, 4 Feb 2017 11:58:08 -0800 (PST), tabbypurr@gmail.com wrote:

>On Saturday, 4 February 2017 18:30:34 UTC, John Larkin  wrote:
>> On Sat, 4 Feb 2017 09:32:13 -0800 (PST), tabbypurr wrote:
>> >On Saturday, 4 February 2017 17:16:45 UTC, John Larkin  wrote:
>
>> >> A dial-primary (120:240 to whatever) transformer makes a good
>> >> isolation transformer. Use the two primaries and ignore the secondary,
>> >> unless you have some use for it too.
>> >
>> >as long as you don't want too much isolation. What voltage breakdown do you get with the 2 primaries wouund one directly on the other?
>
>> Kilovolts, probably. Test it for margin if you plan to float the load
>> way off ground.
>
>that's at odds with my experience of hipot testing transformers. Enamel on enamel failed below 1kV. Unfortunately I don't remember what voltage it managed. Probably over 500v, but that's not a comfortable margin.
>
>
>> It will have more inter-winding capacitance than one of those
>> extreme-expense isolation things.
>
>Yes. I don't know how much shock current that lets through.
>
>
>NT

I made a twisted pair of #36 magnet wire and applied DC until it
arced. 1400 volts.

Somebody should test a few ordinary dual-primary transformers for
breakdown.

Winding-winding capacitance is easy to measure. Grounding the frame
will reduce 3T capacitance.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On 2017-02-04, dcaster@krl.org <dcaster@krl.org> wrote:
> On Saturday, February 4, 2017 at 12:34:34 AM UTC-5, Phil Allison wrote:
>> dca...@krl.org wrote:
>>
>> >
>> > If you really  want to spend very little, get a couple of microwave ovens
>> > and use two transformers to make one isolation transformer.
>> >
>> >
>>
>> ** Completely crazy idea that cannot work.
>>
>> Too many reasons to bother going into here, but it is both stupid and dangerous.
>>
>>
>>
>> .....   Phil
>
> My experience is that it is not dangerous.  If you do a little searching on the internet, you will find numerous articles on making arc and spot welders using microwave transformers.  So apparently others have  had the same experience as -I have had.

microwave oven transformers are not designed to provide safety isolation, only
functional isolation, that is why microwave ovens are grounded.

definitions: http://www.ti.com/lit/wp/slyy063/slyy063.pdf
(last paragraph of first column of page 2)

so yeah, if you don't need safety isolation you can do that.

2 layers of funtional isolation won't get you safety. it takes 2
layers of basic isolation to do that.

Microwave ovens typically have one end of the secondary grounded to
the transformer core, so, if you hook 2 together hoping for two layers
of functional isolation you need to not ground the transformer cores,
else you only get one layer.

Microwave oven transformer cores are typically welded shut, so
changing the windings is tricky (maybe you could open one with a 1mm
thick cut-off wheel and re-close it using a MIG welder). but still
you only get two layers of functional isolation, unless you do a
complete replacement of the windings.

I got my 1500VA isolation transformer for \$8 from the municipal
recycling shop, this was in 2001, before the surge in copper
prices.

--
This email has not been checked by half-arsed antivirus software
```
```On Sat, 4 Feb 2017 00:21:41 -0800 (PST), klaus.kragelund@gmail.com
wrote:

>You should not make an isolation transformer your self if you don't have extensive experience
>
>Isolation transformers must have double insulated design which involves special copper wire,

Special copper wire?

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On Sat, 04 Feb 2017 13:53:14 -0800, the renowned John Larkin
<jjlarkin@highlandtechnology.com> wrote:

>On Sat, 4 Feb 2017 00:21:41 -0800 (PST), klaus.kragelund@gmail.com
>wrote:
>
>>You should not make an isolation transformer your self if you don't have extensive experience
>>
>>Isolation transformers must have double insulated design which involves special copper wire,
>
>Special copper wire?

He's probably thinking of triple-insulated wire, and I don't think
you'll find that in any 50/60Hz isolation transformers. It's useful in
SMPS transformers where you want to put the windings as close together
as possible to link all the flux, but you don't care much about
primary-secondary capacitance.

--sp

--
Best regards,
Spehro Pefhany
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8
```
```On Sunday, February 5, 2017 at 12:27:49 AM UTC+1, Spehro Pefhany wrote:
> On Sat, 04 Feb 2017 13:53:14 -0800, the renowned John Larkin
> <jjlarkin@highlandtechnology.com> wrote:
>
> >On Sat, 4 Feb 2017 00:21:41 -0800 (PST), klaus.kragelund@gmail.com
> >wrote:
> >
> >>You should not make an isolation transformer your self if you don't have extensive experience
> >>
> >>Isolation transformers must have double insulated design which involves special copper wire,
> >
> >Special copper wire?
>
> He's probably thinking of triple-insulated wire, and I don't think
> you'll find that in any 50/60Hz isolation transformers. It's useful in
> SMPS transformers where you want to put the windings as close together
> as possible to link all the flux, but you don't care much about
> primary-secondary capacitance.
>
Nope, for a transformer the basic insulation in case of an earthed laminated core or the double insulation in case of no earth depends on the wire classification

Typical class F wire, which is approved for 155 degrees C.

See for example section 7 in this:

https://www.indiannavy.nic.in/sites/default/files/tender_document/Transformer.pdf

Or first table on this link:

https://en.wikipedia.org/wiki/Insulation_system

During maximum power through the transformer or shorting of the secondary side, the winding gets hot, and must stay below 155 degrees to still have the insulation system in place

Cheers

Klaus
```