# Need an Isolation Transformer?

Discussion in 'Electronic Basics' started by amdx, Sep 24, 2013.

1. ### amdxGuest

I needed isolation from the line today, it reminded me of something I
learned while building a Cockcroft-Walton high voltage generator.

Use two transformers back to back.
Example, you have two 120V to 12V transformers,
(120v in 12 out) connected to (12v in and you have 120V out).
Or any two transformers with the same secondary voltage.
Current limit depends on the transformer size.
It works in a pinch.
Mikek

2. ### Phil AllisonGuest

"amdx"
** Not a good way at all.

The second transformer in the chain has to be driven in reverse and that
leads to large magnetising currents which the first must supply.

You can wind up with almost no capacity left for the load.

.... Phil

3. ### amdxGuest

You may be right Phil, or it might work with some transformers and
not others. I did use the 10kV from the Cockcroft-Walton multiplier to
drive an electrostatic motor. (very little current)
I gave the motor to the local science museum,
and they threw it a away. :-(

Motor plans here;
http://amasci.com/emotor/emotor.html

Mikek

4. ### ShaunGuest

"Phil Allison" wrote in message

"amdx"
** Not a good way at all.

The second transformer in the chain has to be driven in reverse and that
leads to large magnetising currents which the first must supply.

You can wind up with almost no capacity left for the load.

.... Phil

Phil knows this, he has connected to primaries of the second transformer to
his gonads and was not impressed by the small tingle he received.

5. ### Phil AllisonGuest

"amdx"

** It never works very well and is worst with small transformers. So called
1:1 transformers are not *wound* 1:1 - rather their voltage ratio UNDER

For example:

Imagine two 120V to 12V trannies rated at 24VA with regulation factors of
15% and ignoring magnetising currents.

The reverse wired tranny needs 13.8 volts input to give 120 out with NO load
and 15.9 volts with full load.

The normal wired tranny will give 12V when loaded - so is way under the
required voltage.

The output voltage will only be 12/15.9 times 120 = 90V.

Magnetising current in 12V winding of the reverse wired tranny makes the
available VA lower.

Using two dissimilar trannies ( to fix the above) does not work either, it
only makes the magnetising current in the reverse wired one huge, soaking up
all the VA rating.

What you gotta remember is that AC supply trannies are wound with the turns
ratio ADJUSTED to give the speced voltage ratio *under load* - PLUS all
magnetising current is meant to be in the primary, not the secondary.

..... Phil

6. ### Jasen BettsGuest

That's OK if you only need a little isolation.

7. ### Jeff LaymanGuest

Is it the same for toroidal transformers?

About 30 years ago I wanted to power a mains voltage pond pump (I think
50 watts) which was about 10 metres from the house. Not wanting to run
mains cable all the way, I got a couple of toroidal transformers from an
electronics surplus shop (now sadly extinct here in the UK). AFAIR they
were about 12 cm in diameter and 4 or 5 cm high (maybe rated at about
150 watts?), and had several secondary taps. I found I could get the
pump powered by using 33v from the first transformer, connected at the
far end to the 17v tap of the second one. Must have given about 230v on
load. The pump worked continuously for years using this arrangement.