Working on eventually building a isolation transformer with variac.

Looking around in old parts I have laying around. I have one of those euro 240v step down to 120v 1000w transformers, sure be nice to put it to use. Is there a way to get 120-120v with it? I might have an old large auto charger (broke), guessing that might be 120 to 12. These are both large transformers, would like to use them if I could.

 

Sounds like it might be a UK building site transformer, 240 - 120. Normally only one secondary.
Max.

 

You could use the transformer to give 0-60V out or 0-240V out.
The 0-60V out would give twice the current, and the 0-240V out would be half the current of the Variac rating.

 

Sounds like it might be a UK building site transformer, 240 - 120. Normally only one secondary.
Max.

being I live in the US, it's one of those over seas stepdown transformers you use electronic devices. It's just sitting on the shelf and has a nice size transformer in it.

You could use the transformer to give 0-60V out or 0-240V out.
The 0-60V out would give twice the current, and the 0-240V out would be half the current of the Variac rating.

need 120v. you need a 240v input on the variac?

if I had two close match of those euro 240-120 set down trans. I could hook two together. Might have to hit the resale shops and see if I can find a used one. I could take the set down 240-120, plug into 120v and get 60 and take the two secondary hook together and get 120v? Where would the wattage or current fall if I tie 2 transformers together like that?

 

You could use the transformer to give 0-60V out or 0-240V out.
The 0-60V out would give twice the current, and the 0-240V out would be half the current of the Variac rating.

Cruts is right! And for testing things often the full 120 volts is not needed. And it seems that with the 0-240 volt arrangement you will be limited by the wattage of the variac.

 

Cruts is right! And for testing things often the full 120 volts is not needed. And it seems that with the 0-240 volt arrangement you will be limited by the wattage of the variac.

can you vary the input voltage like that into the variac?

 

it's one of those over seas stepdown transformers you use electronic devices.

Those are typically autotransformers and are not isolated.
Have you checked to see that there's no continuity between the input and output leads?

can you vary the input voltage like that into the variac?

You can lower the input voltage but not increase it.
For the 120V-240V connection, you would need to put it at the output of the variac.

 

being I live in the US, it's one of those over seas step down transformers you use electronic devices. It's just sitting on the shelf and has a nice size transformer in it.

I left mine behind in Italy along with the refrigerator it had hanging on it.

Pretty much 240 / 120 volt step down transformer. Problem being there is no way to get split phase 120 volts from it.

Ron

 

Indeed it is important to use an ohm meter to verify that it is an isolation transformer and not just an autotransformer.

 

The baby brother of the Sencore is here: http://bama.edebris.com/manuals/sencore/pr57/ the bigger brother is the PR570

See: https://antiqueradios.com/forums/viewtopic.php?f=8&t=243655

The only thing I miss on my Variac/iso transformer is the ability to know it''s on at low or no voltage. A nice addition would be an electronic fuse.

Should mention that Variac is a brand name, but who is gonna say variable autotransformer or an isolated variable autotransformer?

 

You can add some safety if you plug the variac into a GFI (ground fault interrupter) socket.
That should minimize the big sparks and/or electrocuting yourself if you cross wires somewhere to earth ground.

 

Cruts is right! And for testing things often the full 120 volts is not needed. And it seems that with the 0-240 volt arrangement you will be limited by the wattage of the variac.

When you dig into the details, it's not the best response. While the wattage is one limit, the wiper current is more important.
Say, it'a a 1200 W, 10 A, 120 V Variac. You fuse it at 10A. Partially right. It's not a bad idea to put a 10 A fuse in the line side.
But 20A at 1 Volt is only 20 W, but your Variac will be toast with that load. You MUST fuse the wiper at 10 Amps.

 

When you dig into the details, it's not the best response. While the wattage is one limit, the wiper current is more important.
Say, it'a a 1200 W, 10 A, 120 V Variac. You fuse it at 10A. Partially right. It's not a bad idea to put a 10 A fuse in the line side.
But 20A at 1 Volt is only 20 W, but your Variac will be toast with that load. You MUST fuse the wiper at 10 Amps.

Totally correct! I do recall seeing powerstats fried because of excessive wiper heating.

 

I left mine behind in Italy along with the refrigerator it had hanging on it.

Pretty much 240 / 120 volt step down transformer. Problem being there is no way to get split phase 120 volts from it.

Ron

actually the one I have was made in Italy.

After thinking about what was said here. I am under the assumption that you plug the variac into the isolation transformer. The isolation trans is plug into AC source. Seems like from the conversation, some were saying plug the isolation transformer into the variac, varica plugged into ac source. Not sure makes difference, but all I seen isolation trans are plugged into the ac source.

The step down is only 2 prong, no ground. references my earlier post, cannot I go trans #1 - 240v to 120v, trans #2 120v to 240v, being plugged into a 120v.. = #1 - 60v, 2x current.. take secondary's of #2 120v to 240v and get 120v? What happens to the current after the final transformer?

From this you end up with 120v to feed the varica? Just trying to save buying more parts then I need. Will this work? Most of the circuts have ground, but everything I seen they remove GRS at the variac. The isolations transformer is grounded, that could always be grounded if necessary.

 

The downside of a Variac or auto-transformer is that there is no isolation from mains power. That can get to be a safety factor in many forums. So what you have is a 240 / 120 single phase step down transformer. Here are a few options. You can connect the primary side of the 220 volt / 120 volt transformer which will give you about 60 VAC single phase out (figure about a 2:1 ratio). That 60 VAC can be applied to the input of your Variac or Auto-Transformer. The end result will be a Variac with an output of 0 to a little over 60 volts. Another option is if you have 220/240 residential available the step down as it was designed and they the Variac so now you have Isolation and a Variac out of 0 to about 130-140 volts.

Been some years but if I recall correctly the transformers I worked with taps so we could compensate for Voltage In / Voltage Out. While designed around 50 Hz power they should work OK on 60 Hz. The fridge I mentioned ran fine for 3 years on 50 Hz and was designed for 60 Hz. God knows what else I ran on that thing.

Ron

 

Have checked for continuity from the input to output of the transformer to see if it's actually isolated?

 

Been some years but if I recall correctly the transformers I worked with taps so we could compensate for Voltage In / Voltage Out. While designed around 50 Hz power they should work OK on 60 Hz. The fridge I mentioned ran fine for 3 years on 50 Hz and was designed for 60 Hz.

There were taps on the transformers I used too. Like you said, up to about 140 V with 120 in.

 

There is also "what is an isolation transformer". Here https://toroid.com/Products/Medical-Isolation-Transformers is a medical isolation transformer. In a lot of cases, a real isolation transformer may have a breakdown voltage on the order of kilovolts.
I had a transformer custom wound by them in the 80's for an audio amp.

Isolation transformers can reduce noise in a system when the ground is bonded to the secondary.

I had really good luck with a surge suppressor (ISOBAR) and an isolation transformer made from OneAC, now PowerVar. They say the same thing.

 

The downside of a Variac or auto-transformer is that there is no isolation from mains power. That can get to be a safety factor in many forums. So what you have is a 240 / 120 single phase step down transformer. Here are a few options. You can connect the primary side of the 220 volt / 120 volt transformer which will give you about 60 VAC single phase out (figure about a 2:1 ratio). That 60 VAC can be applied to the input of your Variac or Auto-Transformer. The end result will be a Variac with an output of 0 to a little over 60 volts. Another option is if you have 220/240 residential available the step down as it was designed and they the Variac so now you have Isolation and a Variac out of 0 to about 130-140 volts.

Been some years but if I recall correctly the transformers I worked with taps so we could compensate for Voltage In / Voltage Out. While designed around 50 Hz power they should work OK on 60 Hz. The fridge I mentioned ran fine for 3 years on 50 Hz and was designed for 60 Hz. God knows what else I ran on that thing.

Ron

That might be the best way, if I use that older step down trans. I would have to run a 220 line over to my work area. Might be the better . Then would only need variac.

Have checked for continuity from the input to output of the transformer to see if it's actually isolated?

will check that in a bit, have not checked.

 

That might be the best way, if I use that older step down trans. I would have to run a 220 line over to my work area. Might be the better . Then would only need variac.

Pretty much it. On the bright side 12/3 would be adequate. Those transformers were designed for 240 (about) primary in and 120 (about) secondary out. The one I had was a beast and hardly even ran warm. While they are not cheap they do make GFCI breakers so if you have a few open slots in your panel you could go that route.

Ron