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Variac / Variable Transformer. - Printable Version

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Variac / Variable Transformer. - Benny/W5TYZ - 01-29-2016

I have been looking for a Variac with isolated output.  
I have yet to find one.  Staco has a model 3PN J401,
that is output isolated.  I found one for $289.00. 2 much.

What is involved in going inside a regular unit and set
it up as Isolated Output?
 
I found a couple that have a statement on the exterior
next to the output that states."One side common to line and load" 
What does this mean?
Would that be the Neutral line is common all the way to the
mains?  The hot side is the one being transformed...Right??

RE: Variac / Variable ransformer. - Nathan Slingerland - 01-30-2016

I believe the Sencore PR-57 models are single unit isolated variacs. Those tend to be pricey though because they are intended to test different modes of isolation failure as well.

There's no reason you can't use a separate isolation transformer after the variac (before would work too I think). Just plug the variac into the wall, the isolation transformer into the variac, and the radio into the isolation transformer.

When I was starting out I bought something like this:
http://www.ebay.com/itm/Variac-Variable-AC-Power-Transformer-with-Meter-0-130-VAC-Max-20A-TDGC-2KM/121872830853
A couple things I learned since then: the meter isn't useful (too small). And 20A is really overkill. One with a 5A or so rating would be plenty for radio work. Also, don't trust the dial markings - I just use my DMM to monitor the voltage.

I made an isolation transformer box using one of these that I got off of ebay for ~$50:
http://www.digikey.com/product-detail/en/M4L-3-10/M4L-3-10-ND/1118196

There are also isolation transformer boxes made for hospitals and labs that show up on eBay too.

RE: Variac / Variable Transformer. - Benny/W5TYZ - 01-30-2016

Thanks for the info Nathan.  I still have an isolation Xfmr.
I sold my Variac several years ago and never replaced it.
I thought this time I would try to find one with isolated output.
I thought maybe someone may have reworked a Variac to
make it isolated output.  I may just pickup a 5 or 10 amp
Variac, and use the Isolation Xfmr with it.
I have looked at the PR-57's, and they ARE pricey....
I guess if this was a sho'nuff repair business, the investment
would pay for itself.  After all it is a hobby.  We have a local
Hamfest here the end of next month, and just maybe I'll find
something useful.

RE: Variac / Variable Transformer. - 462ron - 01-30-2016

Benny, I have been using an RCA-Isotap II WP-27A. It isn't continuously variable but instead you can change output voltage in 5 volt steps starting at 25 volts all the way up,to 150 volts and it is an isolation transformer too! I think a different version on of this is a VIZ-IsotapII WP-27A. I've seen them on eBay for around $40.

Ron

RE: Variac / Variable Transformer. - Benny/W5TYZ - 01-30-2016

OK Ron...
Thanks for the tip.

RE: Variac / Variable Transformer. - Warren - 01-31-2016

Here is some good info on an isolation transformer from morzh. might give you some info your wondering bout.

http://philcoradio.com/phorum/showthread.php?tid=4698

RE: Variac / Variable Transformer. - Eliot Ness - 01-31-2016

I gave up trying to find a PR57 because they were all going for more than I wanted to pay.  I did find a similar BK1655 on eBay a couple of years back that was much more reasonable, so you might want to search for one of those.  In the mean time pairing your isolation xfrmer with a variac is a good solution.

I have an old General Radio Company W5MT3 variac that is built like a tank that I bought at a hamfest several years ago pretty cheap.  I still see them on eBay but they aren't quite as cheap anymore:

http://www.ebay.com/itm/General-Radio-Transformer-Mod-W5MT3-/261916002262?hash=item3cfb6907d6:g:~ZAAAOSwpdpVcQVj

RE: Variac / Variable Transformer. - Benny/W5TYZ - 01-31-2016

I have looked at the W5MT3 models and wondered about the statement
on the end "One side common to line and load". Really what are they
saying?

I am still working on radios, and looking for something reasonable that will
still do the job. Looks like finding a Variac to go with my isolationXfmr, will
be the best way to go for now......

RE: Variac / Variable Transformer. - Eliot Ness - 02-01-2016

(01-31-2016, 04:36 PM)Benny/W5TYZ Wrote:  ........wondered about the statement on the end "One side common to line and load".  Really what are they saying?......

A common variac does not provide isolation as this general diagram shows (neutral is common to the both line and load):

[Image: http://www.afcaforum.com/attachment.php?id=53827]

Unless you buy something like a PR57 or BK 1655 that has a built in isolation transformer you'll need to use an isolation transformer in-line with a variac for true (and safe) isolation.

I guess it bears repeating for others that when you buy an isolation transformer don't automatically assume it is isolated.  Ohm it out with a meter (while unplugged of course) to make sure everything is isolated.   Some recent medical grade isolation transformers are actually a leakage reduction transformer.

Like morzh I bought a Dale IT400 and while checking it out discovered like he did that it had a jumper that had to be removed to make it an isolation transformer.  Morzh documents his experience here:

http://www.philcoradio.com/phorum/showthread.php?tid=4698

RE: Variac / Variable Transformer. - TV MAN - 02-02-2016

I'm gonna throw a monkey wrench in this line of thought. I believe it is a good idea to have BOTH a separate variac and isolation transformer. Why? You NEED the protection of the isolation transformer when working on a bare chassis. You need to be able to vary the voltage to bring up, test and watch with the variac. In the real world, a radio/tv or such is designed to work directly off the line. My point? When you want to test something, there ARE times when you NEED to connect it directly to the line, but still have the variability of the variac. On the ac/dc radio sets, the chassis is connected directly to one side of the ac line. Fine when it is plugged in correctly. If it is reversed, then the chassis becomes "hot", and can give you a pretty good shock. The variac connects directly to the ac line, one side is simply a short from your outlet to the variac outlet, the other side runs through the transformer. It works like a dimmer switch that are on ceiling lights. So, you get NO protection using only a variac. Besides, if the variac or isolation trans fails, you only need to replace one or the other. I think it is good plan to have both. My experience anyway. Icon_e_biggrin

RE: Variac / Variable Transformer. - Conelrad - 02-03-2016

Here is my retro-style homebrew iso-var. 

Will handle 1kW as a bare variac, 375VA when isolated up to 600V.

I use it for many tests, not just bugging out receivers.

Dennis


RE: Variac / Variable Transformer. - Benny/W5TYZ - 02-06-2016

I lucked out and found this Variac on Ebay, that has the isolated output.  I went to the company and downloaded the wiring diagram.  When it gets here, I'll check the circuit and make sure no one has changed anything.  I was the only bidder at $49.99, so that made it even better.

RE: Variac / Variable Transformer. - codefox1 - 02-27-2016

Well, Suppose you have two big surplus 115 to 7.5 or whatever hefty surplus transformers and run 'tm back to back, you would get needed isolation, and then hookup your variac whether isolated or not, and do your testing. Yeah, well, I think the next stupid trick may be my last, so it takes me a little longer bench time to sort out the problem of the day. "Belt and suspenders, Gents, we want you here next year!"