Hello All,
The other day, as I was strolling through Radio Shack, I stumbled across two 120 x 25 VAC @ 2-1/2 amp secondary transformers sale priced for $10 bucks each. Since I've been looking for an industrial open frame transformer with a 1:1 ratio and a VA rating of between 250-350 for isolation and haven't been able to find one easily, I bought these, and wired the secondaries back to back to effectively create a 1:1 isolation transformer. I installed these inside of my homemade variac enclosure, with a selector switch off of the outlet to enable the choice of variable 0-130 VAC, or straight (isolated) line voltage, with my volt and amp meter in the circuit. The isolation transformers are first in line, so every bit of AC potential is isolated from the service line.
$20 bucks well spent. Since most of your transformer-less radios draw less than 40 watts, this is more than enough. I also installed a circuit breaker on the incoming AC line for safety.

Thumbs up for wanting to save money! I'm always for that notion.

Two identical transformers "back-to-back" is a reasonable solution for a low power isolation transformer. If the original goal was 200VA, then the dual transformer solution doesn't make it...nominally it is 60VA, but actually a bit less because one transformer is "back fed."

Here is an article from Hammond about "back feeding" transformers.

http://www.google.com/url?sa=t&rct=j&q= ... 0516,d.cGE

$20 is actually an expensive solution for a 50VA isolation unit. I think you could have found something at a ham swap meet with more VA for less... but your idea works.

Rich

Yep, exactly.

I wouldn't run this at more than 50 VA for any length of time.
Inside an enclosure you don't get as much cooling as you would if the transformers are out in the open.

- Leigh

50 VA isolation transformer at Mouser. Lots of windings so you can choose input and output voltages (in case you need to run European radios).

Hard to believe, but it's under $12.

http://www.mouser.com/ProductDetail/Tri ... 1nrh28Q%3d

Rich

Yes, I use the Triad from Mouser often, it is a good value for the money. At 50VA it runs too hot to touch, but within the specs, and I have not had any fail even when operated all day long.

Others on ARF have been doing the back to back filament transformer rig for many years. While it works, I would prefer a single transformer which was actually engineered to be used as an isolation transformer.

The Triad transformer from Mouser might be a good choice as a built-in for a hot-chassis radio if there's room, or in a separate box if not.

Maybe we should add a sticky for it in the resources list.

- Leigh

I couldn't get the back-feeding link to come up. But here it is:
http://www.automationdirect.com/static/ ... ormers.pdf

50VA would be too small for any comfort level. The reality of it, is a 30watt load is only 0.25 amps which allows this somewhat cobbled up arrangement of mine to run very cool. Certainly long enough for initial testing and alignment, but as I've come to realize around here, everybody's a critic.

No. Everybody is concerned with getting accurate information in the thread.

All of these threads are archived, and can be easily retrieved by any search engine.
When somebody pulls this thread up a year from now, we want them to find correct answers.

It's not about just answering a specific question for a specific situation.

- Leigh

Yes I know, sorry for getting "huffy"
My original idea, was a cheap alternative to the $62.00 Mouser 250 VA split primary isolation transformer. I do know about the disadvantages to back-wiring transformers having had to do it in the field many times. I have measured the current draw with an inline Fluke on the first xfmr secondary feed to the second xfmr primary, and at a peak, when you switch on a typical AC/DC set, the highest it goes is 1.2 amps, and then drops down to 0.5 amps when the radio is running. My only use for these, is to make initial testing safe, in conjunction with the variable transformer, and of course, for alignment. It's so much nicer when you don't have to concern yourself with grounding issues between your signal generator and your chassis.

Again, sorry about the attitude, no disrespect intended.

That's what I use them for, on some sets you can mount it on or above the chassis on a bracket, but other times it has to be external. Great for metal cabinet sets with the line connected to the chassis like some of the Hallicrafters models and minimal insulators separating the two.

Triad says they are good for 50VA and supposedly it is safe to run a modern transformer at its full ratings. Older sets with 300 ma filaments sometimes run at about 40 to 48 VA, sets with 150 ma filaments around 30 or so as mentioned earlier, both of which should be just fine for this transformer.

Pete: your back-to-back set-up is only good for about 50 VA. 25V x 2.5 amps = 62.5 VA. De-rating a bit for back-feeding, I estimate that 50VA is a good overall rating. Not much different than the Triad unit selling at Mouser.

Rich

I concur. It will run pretty warm even at that loading.

- Leigh

Well...It was still a low-cost nifty idea... at least you were thinking and having fun conjuring up stuff.
... it's just that the anticipated numbers needed some tweaking... that's all...

I have seen that done for small tube projects since small 125-150V trannies with filiment windings are getting scarce. Someone just takes two transformers with 6.3 or 12.6 Volt secondaries, ties the secs. together, taps off for the filiment there, and then rectifies the output of the output "primary". Can easily work for small projects using tubes that draw smallish currents. Wastes space with two trans. but saves money if you can get the two trans really cheap or free.

I had the bright idea once of using my UPS or my portable battery car jumpstarter that has a built-in inverter. No connection to the mains at all, right? Except they were so RF-noisy, they were useless for anything but dead-set analysis. Then they doubled as signal injectors.
-Ed

Nowadays there's a lot of dual primary & dual 2ndary transformers available that you can do that with if rated properly.

I recently used one exactly that way for my audio amp power supply.
I fed the 120AC into one primary and used the other primary for 120v 2ndary.
Then the two 12v 2ndaries were used for the filaments.

Pbpix: good solution. I use the same idea for some of my two-tube radio projects.

It works well as long as you don't need to utilize the full VA rating of the transformer. Usually, the full VA rating is based on using both 120V primaries in parallel, or in series if you have a 240V line. I was checking the Signal data sheet and they don't explicitly say the primaries have to be paralleled. I wonder if other members have any further info on this?

For light loads, using that other primary as a B+ option is a good solution.

Rich

It could be unsafe to do that unless the two primaries are on separate bobbins as there is probably not enough insulation between the two windings to have them isolated using one as a secondary since the transformer was not engineered to work like that. I wouldn't do it.

I agree that you will end up with somewhere around half the original VA rating of the transformer by using only one section of the primary as a primary.