Cheap Isolation Transformer ??

Now would *I* do something like that ?:-)

...Jim Thompson

Why waste two transformers? MCM 28-3480 1A 120 VAC Isolation Transformer $14.23 each. page 169, catalog #48.

A lot of power transformers these days have dual primaries -- I use a small one to float an audio preamp...

Jim, why are *you* asking *this*? It sounds like a newbie question.

Unless you have some critical requirements, I can't see why it's not obviously a reasonable thing to do. But you know that.

Strange. Have you been crawling around the insides of chips for too long? Heck, you could have simulated two trannys and found out it'd work, with just a somewhat more mediocre regulation than with a dedicated one, right?! :-D

Anyway, a respectable Stancor P-6411 straight isolation transformer (with leads), 115VAC, 0.13A, 15VA is $15.37 from Allied, catalog # 928-3033.

If you want one with a cover, plug and socket, they get a little more expensive, the Stancor GIS-100 0.87A 100VA is $63.05 catalog # 928-0325.

I recently bought a 500VA Stancor, and it's real nice. I returned a Hammond 500VA because it got too hot.

Good day!

If you also need low voltage power, try one of the universal dual-primary

Or rip the isolation transformer out of the shaver outlet in your bathroom.

I don't handle transformers every day, thus the question.

Bought two 25.2V/450mA filament transformers for $11.05 including tax.

...Jim Thompson

The most problem with identical filament transformers back-to-back is the regulation, which can easily be 10-20% for each small transformers, so 20-40% for the pair. So with 115VAC in, the output starts out with 110-114VAC at no load, but might drop under 80VAC on load.

At the higher power end, I used a pair of 60+60VAC/500VA toroids (both c>>

Tony (remove the "_" to reply by email)

The only thing you might have to keep in mind is that the primary windings are sized to carry both the rated VA product current and the magnetising current.

The data sheet doesn't usually tell you what the magnetising current is, and it can be quite big for small transformers. A preliminary measurement to establish the magnetising (no-load) current through the primary might be in order to let you work out how much you have to reduce the VA rating for the pair.

Since I'm on your kill-file, you aren't going to get to read this unless I've made a mistake and someone posts a correction.

------ Bill Sloman, Nijmegen

There's another probem waiting for you too Jim. Transformers have copper losses, so your open circuit V_out for the first TF might be say 10% or 20% high off load. This means the 2nd TF dosent have the turns ratio one would think, and when driven at nominal V on its secondary will never produce V_mains at its primary, it will always be low.

The quick dirty best avoided way is to use TFs with primary taps - here TFs had primaries with 200,220,240v taps for a long time: adjusting the tap setting can correct the problem. But to do that one must ensure the thing is properly fused, thermally cut-out-ed, and fireproofly housed, since doing so can result in substantial overcurrents, shorting metlting and fire. So for those that dont understand how to engineer safely round the implications of that, dont.

Using TFs rated at well over what youre drawing helps, since the deregulation drops to only a percentage of the nominal regulation figure.

A better way is to use a variac with your iso setup. A better way than that is to add a few more turns onto your first TF's secondary to get the right off load V_out on TF2. And an even better option is to add more primary turns on TF2, long as you understand how to do it without compromising insulation.

At best back to backing is a rough hack. Is that what you need?

Regards, NT

If you genuinely did need 100mA, (or a lower current dc from a rectifier+capacitor), then you could be disappointed. To get a reasonably good 115V from back-back transformers for those loads would require transformers of around 50VA nominal rating.

If you only need (say) 10mAdc or so for neon tube experiments then you will probably get away with it.

...(waits for smoke to clear ....)

Jim? Jim? Is that you Jim?

I *have* been known to clear a lab ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

Bingo! You guessed my application.

...Jim Thompson

yes, which will load up TF1 considerably, further dropping the V_out.

there is that.

Regards, NT

It's running with no noticeable heat... 116VAC input gives 111VAC output... must be better than average Radio Shack transformers.

...Jim Thompson