Isolated transformer ?

[bianchi77]

Everyone,

Does a DIY isolated transformer safe enough for fixing job ?
I saw :
http://www.instructables.com/id/Test...vage/?ALLSTEPS

Cheers

[eccerr0r]

Re: Isolated transformer ?

Two filament transformers back to back will work and is safe, but the power rating is likely too low to be very useful. Unless you find some really beefy (and identical) filament transformers, probably should probably just get a real isolation transformer, unless you're just working on, say, very low power wall warts.

[bianchi77]

Re: Isolated transformer ?

Two filament transformers back to back will work and is safe, but the power rating is likely too low to be very useful. Unless you find some really beefy (and identical) filament transformers, probably should probably just get a real isolation transformer, unless you're just working on, say, very low power wall warts.

for example I have two identical 8A 220V primary and 15V secondary..
Will I get 120VA on the isolated transformer ? or how much is the loss ( efficiency ) ?

[eccerr0r]

Re: Isolated transformer ?

Usually the secondary is limiting, so if you have a 15V 8A secondary you'll probably get just about 100VA depending on transformer losses, and both would get some losses.

I was wondering about my 12V 1A filament transformers which wired as an iso transformer I'd only get 100mA 120VAC or less before possibly frying the transformers... and indeed this is only good enough for small wall warts...

[bianchi77]

Re: Isolated transformer ?

Usually the secondary is limiting, so if you have a 15V 8A secondary you'll probably get just about 100VA depending on transformer losses, and both would get some losses.

I was wondering about my 12V 1A filament transformers which wired as an iso transformer I'd only get 100mA 120VAC or less before possibly frying the transformers... and indeed this is only good enough for small wall warts...

so it's 10% efficiency ? 1A to 100 mA ? how do you know it's 100mA from 1A...did you test it with bulb ? 60W bulb with 120V ...it will need 0.5A

[eccerr0r]

Re: Isolated transformer ?

It's the volt-amps of the inner section that limits, plus losses.

No, this isn't "efficiency" but rather max power output

So you have pri:sec -> sec:pri of two filament transformers. When you go through the first transformer you have a max of 12VA because of the 12V 1A. Now that 12VA gets fed into the second transformer, and you get an output of 120V (about). But since you fed in 12VA, you get only 12VA so that means you only can get 100mA out at 120V.

This is just hypothetical, I didn't actually do this but the math works for this kind of iso transformer..

If my center voltage stayed at 12V and I need to light a 60VA light bulb =~ 60W, then that center section needs to be 60VA (or more) which means at least 5A needs to be flowing in the secondary.

Transformer efficiency is a different beast, though as long as the transformer doesn't get warm or leaks too much magnetic field, it should be fairly efficient, at least 80-90% efficient if not more. Multiply the two efficiencies together for the two transformers and get 64% to 81% efficient isolation transformer. However I'd guess that the 80% efficient transformers are kind of rare (and unwanted). Transformers can be built quite efficient.

A real 1:1 winding iso transformer is definitely more efficient as you only lose on one core and two windings, instead of losing on 2 cores and 4 windings.

[redwire]

Re: Isolated transformer ?

Tried back-back filament transformers but they each have 10-20% regulation and I believe the secondaries are overwound extra turns to make up for copper losses. End result is twice the mush, 30-40% regulation. It depends on the VA rating but too sloppy for some gear.
I use a transformer with dual 115VAC primaries for isolation- one winding for input, the other winding for output. Works OK but I de-rate by 50% because one winding is feeding the core instead of two.
Lately I use a GFIC on my workbench to stop any drama. But to 'scope around a primary switcher, gotta have an isolation transformer.

[R_J]

Re: Isolated transformer ?

I built one a long time ago using an old microwave oven transformer, I chopped the highvoltage secondary out of it and rewound it (can't remember the wire guage). aprox 1 volt per turn, it worked quite well.

[eccerr0r]

Re: Isolated transformer ?

That MOT modded as a isolation transformer probably handle more watts than any of these filament transformer solution, probably better than my true blue iso transformer (though the one I have is tapped so I can somewhat change the voltage).

However I've heard that MOTs were designed on the cheap side and tend to run slightly saturated as the duty cycle of microwaves tend not to be 100% - does the transformer get warm?

[Andyy]

Re: Isolated transformer ?

If you can find a couple dead computer UPS, you can extract the transformers and wire the secondaries to each other. Choose transformers from higher VA rating, and derate when using.

[R_J]

Re: Isolated transformer ?

that microwave oven transformer did run a bit warm, the laminations are welded together on those. I ran a 1500 watt heat gun off it (for a load test) and the voltage hardly droped. but I did'nt run it all day so it worked fine, I have a hammond 1500va now that I use to isolate the whole bench.

[bianchi77]

Re: Isolated transformer ?

that microwave oven transformer did run a bit warm, the laminations are welded together on those. I ran a 1500 watt heat gun off it (for a load test) and the voltage hardly droped. but I did'nt run it all day so it worked fine, I have a hammond 1500va now that I use to isolate the whole bench.

Can I use 2 transformer with different ampere but same voltage ?

[eccerr0r]

Re: Isolated transformer ?

yes but the overall rating will be of the lower rated transformer.