Bridge rectifier output

[electronics-whiz]

I got an old variac from a friend. I made a rectifier unit for it so I can have 0-140V DC. I'll probably be sticking around 0-30V though most the time. I took a 1500UF 250V filter cap I had pulled from an old computer PSU to use as my filter cap. I told him later what I made and he said the cap could blow due to over voltage even though the variac only goes to 140.

Said the output would be like 1.5 times the input, then did some other calculation and somehow at like 120 it came to like 280V. Then he tried showing me something with a sine wave and drew a line like 3/4 the way up it and that represented something. I didn't really understand why or where he was getting these calculations.

Seems odd too because the cap was on the 120V side after a rectifier. He kept saying I was confused and had to be the secondary side.   

Just seems odd because every time I dealt with rectifiers you put like 9V in you get like 8.75 out. Maybe a capacitor changes that somehow and all this extra stuff comes in.

Looking for clarification on this, and possibly an idea of what I should look at for a capacitor that I don't have to worry about issues should I run it up to 140V off the variac. I know caps in series the ralue divides in 1/2, i'm not sure the voltage rating changes though. Most computer PSU use two caps I assumed they were parallel, but maybe there series.

[retrolefty]

Your friend probably meant well but either misunderstands or you misunderstood. A 140 vac (RMS rating)variac rectifier/filter can charge up to a maximum 198 vdc (peak rating), so your 250 vac rated cap should be fine.

[tautech]

Have a Google look for:"ac rectifier to dc voltage"

This image shows whats happening:

[LukeW]

The peak voltage is sqrt(2) * the RMS voltage, so if the input is 140V RMS the output will be about 200V DC. So your 250V cap will probably be fine but a voltage rating a bit higher would not hurt.

A common "rule of thumb" you see with rectified mains AC in offline SMPS, for example, is that in the 240V countries like Australia you get about 340V out and use a 400V capacitor, and in the 120V countries you get about 170V DC and a 250V, or so, capacitor will probably be acceptable.

[PA0PBZ]

That is why the filter cap you took out of the old psu is rated 250V. But I'm more worried that your variac is non-isolating, which would mean that your DC output will be directly connected to the mains like this:



Be very careful and make sure you understand fully what you are doing!

[electronics-whiz]

I know the variac isn't isolated. For what i'm doing I don't really see any issue with that. I have an old isolated power conditioner I can put between line and variac if I have some reason for that. I like this because can power lamps, motors, etc from whatever voltage I want up to 7A. I have bench supply if set in series will do 0-60V at 1/4A or parallel 0-30V at 1/2 amp.  Pretty much all that is good for is logic stuff.

I think rectifier will give some degree of isolation too (or so seems to me).

[Stonent]

My favorite explaination: