Full bridge rectifier trouble / variac

[Etesla]

I have a very simple full bridge rectifier(4 diodes + capacitor) rectifying the output from a variac. The rectifier seems to be working fine, as I can see a dc voltage difference between the negative and positive output that is a little higher than the ac input from the variac. The problem I am having is that there is a 120v ac voltage difference between the negative output from the rectifier and earth ground. This makes probing my circuit pretty much impossible. Is this what is supposed to be happening, or is something wrong?

Extra notes:
I did not wire the variac, and there is a possibility that the grounding is not done correctly somehow...
Touching earth ground to the negative output of the bridge is a dead short, and my breaker trips immediately
I don't really want to float the variac or the scope cuz safety

Thanks!

[Benta]

It's quite dangerous what you are doing.
Variacs are normally autotransformers and provide no isolation between mains and secondary. Think carefully about what you are doing.

[madires]

If you want to use your variac as a simple PSU place an isolation transformer between mains and the variac. Otherwise you might win a Darwin award.

[Natee]

That breaker-pop was really best case scenario in this situation. Be thankful you have a good enough earth ground there! Your hesitation with floating the scope is at least commendable. It's really worth reading up and understanding galvanic isolation and circuit protection if you're running stuff directly off the mains.

To answer your question though, this behavior is indeed normal. You can get around this from an appropriate isolation transformer inserted before the variac and grounding its output, but... Do you really need the power this is going to deliver? It's going to be a pretty low impedance source so I would avoid plugging it in to circuits you're not sure about.

[Zero999]

Touching earth ground to the negative output of the bridge is a dead short, and my breaker trips immediately

What else would you have expected to happen?

Look at the flow of current through a bridge rectifier circuit and take into account that the earth ground and 0V side of the transformer are connected together either at your property entrance or local distribution transformer.


http://www.reuk.co.uk/wordpress/electric-circuit/bridge-rectifier/

Notice that connecting any connection on the AC side to the DC side will cause a short circuit through the diodes? If you've not killed the bridge rectifier then you're very lucky.

As others have mentioned, this is very dangerous to do, if you don't know what you're doing and you obviously don't otherwise you'd wouldn't have asked why the breaker tripped.

[Siwastaja]

Stop RIGHT NOW.

You are rectifying mains directly, and are surprised about what you are seeing.

Learn first. Don't kill yourself.

Get a transformer to work with extra low voltage. Variac is not one.

[Ian.M]

The order "Stop RIGHT NOW." is good, but the explanation "Get a transformer. Variac is not one." is not. 

Corrected:
Get a transformer with an isolated secondary . Variac is not one.

Depending on the DC voltage you need, either put an isolating transformer before the Variac or use the Variac to feed a transformer with a lower voltage secondary.  Assuming you have a transformer with a high enough secondary current rating, this can be a good way of getting more current at a lower voltage than the Variac can safely supply.  If you use a Variac to feed a transformer, *NEVER* use the Variac to boost the voltage higher than the transformer's primary rating.

[tecman]

By the way, the DC voltage w/o load will be approx 1.4 times the AC voltage (AC peak value)

paul