guys, pls see pictures of curcuits and advice me .

by 1,2 cutcuits , it is ok , all working good.
but by circuits 3 , problem .
bridge rectifier gets too hot very fast , and variac is humming, ( seems a lot current draws from variac to capacitor. )

i have no idea how to fix this , and can't understand what is going wrong

pls ???

 

A variac does not give any isolation. (It is an autotransformer.) One side of your signal source is probably connected to ground which will be shorting out diodes in the bridge rectifier. Use a proper isolating transformer.

Les.

 

A variac does not give any isolation. (It is an autotransformer.) One side of your signal source is probably connected to ground which will be shorting out diodes in the bridge rectifier. Use a proper isolating transformer.

Les.

Les , thanks

so, you mean , don;t use variac, instead of variac use simple step down transformer??

 

Yes, Or have a 230 V to 230 V isolating transformer between the mains supply and the variac. The problem is that the neutral side of the mains is connected to ground at some point in the power distribution system. So as you are grounding the negative output of the bridge via the signal source some of the diodes will be shorted out.

Les.

 

Yes, Or have a 230 V to 230 V isolating transformer between the mains supply and the variac. The problem is that the neutral side of the mains is connected to ground at some point in the power distribution system. So as you are grounding the negative output of the bridge via the signal source some of the diodes will be shorted out.

Les.

Aaaan , I seeee, Understood well. Les , thank you very much.

may i put isolation transformer between the variac and capacitor ?

 

no no , between variac and bridge, , bridge will be after isolation transformer

 

You can use any solution that provides a floating AC supply to the input of the bridge rectifier.
NOTE you may have allready destroyed some of the diodes in the bridge rectifier.

Les.

 

The Vce polarity of bjt were wrong in the circuit 1, 3.