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#### sisesa

##### New Member
I have the attached circuit and have the problem that the 10K pot
always burn even with open circuit. What is the mistake ?

I believe this is the circuit you meant to post

Assume that the SCR, the 2-100ohm resistor and the 2-50uF caps aren't there. Since the SCR doesn't conduct when first start up. Then the only path to your electric drill (if it is a drill conected to the AC socket) is thru the POT.

See if you follow me, out of the rectifier into the POT, out of the pot into the AC socket and back to the rectifier. Lets say whatever you connect to the AC socket (like the drill) pulls 5 amps. So you have pretty much a 10K resistor limiting the current of the drill. At 120Vdc and the drill trying to pull 5A......and the POT limiting this current , converting it into heat......

120V @ 1A is like a 120W bulb to relate it to something.
So I believe this is why your POT keeps on burning.

Ivancho

The explanation of how the circuit is going wrong is described by ivancho.
But in the HTML it says it is rated higher than 5amp.

Suggestions
1) Make sure your SCR is connected correctly.. if the gate is not fed with
the voltage, then the current will go through the pot as described.

2) Make sure the SCR is WELL heatsinked.

3) Make sure the SCR is not open circuit (faulty)

4) Check orientation of the Diode D1 make sure the connections are not reversed.

Basically any time that the SCR does not turn on would cause this problem.
I think that if you have no load you may also have a problem too.

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