i have a variac and a 100watt light bulb built in series in a short extension cord to limit the wattage to 100watts. i was builing a power supply and went to test it throught the variac and bulb. the bulb started to light up which made me glad i pulled the variac out. as i was trying to find out what was wrong i turned the variac up a little more than before and blew a breaker. that shouldn't happen should it. i thought that was the idea of the bulb

 

The idea of the bulb in series is to limit the currentflow.

In case of a 100 watts bulb at 230 Volts you limit the current to about 450 mA.
for 110 Volts nearly 1 Ampère.

When you have the bulb in series with the variac it will glow because of the magnetising current of the variac.

When you test a power supply you don't need to use the variac but put the 100 Watt lamp in series.

Phase, 100 watt lamp, powersupply, Neutral.

Bear in mind when your powersupply draws more than 100 Watt the lamp will be at full brightness and you may have to put a 500 watt lamp or 1 kW heater in series to limit your current in the circuit under test.

Regards, Raymond

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If above works satisfactorily then the bulb may be taken out of the circuit.

If your braker trips then there could be a fault in your powersupply or variac.

An insulation check is a good idea for your powersupply if it blows fuses.

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There are more ways to get to Rome.

Electricity, Electric clocks, Meters and Trains are great.

 

i have the bulb after the variac. and no load on the supply. ony rectifiers and caps. not even a regulator.

 

If the variac is in series with the power supply (or other load) it would seem that the worst case condition, in terms of load on the variac, would be 100 watts if the variac, at full voltage, is limited to the nominal line voltage - presumed to be 120 volts. That condition would occur if you short circuited the load. The fact that the breaker tripped suggests something is wrong with the apparatus or the 100 watt load is exceeding the capacity of the breaker.

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stevez

 

Instead of everyone guessing, can you be more exact to tell everyone what does "a little more" means?

130V? All the way clockwise? Can you show a picture of your variac?

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L.Chung

 

i started at about 20% of full voltage and a little more was about mabey 25%. i will take a pic when i get home from work.

 

Why would you do that?

I'd put the lamp in series with the live so it will limit the current if it shorts to the case.

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