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\Dim light short circuit tester

paddy ryan

New Member
May sound silly, but having just built a basic dim light short circuit tester, and being in the UK on 240V, can I use this on a 112V Projector to test for a short circuit as well?
 

Ylli

Active Member
Assuming you are referring to what is normally called a Dim Bub Tester (DBT). If the projectors is designed for 112V, then you do not want to plug it into a 240V Line. Depending on how much the unit under test draws, and the wattage of the *incandescent* bulb used, the voltage to the UUT can get quite close to the applied line. You would need to add the appropriate transformer or a variac between that 240V line and the DBT.

And a DBT is not exactly a 'short circuit detector'. It is more of a non-linear dynamic current limiter. An incandescent bulb has the characteristic that it has a low resistance when cold, and the resistance increases substantially when hot. If the UUT is operating normally, and the bulb is chosen correctly, then the UUT will see very near full line voltage and operate pretty much normally - and the bulb will be 'dim' or totally out. If the UUT is drawing excess current, then the DBT will glow brightly, causing it's resistance to increase and reducing the voltage applied to the UUT. Not perfect, but it can often prevent damage to a device that would go up in smoke if connected directly to the line.
 

paddy ryan

New Member
Okay understood, thanks. Build was taken from u tube demo. Followed schematic from there. So I could use it through a transformer (at 112volts) to my 112 volt projector?
 

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