Protection bulb tester

Light bulb tester

This tool is recommended for testing amplifiers (particulally the psu) in early development stages.

Do NOT try to bias up a ClassA Power Amplifier through the protection bulb tester.

Do NOT try to bias up a high bias ClassAB Power amplifier through the protection bulb tester. By high bias I mean Iq>=40mA.

How does it work ?

Provided by pacificblue

Light bulbs are temperature-dependant resistors. The higher the temperature, the higher the resistance (PTC - positive temperature coefficient). That is, why they usually blow the moment, they are switched on. Their resistance is low then.

Now connect an amplifier in series and switch it on. The inrush current is high and will probably make the bulb light up for a short moment. If the amplifier is okay, there will not be much current draw after that -> low current through the lamp -> no heat, no light -> low resistance -> no voltage drop across the lamp -> full voltage at the amplifier. If the amplifier has a fault, it will draw a big current -> filament heats up, lights up -> resistance goes high -> high voltage drop across the lamp -> reduced voltage at the amplifier. There is your cheap and easy current limiter.

Provided by Redshift187

If the bulb is wired in series with the Live as it should be, all current must pass through the bulb. At low current, the filament will not heat much, and resistance will be low. As current rises, the filament heats, and resistance rises. It acts as both a current limiter and a warning device (if the bulb is full on, there may be a short). This protects your equipment (and you!) while allowing you to check voltages.

.........from diyaudio forum members