Current limiting "bulb" tester for battery powered preamp

[Hikari1]

When powering up AC powered devices for the first time I always use a "bulb" tester to limit current if there is a fault in the circuit.

I am working on a battery powered phono stage...about 30V (20xAA). Any similar devices I can put together for DC powered testing? Maybe a bulb in series? Any ideas?

 

[Enzo]

I would power it up with a bench supply during testing, that way you can monitor current draw.

 

[Speedskater]

With an AC powered system you use an appropriate line voltage rated bulb. So you might try a 30V bulb. (not many of those around)

 

[sofaspud]

Two 12v (automotive) bulbs in series?

 

[simon7000]

With a preamp the current draw is so small a light bulb limiter is not very useful. Try a 10 ohm 1/4 watt resistor. If you have a power rail short it will smoke.

The best technique is to use a power supply with current limiting. A worthwhile DIY project by itself. It can be as simple as a series pass MOSFET fed from a potentiometer across a Zener, with a current sense resistor in the ground leg feeding a transistor that can the shunt the pass transistor's drive. A meter across the output will show the voltage and one across the current sense resistor will show the current.

 

[Hikari1]

With a preamp the current draw is so small a light bulb limiter is not very useful. Try a 10 ohm 1/4 watt resistor. If you have a power rail short it will smoke.

The best technique is to use a power supply with current limiting. A worthwhile DIY project by itself. It can be as simple as a series pass MOSFET fed from a potentiometer across a Zener, with a current sense resistor in the ground leg feeding a transistor that can the shunt the pass transistor's drive. A meter across the output will show the voltage and one across the current sense resistor will show the current.

Thanks. I might try the 1/4 resistor...

 

[Elvee]

Thanks. I might try the 1/4 resistor...

A basic lab PSU is a must for experimenting. The light bulb is only useful for mains supply because there is no affordable substitute for an amateur but it has many drawbacks, like unpredictable drop when things are alright, slow response, undefined threshold (if there is even such a thing as a threshold in this case).

In short, for low-voltage DC, a limiting supply is vastly superior to bulbs, resistors, PTC's, etc.

It can be as simple and cheap as a low-power version of LM317 configured for 30V, which will also give you excellent regulation as a bonus