Another happy TAG light bulb current limiter user.

[armillary]

Thanks to TAG for the great instructions on building a light bulb current limiter.

It's been a great teaching tool for current draw on amp startup. And great for checking the status of an unknown amp, after checking for obvious safety issues/damage/etc. This thing is definitely going to get some use.

Anyway, here's another happy TAG light bulb current limiter user. Thanks again!

[Structo]

Yep, great cheap tool!

[armillary]

I wanted to share a misconception I had about the light bulb current limiter. I had assumed that if the current limiter light bulb is not glowing fairly brightly, then the current to the load (the amp under test) was not being limited. But it is.

For example, if I plug a Champ into the 40W light bulb current limiter and turn it on, the bulb burns white briefly as the startup current charges the electrolytics, then the bulb goes out until the heaters warm up, then glows orange (as in the picture) once idle current flows.

Then today I was taking voltage readings on the amp while it was plugged in to the 40W light bulb current limiter. After warmup, the B+ was 282 VDC. That seemed low, so I plugged in the amp into wall power and read 393 VDC on the B+, which was more like I was expecting.

It's still a great tool, but I'll keep it away from my work area so I don't use it by accident.

Comments/corrections appreciated.

[xtian]

Sure. If you plug 40 watts and 120 volts into Ohm's Law, you get 0.333 amps. So that's the max current the bulb will let pass into your amp.

[armillary]

Thanks xtian. And that gives us 360 Ohms as the resistance of the bulb at that point. But a cold light bulb has 0 Ohms resistance, so we're talking about a variable resistor. I guess what surprises me is how linear the resistance of this variable resistor is. I assumed it would be a knee curve based on the temperature of the filament, or something like that.

[martin manning]

Here's the results of an experiment using a #47 bulb and a bench supply, going from 0 to 7VDC. Resistance is essentially a linear function of current.

[armillary]

Thanks Martin! That's pretty linear. I'm learning a lot here. Thanks to all.