Basic Dim bulb Tester Questions

[Superman541]

I can find various suggestions and configurations for building these, but I have a couple of more 'basic' or general questions:

1. Does this require using an incandescent bulb? These are harder to find now. Or can a 'dimmable' LED bilb be used? I question whether it can, simply because LED bulbs draw almost no current. Even a properly working amp, with no short, would bring that bulb to full brightness.

2. Can a DBT be used to estimate (not measure, but just estimate) the amount of the over-current? A DBT can, I assume, show that the amp will not likely blow a 3A fuse, right? Could it identify a frightening 10A over-current situation? Using different bulbs, how much can we learn about an amp's current draw?

 

[whoaru99]

The amp can only draw what is relative to the lamp allows.

In other words, if you used a 60W lamp on a 120V circuit, the max the amp can draw is limited to roughly 0.5A.

Unfortunately, I think brightness of the lamp may not be straight proportional to current draw. IOW, if you perceive the lamp to be "half bright" doesn't mean there is half current, so to speak.

 

[Superman541]

Trying to think this thing through. With limited knowledge of electrical theory. V * A = Watts, right? So then, if line voltage is 120 VAC, and if I used a 120-Watt bulb, then one Amp of current draw would fully light the bulb. Correct?

 

[Superman541]

In looking at a nearby related thread, it sounds like current draw at idle, for many amplifiers, is rather low. Nowhere near the fuse value. At idle, perhaps the unit would draw 30 Watts. This would light a 30 Watt bulb fairly brightly, but would light a 60 Watt bulb more dimly. But if the amp, at idle, has a shorted circuit and wants to draw 3 Amps, then the DBT would fully light a 30W bulb, or a 100W bulb, or even a 300W bulb. The main purpose of a DBT is to disallow the unit from drawing any more current than the bulb will pass, not to allow me to measure or estimate current draw. But some observations can be made using a DBT about current draw. If idle current should be only 30W, but a 100W bulb is fully and brightly lit, then the unit os drawing far more than 30W and something seems wrong. Am I understanding this properly?

 

[whoaru99]

Am I understanding this properly?

Yes, well enough in my opinion.

Kill-A-Watt meters are not too expensive for keeping an eye on current, watts, VA, voltage, power factor, and frequency. Although, if used after a Variac or DBT, the display blanks out at some point of low voltage although I don't recall that point.