Different wattage bulbs in a current limiter produce what results?

[Mig29]

I built a current limiter recently and am curious what different wattage lamps in it will produce what results?  For instance what would the difference be between a 100 watt bulb and say a 75, or 60 watt bulb?

[jjasilli]

The difference is the current draw  / resistance of the bulb; and the voltage drop it causes to the amp; neither of which I think matters.  Let's look at two examples at either extreme.  watts = volts X amps:

 20 watt bulb = 120 VAC X 167mA
100 watt bulb = 120 VAC X 833mA

So a 100 hundred watt bulb draws more current - or allows more wall current to pass though it - than does a 20 watt bulb.  (My wife knows this from looking at the electric bill!!!).  This is another ay of saying that the 20 watt bulb has more resistance than the 100 watt bulb.  So if a 20 watt light bulb limiter goes dull in use, it will cause a greater voltage drop (Ohm's Law) of wall supply current to the amp. This will put a strain on the amp, filament supply, etc.  The drop in supply voltage to the amp will be bigger as the wattage rating of the chosen bulb gets lower. 

I don't see how any of this matters too much.  This is a quick test.  If the bulb is glowing full bright, then your amp is frying, whatever the wattage of the bulb.  If the bulb goes dull, the amp has passed a test, but is under strain.  Either way, turn off the amp quickly.  Hence it seems OK to use any bulb from 20 to 100 watts, whatever is handy.

[Geezer]

Either way, turn off the amp quickly.

Yes, all you are doing is quickly seeing if you have a short...If the light is bright, turn it off & find/fix the short/problem....if the light goes quickly dull, turn it off & fire up the amp "normally" without having to wonder when/if the smoke is going to start!

[Mig29]

Thank you - I've seen mention of different wattage bulbs used and thought there might be something to it.  I built mine with a switch so I wouldn't have to yank a plug out.  I have some small 75 watt bulbs I'm going to use then. 

[PRR]

The lamp must be small enough to limit disaster, yet large enough to let "life" into the amplifier.

A 3,000 Watt lamp would easily light-up any amplifier; but if the amplifier were shorted, 3,000 Watts would overload a home outlet. 1,500 Watts is safe for your wall, but may be too much power to let-loose in a sick amplifier.

A 3 Watt lamp would be no strain on your wall, but also let so little power flow that you could not tell a healthy amplifier from a sick amplifier.

So there is no "right" answer.

For first smoke-test, use a bulb "somewhat" bigger than the expected normal power demand of the amplifier. A little Champ sucks 59 Watts. A big SVT could suck 600 Watts. Most every-day amps will start to work with a 75W bulb, enough to know if the amplifier is SHORTED. Bigger amps, check with a 75W, then if you are paranoid, check again with a 200W.

> If the bulb is glowing full bright, then your amp is frying

Actually, if you have "120V brightness", then the amplifier is flowing full lamp current at zero voltage. The amp is very sick, dead-short, but not heating or frying.

If the amp and lamp were pure resistors, the Worst Case would be when the lamp was "60V brightness". That's dim-red. For a big amp on a small lamp, that may even be "normal" and "safe", though you should not leave it that way. And for complex reasons, I think the lamp won't fence-sit. It will either be too dim to see, or so bright there is no doubt "we gotta problem".

What actually happens is: the cold lamp has LOW resistance. If the amplifier comes up at fairly low current, the lamp stays cold, low resistance. It may drop 20V, which is too dim to see. The amplifier gets the other 100V, which is not right, but enough to show it isn't horribly sick. You don't want to leave an amplifier on 100V all day, but you should get a dull glow and probably some sound.

If the amp is shorted, or lacks bias (HIGH current), the lamp will heat-up, increase resistance, start to take 100V, glow bright red. Only the 20V leftover heats the amplifier. You should not leave the amplifier this way, but it probably won't burn-up for minutes.

Tube amps may cause two limit-lamp flashes. At turn-on, the cold heaters suck more power, this will fade in a second. The main cap must be charged, instantly for solid-state or 5-10 seconds later for tube rectifier. After that a reasonably large (compared to amplifier) lamp should be no-glow.