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From:
Mark Lavelle ([email protected] )
Date:
7/4/2001 4:54 PM
Subject:
Page 1 of 2
"Light Bulb Limiter" questions
There have been a few mentions of light bulb limiters (let's call 'em LBLs) lately, and I've got some questions about them. Just
to be clear, I'm talking about a light bulb in series with a 120VAC socket that your device under test (DUT) plugs into. This is
supposed to limit the power available to the DUT. The questions:
1. Other than protection of your PT & other components against a short to ground or similarly disastrous wiring, what can an
LBL do for you? In other words: I can see why you'd want to use one when you first power on your latest creation or mod, but
are there other times when it's a good idea?
2. Is there a useful calculation/rule-of-thumb/??? for determining the amount of limiting for any particular wattage bulb? (Have I
just declared myself ignorant of the practical application of Ohm's Law? Yeah, I know the formula, but it just ain't intuitive to
me...)
3. How do you decide how much to limit (see #2) in any particular situation?
4. Does an LBL have any effect on a properly functioning circuit after it's 'tamed' the initial power surge?
5. When is it better to use an LBL than a variac, and vice versa?
Thanks in advance,
Mark
From:
kg ([email protected] )
Date:
7/4/2001 5:07 PM
Subject:
Re: "Light Bulb Limiter" questions
1. Other than protection of your PT & other components against a short to ground or similarly disastrous wiring,
what can an LBL do for you? In other words: I can see why you'd want to use one when you first power on your
latest creation or mod, but are there other times when it's a good idea?
i use it when testing ANYTHING, whether it be an unknown tranny, an older amp that hasn't been power on recently, etc.
2. Is there a useful calculation/rule-of-thumb/??? for determining the amount of limiting for any particular
wattage bulb? (Have I just declared myself ignorant of the practical application of Ohm's Law? Yeah, I know the
formula, but it just ain't intuitive to me...)
yes, ohms law... actually you've got to use his pal watt as well:
wattage of bulb = current * 120v
current will be in rms amps, so 100w bulb will allow 0.833Arms to pass--and nothing more.
3. How do you decide how much to limit (see #2) in any particular situation?
well, it would make sense to limit the current to the lowest value you can get away with, and start increasing the current if
necessary. check the back pannel of the device in question to determine current draw, and go from there.
fwiw, i use a 150w bulb and it pretty much works for everything. you know you need to increase bulb wattage when the bulb
lights up and the amp does not, and you're pretty sure the circuit is functional.
4. Does an LBL have any effect on a properly functioning circuit after it's 'tamed' the initial power surge?
if the light bulb is glowing, it is dropping significant voltage... the brighter the glow, the greater the voltage. the best thing to do
is wire a normal spst household light switch in parallel with the light bulb socket so you can switch it off for "full power"
operation.
http://firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=124035-000000.msg
7/4/01
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Page 2 of 2
5. When is it better to use an LBL than a variac, and vice versa?
a variac does not limit current any more than its built in fuse does. your best option is to plug the variac into the current limited
socket.
ken
From:
Mark Lavelle ([email protected] )
Date:
7/4/2001 6:14 PM
Subject:
Re: "Light Bulb Limiter" questions
Thanks for the detailed answer, Ken - it'll definitely go into my binder of Ampage 'keepers'! I hadn't thought of plugging the
variac into the LBL (doh!)...
So, to summarize: If you plug in a DUT with a short somewhere, the bulb just stays on full brightness, right (while your circuit
fries much more slowly than it would at full power)?
From:
PaulC ([email protected])
Date:
7/4/2001 6:25 PM
Subject:
Re: "Light Bulb Limiter" questions
Those things are great. I've got 3 benches at work and each one has one plugged into a variac. SS pwr amps and everything
go into that before anything is fired up for the first time. I've got mine with a switched and fused also. Just make a cool box and
mount 'em up.
From:
kg ([email protected] )
Date:
7/4/2001 7:58 PM
Subject:
Re: "Light Bulb Limiter" questions
So, to summarize: If you plug in a DUT with a short somewhere, the bulb just stays on full brightness, right
(while your circuit fries much more slowly than it would at full power)?
yes.
with a dead short, there's no voltage across the load, and all the voltage across the light bulb, which glows with full intensity.
with minimum current draw in the load, the filament in the bulb does not have enough current flowing to cause it to heat up to
the point where it glows. very little voltage is dropped across the bulb, which means most of it appears across your load.
the key is the fact that the more current flows through the bulb, the greater its resistance...
small current, very low resistance, no glow, voltage across the load
high current, high resistance, full illumination, voltage across the bulb.
ken
http://firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=124035-000000.msg
7/4/01