Light Bulb continuity

[MiketheEngineer]

Dumb old structural here with an $8.00 Radio Shack tester.

I would have thought that a light bulb (known to work) would show continuity - but it doesn't. IS the resistance too high??

Or do I just not know what I am doing?? Has happened before.

 

[electricpete]

I assume it is a standard household incandescent lightbulb and verified not burned out.

The resistance of a cold light bulb is much less than you’d calculate by R=P/V^2 (where P = watt rating and V =voltage) as a result of temperature coefficient of resistance.

I don’t know what the cutoff is for your tester, but I’d have thought you’d see an open.

Does your tester allow you to check resistance?


=====================================
(2B)+(2B)' ?

 

[MacGyverS2000]

Isn't a cold filament like 10-15 ohms, or thereabouts? I think hot filament resistance jumps a good 10-15x...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

Test your test equipment.

 

[IRstuff]

Continuity testers are intended to verify the integrity of WIRES, where the greatest resistance in a two-probe measurement is the contact resistance that's about 1/2-ohm.

A typical continuity tester might reject anything even close to 10 ohms:

http://www.elcomcommunications.com/continuitytester.pdf

Obviously, there are others where 2K is acceptable:

http://www.extech.com/instruments/resources/manuals/CT20_UM.pdf

And others that are in-between at 400 ohms:

http://www.fluke.com/fluke/usen/electrical-test-tools/electrical-testers/fluke--t5.htm?PID=55986

One Radio Shack instrument buzzes at less than 50 ohms:

http://support.radioshack.com/support_meters/doc64/64777.pdf

I don't know what kind of tester you have, but presumably, you're just using the continuity position of the ohmmeter function, in which case, you should just measure the resistance directly.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MiketheEngineer]

Bulb is known to work - tested it one second before I put the continuity tester on it. But the tester is a simple go/no-go type.

Just trying to find a fault in a circuit.

THANKS for your help!!

 

[VEBill]

Post the Radio Shack SKU (stock number) for the tester so that we can look up the details.

Describe the light bulb in more detail. Is it household, or is it a small flashlight sized bulb.

Why are you testing a light bulb if it's known to be good? ;-)

 

[OperaHouse]

Lots of bulbs (a friend at lamp division goes balistic every time I call it a bulb) fail mechanically in the act of unscrewing them if they have lots of hours on them. Did you try the LAMP again to see if it still worked?

 

[MiketheEngineer]

Plain old 40W - 120V bulb - tested good BEFORE and AFTER continuity check. I know I am an SE - but I am not that dumb. Not even sure if it is a Radio Shack. If you have continuity like when you touch the leads together - it goes BEEEEEEEEP!!

It must be a resistance thing. Again - THANKS!

 

[btrueblood]

watts = V^2/R,

40 watts at 120 volts gives R = 360 ohms

Typical tungsten filaments will undergo a 10:1 to 12:1 increase in resistance from cold (unlit) to hot (lit), so a typical cold/unlit resistance might be 30 to 40 ohms.

 

[IRstuff]

Well, so don't use a continuity tester; use an ohmmeter.

Since we (and you) know nothing about the "tester" and its performance specifications, discussing it further seems rather futile.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MiketheEngineer]

IRstuff -

AGREED - just thought a light bulb would show continuity - NOW I KNOW better. Obviously a resistance thing. Now I can go out and try not to kill myself and come back with some other dumb questions.

 

[IRstuff]

Continuity is in the eye of the beholder...

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize