Basic basic electrical question (Ohm's law)

[cowtown]

What am I doing wrong here guys-

I have a tiny auto light bulb and want to know its wattage so I can go shopping.

It draws 120mA as measured by an in-serries ammeter at 12VDC. Ohms law says that means it is approx. a 1.44 watt bulb and its resistance is 100ohms.

But measured filiment resistance is 9.5 ohms...huh? What's wrong here?

[slakjaw]

Try a diffrent DVOM.

Thats all I can think of.

[red-beard]

As the wire heats and glows, the resistence goes up.

[slakjaw]

Oh yeah thats right. I forgot about that.

[cowtown]

Red-beard -so it's normal that cold resistance would be that low and the 1.44W calc is OK? Thanks.

[fastpat]

Originally posted by cowtown
What am I doing wrong here guys-

I have a tiny auto light bulb and want to know its wattage so I can go shopping.

It draws 120mA as measured by an in-serries ammeter at 12VDC. Ohms law says that means it is approx. a 1.44 watt bulb and its resistance is 100ohms.

But measured filiment resistance is 9.5 ohms...huh? What's wrong here?

What does the manual call for in the position the bulb goes in? You're not working on a car without a maintenance manual open are you?

[slakjaw]

Originally posted by cowtown
Red-beard -so it's normal that cold resistance would be that low and the 1.44W calc is OK? Thanks.

http://www.geocities.com/SiliconValley/Circuit/1858/resiscale.htm

Yes it is normal.

I used to know that, but I never need to use some of that knowlege anymore. so you know, it gets forgotten.

[red-beard]

The nominal wattage may be 2 watts. Where is the bulb located? The speedo/guage cluster?

[red-beard]

Also remember, car are 12v nominal, 13.8V at the battery and about 15vdc when charging.

[cowtown]

It's a bulb from the 3rd brake light array on my 993. It's a little one, and there are 20-something of them.