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Need resistor help please (35w x 12v)

Here is sommore tests I've done, which produce some odd results, at least I think.

Today I hooked up my 5watt lightbulb to a voltimeter to measure it's ohm resistance, it came back at 0.5ohm. By my calculation that's near

300watts at 12volt.. doesn't that seem pretty unreasonable? Maybe my ohm meter sucks. I measured some of the resistors I have 50ohm, 15ohm, etc, and it came back with perfect results on them.

So I put sommore tests into the light circuit to see which would make my lightbulb monitor go away. All tests are with resistors in parallel.

50ohm + 50ohm + 15ohm made the car hesitate an extra couple seconds before giving me the warning. 50ohm + 50ohm made the error come on right away 15ohm + 15ohm gave me a few second delay before the error came on

Plugged my 5watt bulb in again (the one that measures .5 ohms on my meter) and no warning. Anyone make any sense of this?

- Nate

Reply to
Locu
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YOu can not use an ohmmeter to directly measuer the resistance of a lightbulb when it is giving off light. When using a standard ohmmeter you are only putting a few miliamps or less through the bulb. It will not light. The resistance will be very low. As it heats up due to having the correct voltage and current applied to it, the resistance goes up. OUtside of maybe some special metals , they all increase reisstance as they heat up. Carbon is the only common conductor I can think of right off that the resistance goes down as it heats up.

If you measuer the voltage and current of the bulb at its normal brightness you can calculate the wattage and ohms of the hot wire in the bulb.

Reply to
Ralph Mowery

No that could be quit reasonable. A lightbulb is not a pure resistor, but a PTC (a resistor with a Positive Temperature Coefficient). So when the lightbulb heats, the resistance will increase. If your calculations are correct, your lightbulb resistance will increase by a factor 300/5=60, or in other words 30 Ohm.

Stef Mientki

Reply to
Stef Mientki

It makes sense - it is the cold resistance of the bulb. My previous reply told you that, and offered a possible solution. "The bulb, when cold, presents a very low resistance, far lower than 15 ohms. "

Can't help you if you don't read the replies.

Ed

Reply to
ehsjr

Sorry Ed,

Upon first reading your reply it was a bit too technical for me and made my eyes glaze over. After re-reading it now, and in conjunction with my real world testing in the garage, it make perfect sense what you said. I appreciate your input.

I'm a bit confused on the time delay circuit, but I'll try to look into that further. Sounds like a bit of work. =)

- Nate

Reply to
Locu

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