Determine Resistance of a Halogen Light

Newbie Disclaimer: I am not an electrical engineer, but I am very interested in the field, so forgive the naive question.

I have a set of halogen lights, like off-road lights for a car. I know nothing about these lights. I would like to figure out the amperage they'll put off when connected to a 12V DC electrical system.

So I was thinking all I need is to determine the resistance and then I could do the I = V/R formula and poof there I go.

I don't have the lights at home with me, but for a quick proof of concept I grabbed a household light bulb, 120V 60W, regular old bulb. So I figured out the amperage for that with I = P/V (60W / 120V = 0.5 amps). So to double check those numbers I grabbed a DMM, switched the range to selector to the OHM symbol, and touched the leads to the contacts on the light bulb, it finally gets a steady reading of 17.5 OHM. My calculation says that for the above formula to be correct you would need to have a resistance of 240 OHM.

Is there something that I'm not aware of in all of this? Is a digital multimeter not the right tool for this? What is the best way to determine the amperage for these lights? I want to choose the correct switches, relays and wire gauge for all this.

Thanks for any advice in advance.


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Only one teensy little detail - the resistance of a filament when the lamp is off (room temperature) is very different to its resistance when on (glowing white hot).

So, get the filament glowing white hot and repeat the measurement and all will be fine.

Now, if the cost of a solar kiln, laser heat gun (or whatever other means you were thinking of using to raise the filament to white heat in order to measure its resistance) is a bit of a problem - there is a cheap solution:

Put the multimeter on its 10A range and connect it in series with the lamp and a 12v battery.

You are then reading the current that you need to design, strangely enough, the filament will be at its operating temperature.

Yep, on the 10A range, not the ohms range. Unless you have a spare solar kiln, that is...

Then *measure* it, dear William, dear William, dear William. ... With what shall I ....

With a 12v battery and the multimeter set on 10A range, dear William...

Bear in mind that this is DC, not AC - so your switches and relays need to be rated for that current DC (irrespective of what their AC rating is). But that is a different question..

-- Sue

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You can not measure the resistance of the lamp when the filament is cold. It goes up as the filament heats up. Instead, you have to measure the current through the bulb at full voltage, and calculate the resistance. You have to be careful at 120 volts. You could get a nasty shock, or worse.

Reply to
Michael A. Terrell

Despite my age I seem to to remember that was the way to do it back in college in 195?. But 240 volts needed 2x the care or is that 4X.

there is some sense in proper schooling in these technologies.

John g.

Reply to
John G

Another way if the wattage of the bulbs is shown anywhere, is to use that for your calculations. Cheers ....... Rheilly P

Reply to
Rheilly Phoull

Others have mentioned using an ammeter to measure the current then calculating the resistance with ohms law. Here is another method:

You can measure the hot resistance of a lamp, heater or any other such device by placing the lamp or heater in a bridge circuit. On the right side of the bridge place the lamp in series with a low ohmage resistor say 2 ohms. Lamp at the bottom, resistor at the top connected to a voltage source.

On the left side of the bridge at the top place a resistor 10 times the value of the right side resistor, say 20 ohms in this example. Below this resistor in series with it, opposite the lamp place a variable resistor like a 10K pot.

Place a voltmeter across the lamp. Adjust the input voltage at the top of the bridge until the lamp voltage is the rated value or any other value where you want to measure the lamp resistance.

With a voltmeter across the center of the bridge, adjust the pot until the center voltage is zero. This balances the bridge.

Remove the pot and measure its resistance. The measurement will be ten times the lamp hot resistance.

Reply to
Bob Eld

This sounds like a fun experiment.

Reply to

I have done this 3 times. The actual halogen light is in a lens case, similar to a fog light, sealed up. There are no specs on the lamp, at least that I can see with my naked eye.

I agree, thats why I'm so baffled that they didn't print any specs on the lamps themselves.

*should* be indeed.

Do you know if this is true for the lens casing itself? Or just the layer of glass that contains the filament?


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It is the silica glass envelope containing the filament that has to be kept uncontaminated. It gets to such a high temperature that any surface contaminants will fuze into the glass, which greatly weakens it.

-- Sue

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Are there any numbers or letters on the lights at all? If so, maybe asking Mr. Google what they are would work.


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Reply to
Dean Hoffman

Hard to believe that folks in the group would actually think that you have anything at all to add to the group.

Your horseshit doesn't count as an addition, dipshit.

The folks "like us" that you met years ago were trying to tell you something, you retarded f*ck.

Nice projection job, since it is you that is the inbred numbskull.

I think you are just pissed because you never made it past E-1.

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