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Can anyone offer suggestions as to where I am going wrong please?

In my circuit (diagram below) the system voltage is 12.03 V and the lamp resistance is 2.9 ohms (measured with power disconnected through circuit). I was expecting to measure a current of 4.1 Amps (12.03/2.9 = 4.14)

However the DMM ammeter reads 0.450 A DC (positive lead to positive side of circuit). Screenshot below.

This doesn't tally with Ohm's law so I suspect that I am doing something wrong rather than Mr Ohm!

Thanks in advance for any help (first post here)

schematic diagram

photo of DMM current measurement display


thanks for you comments. It appears then, that one cannot test for actual operation resistance from cold - I take it then one would need to operate circuit for some time, then disconnect power and then test resistance to gain a true reading. Haven't seen a mention of this real world scenario when reading about ohms law!

Once again thanks - off to re-test! N


Reply to all again. Re-tested. Voltage has dropped to 11.5 Closed the circuit and left it for 5 mins to heat up, tested current again, now showing as 1.74Amps. Took battery lead off and quickly tested circuit ohms and they momentarily showed a peak of 9-10ohms before quickly falling away (as bulb temp drops). So expectation might be 11.5v/1.74A=6ohms - something much nearer. I was (naively?) imagining this was all rather simpler than the real world appear to show! Thank you all - i have adjusted my expectations accordingly

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  • \$\begingroup\$ Which range have you got the meter set to? \$\endgroup\$
    – HandyHowie
    Commented Jan 26, 2018 at 12:56
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    \$\begingroup\$ This is a really nice question. Because it contains all the knowledge you have about the specific topic as well as your thoughts and ponderings about possible misconceptions of the matter. On top of that, you tried to include all details which might be of interest for others who tried to solve the riddle. This is best practice for a first post! \$\endgroup\$
    – Ariser
    Commented Jan 26, 2018 at 15:02
  • \$\begingroup\$ Might also be worth checking your DMM leads. I came across a bad one in college that took me a while to find. It had several ohms of resistance which would really throw off a current measurement. \$\endgroup\$
    – mjh2007
    Commented Jan 26, 2018 at 15:19
  • \$\begingroup\$ Thanks, i tried to set out the problem and the circumstances. \$\endgroup\$ Commented Jan 26, 2018 at 16:10

3 Answers 3

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I am using this exact circuit at the moment to test load a battery and using a car headlamp. I measured the cold resistance of the headlamp and (from memory) it was about 0.75 ohms. In the circuit it is bright and from the current readings I get (typically 4.5 A) it has a resistance of about 2.7 ohms when hot.

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    \$\begingroup\$ @NickHudson Curious about your selection, here. \$\endgroup\$ Commented Jan 26, 2018 at 17:30
  • \$\begingroup\$ I am trying to "repair" a bad cell in a 12V lead/acid battery, one of five from a 60V electric scooter. On line several people have advocated repeated charge with an old fashioned charger (which can get the voltage up to 15V) not the "Intellegent" one, followed by deep discharge. The scooter motor is 500W so at 60 volts it's working current is about 8A. The headlight gives a comperable current and was there in my old stuff I didn't throw away years ago. \$\endgroup\$
    – blairsdad
    Commented Jan 28, 2018 at 1:52
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2.9 ohms (measured with power disconnected through circuit)

As the lamp heats, its resistance rises sharply. Ohms law is never wrong except when misapplied.

enter image description here

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  • \$\begingroup\$ If the resistance increases, then... shouldn't the current decrease? 4.5 A is more than 4.1 A. \$\endgroup\$ Commented Jan 26, 2018 at 12:56
  • \$\begingroup\$ @HarrySvensson the OP said 0.45 amps \$\endgroup\$
    – Andy aka
    Commented Jan 26, 2018 at 12:58
  • \$\begingroup\$ Oh woaw, then it makes sense. \$\endgroup\$ Commented Jan 26, 2018 at 13:03
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    \$\begingroup\$ Saying Ohm's law is never wrong is a bit misleading. Ohm's law isn't specific on what it applies to so when it doesn't apply to a certain case then I would say by definition it's wrong in that case. \$\endgroup\$ Commented Jan 26, 2018 at 19:10
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    \$\begingroup\$ @Andy To me "misapplied" sounds more like "used incorrectly" and not "used on a device Ohm's law does not apply" as obviously anything is correct except when it's not. I know this confused me when I first learned about diodes. \$\endgroup\$ Commented Jan 26, 2018 at 20:41
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I assume you measured the lamp when it was cold. However when it lights up the resistance is much higher.

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  • \$\begingroup\$ About 26.7 ohm, I'd say \$\endgroup\$
    – MrGerber
    Commented Jan 26, 2018 at 13:23

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