The 21mm 12V car outlet (originally invented for cigar lighting) can be measured using a multimeter.

While the car is in stand-by mode, the multimeter indicates 12.7 Volts.

But when the key is pulled, I still have 4.2 Volts.

Is that intentional?

And what are the technical background aspects of it?

Edit 1:

When connecting an 5V .6A 3W USB LED lamp directly to it, the voltage drops to 2V immediately when switching it on. The lamp also gets dimmed severely.

Edit 2:

The voltage varies between 3.9V and 4.1V. Not always the same.

  • What kind of car, and year? – CrossRoads Oct 19 '18 at 12:25
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    I wonder if there is capacitance that needs to bleed out. – Bort Oct 19 '18 at 12:31
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    WAG is a common term "Wild A** Guess"... due to no make, model or year so it's just speculation... – Solar Mike Oct 19 '18 at 13:26
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    @soosaisteven - What do you mean by stray voltage? Do you mean an incomplete "opening" of a switch? It could be, though I imagine the 12V line is switched via an electromechanical relay, and those usually open completely. – Bort Oct 23 '18 at 2:06
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    If open completely there shouldn't be any voltage. I think there must be leakage thru some dirt bridging the wiring which otherwise should be fully isolated. – soosai steven Oct 23 '18 at 2:11

Try different scales on the multimeter: does the lowest millivolt scale overflow? Is the value the same on the 1000 V scale? It's genuinely possible you obtain different voltage values at different scales.

Put some resistor, let's say 1000 ohm - 10 000 ohm, between the 12v outlet leads. Then measure. The voltage should be much lower now, very close to 0V.

A multimeter is a very high valued resistor, usually in the high kilo-ohms to low megaohms range. The resistance may depend on the measurement scale.

Or if you dare, you can measure the maximum current you can obtain by using the current position in the multimeter (warning: don't do this unless you know what you're doing! NEVER do this if the key is in the "acc"/"on" position!). I bet the maximum current you can obtain from those 4.2 volts is a very low value, so low that you won't damage anything by using the multimeter.

Yet, there is a genuine possibility it's really outputting 4.2 volts and can provide some significant current, so to avoid blowing a fuse, do try with 1000 - 10 000 ohm resistor first and proceed to the second test only after you have verified the voltage is essentially zero with the resistor.

  • A connected USB LED lamp dropped voltage to 2V immediately. – neverMind9 Oct 19 '18 at 16:34
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    @juhist multimeters tend to have a "resistance" in the region of 50,000 or more Ohms per volt... does depend on the manufacturer though - cheap ones... Well, just say I don't have a cheap one...(they don't last long enough...) – Solar Mike Oct 19 '18 at 16:46
  • @SolarMike How much is "cheap"? – neverMind9 Oct 22 '18 at 20:41
  • @SolarMike Surprisingly, I have seen 10 year old multimeters still working. How does a multimeter break in your experience? – neverMind9 Oct 22 '18 at 20:42
  • Dropped etc Buy Fluke or Avometer then see what cheap ones are like in comparison. And yes some of mine are more than 10yrs old... Look after good kit... – Solar Mike Oct 22 '18 at 20:46

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