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I decided to test some diodes that I have, using a multimeter to measure their forward bias voltage. I set my multimeter to voltage test and connected the diode across:

enter image description here

I tested a bunch of different diodes, both germanium and silicone type, so I expected to see voltages around 0.2 - 0.7V. However, all diodes showed 0V!

enter image description here

I tried reversing the polarity of the diodes, as well switching multimeters. Always my measurement came out as 0V. So am I making some kind of mistake in measuring diodes, or are both of my multimeters/all diodes broken?

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  • \$\begingroup\$ Short answer : No \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 13 at 4:35
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    \$\begingroup\$ Dunno but you're certainly not taking photographs properly: those are incredibly dark and hard to see. \$\endgroup\$ – David Richerby Jul 13 at 8:37
  • \$\begingroup\$ It's worth noting that your question indicates some misunderstanding about how diodes work. They only exhibit their characteristic forward voltage when energized in forward voltage mode. The accepted answer correctly identifies how to measure Vf with your multimeter, but for a more experimental approach, you can add an inline resistor and constant voltage, and keep the DC measure mode. You should then see the voltage in the multimeter. You can also try varying the voltage and see how it increases linearly up to Vf then stops. Or switch to current mode and see it stay near zero until Vf. \$\endgroup\$ – MooseBoys Jul 13 at 14:46
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In voltage mode, a multimeter just measures what voltage is present between its leads. What you want is diode test mode, which is usually indicated on the dial with a diode symbol. On your meter, it's the option one to the left of your voltage mode--set the dial to that and press the mode button a few times to put it in diode mode; it'll say on the LCD. In diode mode, the meter applies a known current to the diode -- you can check the meter's datasheet or instruction manual to know what current it uses, and fancier meters might even let you select a current -- and then measures the voltage across the diode.

Note that this usually won't work for LEDs, as most meters limit their diode test voltage to only one or two volts, which is too low to turn on any LEDs except maybe some red or yellow ones. But for a conventional diode like the one you show in the picture it will work fine.

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The guys in forensics had a difficult job enhancing the dodgy photos but the problem is clear.

enter image description here

  1. The meter is set to measure DC.
  2. Volts.
  3. (Barely visible in this rendering) the range selected is 'V'.
  4. The correct switch setting.

Since you selected DC V and a diode doesn't generate any voltage the reading is zero. This is correct.

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    \$\begingroup\$ wait, back up... there... zoom in and enhance.... Seriously though, good job on the photo! \$\endgroup\$ – mkeith Jul 13 at 0:57
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    \$\begingroup\$ @mkeith ENHANCE! \$\endgroup\$ – Harry Svensson Jul 13 at 11:25
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Nope. You need to set your multimeter to diode test mode. One click anticlockwise, and then press the "mode" button until the LCD shows a diode symbol.

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Perhaps you are being misled by the fact that circuit simulators model a diode as a voltage source in series with a resistor. That is a fiction that is required to make the circuit simulate correctly.

A Real diode does not produce a voltage across itself, but will produce a voltage drop when you pass current through it in the forward direction.

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Brand New Fairchild 1N4004 Diode.

Brand New Fairchild 1N4004 Diode. Does that answer your question?

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    \$\begingroup\$ Note the fact that the meter is in "diode test" mode, which applies a current and displays the resulting voltage. \$\endgroup\$ – Dave Tweed Jul 13 at 12:59

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