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Please take a moment to look over the circuti below:

enter image description here

As you can see, the circuit is nothing more than a 10 volt power supply and a diode. Note that the power supply negative terminal and the diode anode terminal are not connected to anything. Also note that a multimeter is connected across the diode with the purpose of measuring voltage.

Based on the circuit above, what voltage amount would you expect the multimeter to measure? Zero volts? Well, as it turns out, the multimeter actually measures around 850mv. This is puzzling to me because the circuit is not complete so no current should be flowing through the diode therefore the voltage drop on the diode should be zero right?

Could someone explain this phenomenon?

Thanks.

EDIT:

Per user request, here is a picture of the setup:

enter image description here

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    \$\begingroup\$ Diodes are sometimes modelled as a voltage source - it could be measuring that. \$\endgroup\$ – Majenko Mar 21 '15 at 22:19
  • \$\begingroup\$ If you disconnected one lead of the multimeter from the diode, what voltage does the meter measure? AC or DC numbers please and also what was the 100mV? AC or DC? \$\endgroup\$ – Andy aka Mar 21 '15 at 22:47
  • \$\begingroup\$ Just put one multimeter lead to + of your voltage source and look waht it reads \$\endgroup\$ – PlasmaHH Mar 21 '15 at 22:48
  • \$\begingroup\$ Also, a picture of your meter would help or a link to it. \$\endgroup\$ – Andy aka Mar 21 '15 at 22:49
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    \$\begingroup\$ @Alexander Sabiona: Per your request, a picture of the setup has been added. \$\endgroup\$ – T555 Mar 21 '15 at 23:32
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You know what you have there?

A radio receiver.

The power supply isn't supplying power (as such), but the wires and things are acting like an antenna.

Look, same effect using just me and a diode:

enter image description here

It's kind of like the old crystal set. No power source there - just radio waves:

enter image description here

Since there is no (intentional) tuning and filtering you are most likely seeing the most powerful source of EM waves in the locality - the ubiquitous 50 (or 60) Hz hum from the local mains system.

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  • \$\begingroup\$ Ok, this explains a lot. I tried using another multimiter and power supply and the number were even bigger. This is differently noise. I guess this explains this other question that I had: electronics.stackexchange.com/questions/159578/… \$\endgroup\$ – T555 Mar 22 '15 at 0:38
  • \$\begingroup\$ For bonus points, set meter to Hz \$\endgroup\$ – PlasmaHH Mar 23 '15 at 22:16
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Unless you have taken extraordinary precautions that seem unlikely, your circuit is exposed to stray radio signals, including radiation from the digital circuit of your DVM. The cathode side of the diode has an ac route to ground via the power supply but the anode side acts as a wideband antenna. Current flows in the diode until the input capacitance of the DVM is charged up to the 100mV that you see.

For an experiment, you can put the diode and a battery (conveniently 9V) in a small metal screened box and see whether a nearby radio transmitter affects the measurement. Also, adding 10nF ceramic capacitors between the negative supply and each leg of the diode will remove the puzzling effect.

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  • \$\begingroup\$ It is most likely AC mains hum, and not a radio transmission, that is being rectified by the diode. \$\endgroup\$ – Roger C. Mar 21 '15 at 23:39
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If your power supply is not grounded and it uses an unscreened transformer (or a screened transformer with a floating screen) you can get weird stuff like this happening quite readily. Grounding the power supply will probably make it mostly or entirely go away.

The problem is the the DC outout of the power supply can be floating at perhaps half of your mains voltage (or more) due to capacitive coupling across the winding or maybe a deliberately installed Y capacitor, especially on switching supplies.

Grounding the output, using a grounded outlet, or reversing the plug may make a big difference.

Think of grabbing the input of of an audio amplifier with your hand- even if the amplifier is properly grounded you will get a lot of mains frequency hum. Without the diode, your meter averages the hum to 0V on a DCV range, but the diode rectifies it and you see a bit of DC (almost no current, but some voltage). If you switch your meter to ACV and spread out the leads one to the power supply output and one floating (or touch the one that is floating) you may see quite a few volts AC.

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Is it a physical circuit? If yes, then the diode is picking up the ambient light through the glass package.

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  • \$\begingroup\$ Yes it is a physical circuit but I don't think this has to do with ambient light. The diode is a 1N4004 rectifier covered under a black plastic case. Most importantly, if I disconnect the diode then I get zero volts, so clearly, the voltage source is what is driving the voltage. \$\endgroup\$ – T555 Mar 21 '15 at 22:33
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I suspect that the psu is a switched mode model so it could be generating quite a field near by. It the psu is switched off, does the displayed voltage on the DMM drop to zero?

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  • \$\begingroup\$ It does not drop to zero but pretty close, it goes down to 5mv. There is definitely all kinds of interference going on with this setup, this morning I ran the same test and now get 1.7V across the diode. Crazy stuff. \$\endgroup\$ – T555 Mar 22 '15 at 15:44

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