A Diode Question

[H2814D]

Hello all. I have a question concerning a diode on a 12vdc to 12/8/5vdc power supply for an RV Stereo/CD/DVD Player (Quest QX3202DVD). One of the diodes (red arrow) on the PCB has a 12 volt dc charge on both sides of the diode (anode and cathode) when tested from one of the legs to ground while it is in the circuit. Removing the diode from the circuit and testing it as a diode shows it is operating properly, with voltage only flowing one direction. All of the other diodes on the PCB do not have this same characteristic and show a normal 12vdc voltage on only one of the legs to ground. The other legs show a random mV. My question is this: Can anyone think of a reason the diode I have described initially would have 12 volts on both sides of the diode?

Thanks in advance for your reply.

 

[Bluejets]

Pics too far away to make out anything.

 

[Harald Kapp]

To realistically test a diode (or any component) you need to remove it from the circuit. In the case of a diode remove oneleg and test the diode for conduction in forward direction and non-conductin in reverse direction. This you have done and the diode seems to be o.k. (by the way: its current that flows, not voltage).
Being in the circuit opens many possibilities for faulty measurements. The 12 V on both sides of the diode may be present (just for example) due to another current path. Impossible to tell without having a schematic of at least the relevant part of the circuit.

 

[H2814D]

Thanks for each of your replies.

I've tried to locate a schematic for this thing and have had zero luck finding one. Anyway, the power supply is supplying the correct voltage (12/8/5) at each of the supply points after some minor component replacement.

So, based on the fact that the diodes check ok while out of circuit, and because the power supply is supplying the correct voltage, I am going to rely on your expertise in the statement you have made about the "another current path" explanation. I just thought it odd that a diode in an energized power supply would have the same voltage on both legs.

 

[Harald Kapp]

I just thought it odd that a diode in an energized power supply would have the same voltage on both legs.

One use of a diode in such a configuration can be e.g. to protect the circuitry on the anode side if the voltage on the cathode side falls below 12 V. The diode will then become conducting and e.g. discharge a capacitor on the anode side. See e.g. this article (albeit in normal operation the diode is reverse biased and the voltages on anode and cathode are not the same, it's the prionciple that I'd like to point at).

 

[H2814D]

Thank you, Mr Kapp.