Pauper - Whacky Voltages

[Spiff4565]

I am getting odd voltage reading on this build. On the 5817 diode, I am getting 9.44V going in. When I check the voltage coming out, it slowly increments up to around 135 mV coming out. R12 matches this voltage and R13 is roughly half of R12, and pin 5 on the IC is roughly equal to R13.

If I pull the 5817 and use a jumper, the voltages are much more what you would expect.

At this point, I'm simply perplexed.

 

[Brett]

Can you test D5 for voltage drop outside of the circuit or have you tried another 1N5817 diode in that position?

 

[Spiff4565]

I have tried several 1N5817's.

How I would test D5 outside of the circuit?

 

[Brett]

I have tried several 1N5817's.

How I would test D5 outside of the circuit?

Does your DMM have a diode testing function? Many do.

If all of your off board wiring is correct (specifically, the power/ground connections) and a jumper in place of D5 gives you the correct voltage at the cathode of D5, the most likely suspect is the diode.

Try pulling D5 and measure the resistance between D5's cathode pad to ground on the board, what resistance do you get?

 

[Spiff4565]

Duhh, yeah, I get it now. Yes, when I measure the diodes I am reading .220V.

When I place it on the board I am getting 9.44 going in and about 60mV coming out.

As you can see in the pic, the diode solder pads are not close to anything so there isn't a way for voltage to leak of the negative side to ground or elsewhere in the circuit.

I'm at a loss as to why 9.44V goes in but only 60 mV comes out and the 1N5187 measures just fine, off board and in circuit.

It just doesn't add up.

 

[Brett]

Try pulling D5 and measure the resistance between D5's cathode pad to ground on the board, what resistance do you get?

You can just lift the cathode side of the diode if you'd like. I'm asking you to do this because it'd be helpful in determining just how much of that power rail is being sent to ground.

 

[Spiff4565]

Getting about 8.14V at the lifted cathode.

 

[Brett]

And the (unpopulated) cathode pad's resistance to ground?

 

[Spiff4565]

About 93kΩ

 

[Brett]

About 93kΩ

Okay, that sounds right.

You should have infinite resistance to ground on the anode side of D5, but did you happen to double check that to be sure?

Honestly, I'm at a bit of a loss here as well. I'm not sure what else to recommend, hopefully someone that's seen this before will respond with other suggestions.

 

[Spiff4565]

OL from anode to ground.

It is odd for sure. Voltage looks OK at the cathode until you put it in the circuit, then the voltage drops to virtually nothing.

Yeah, hopefully someone can identify the source of the phenomenon.