Ghost voltage on diode
Ghost voltage on diode
(OP)
This really has me at a loss. I am reading a voltage across diodes when the diode isn't connected to anything. I am testing a circuit, measuring the voltage from the back side of the diode to ground (AC input). I switch off the power and the diode still has around 150mV across it. I unplug the souce, still there, I remove the diode from the circuit, still there. I try another diode that wasn't part of the circuit, same reading. I try a different mmultimeter, no change. Any ideas? I tried shorting the diodes thinking maybe some residual voltage across the junction, some capacitance, didn't stop the reading. I tried multiple meters so that rules out feedback from the meter, although they were both digital FLUKEs.
What could be causing this and how do I eliminate it?
What could be causing this and how do I eliminate it?





RE: Ghost voltage on diode
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ghost voltage on diode
Keith Cress
kcress - http://www.flaminsystems.com
RE: Ghost voltage on diode
RE: Ghost voltage on diode
I just tried it with a 1N4148 diode and a little LED flashlight. Essentially zero volts in the dark. Easily 35 mV with light shining on it.
Not necessarily what you're seeing, but I offer this as one of those oddball things that can cause no end of headaches until you get to the "a-HA!" moment.
RE: Ghost voltage on diode
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ghost voltage on diode
I also had such an experience. We needed a very sensitive differential pressure gauge for dynamic pipeline leak detection/location. I whipped up a bread board unit. Worked great in the lab. I hauled it up to my bosses mountain top home to test with his well pump line. It had hideous offset that varied constantly. I finally had to go back and get a scope. It had a period of about 7 seconds with a large repetitive peak. As I was scratching my head gazing across the spectacular view there on a far hill was a huge NORAD radar rotating about once every 7 seconds... Wrapping the whole thing in tinfoil reduced the problem just enough to allow testing to proceed.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Ghost voltage on diode
I know you are a guy with resources - and an attitude. But wrapping a whole radar in tin foil seems to be a little OTT, even for you!
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ghost voltage on diode
Keith Cress
kcress - http://www.flaminsystems.com
RE: Ghost voltage on diode
RE: Ghost voltage on diode
BTW most black electrical tape lets a lot of light thru even though you'd think not. You can see the sun thru it. I took a long Sierras backpacking trip with a camera that had a plug missing behind the film's aperture location. I put a piece of black electricians tape over the 1/4" hole. On getting the pictures of the whole trip back it was found that every single one had a huge, massively overexposed, circle in the dead center. It looked like a bunch of shots of a nuclear apocalypse.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Ghost voltage on diode
The slightest pin-prick in the coating on the case and these things would act like a photo-transistor.
RE: Ghost voltage on diode
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ghost voltage on diode
RE: Ghost voltage on diode
I am doing overtime - severe!
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ghost voltage on diode
There were OC 81 also. Only, I seem never to have met them. See this link http://
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ghost voltage on diode
Yeah! You can say that again! This was the diode Gunnar made his first VFD with.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Ghost voltage on diode
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ghost voltage on diode
I remember when the OCP 71 (Phototransitor - clear envelope) was 5 times the cost of the OC71 with the paint removed!
Might "outgun" Gunnar here!
H
ps: for those of a "certain age" I still have 8 off 4CX250B's in the cupboard! Just in case an EMP arrives...
RE: Ghost voltage on diode
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ghost voltage on diode
RE: Ghost voltage on diode