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Ghost voltage on diode 1

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cmb042

cmb042

Geotechnical
Apr 28, 2008
39
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?
 
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Use an oscilloscope to see what your DMM is reading. You probably have a magnetic/electrostatic/or electromagnetic field which the diode rectifies. Does the reading change when you touch anode or cathode? Close to a VFD, radar or radio/Tv transmitter?

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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I've been amazed at how much noise a digital multimeter can impress back on a signal when I look at it with an oscilloscope.
 
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Years ago I had a circuit I was debugging. Every time I opened up the box, an integrator would saturate. It worked fine when the box was closed. There was a diode across the integrating capacitor to prevent operation in one polarity. The diode was in a glass case. Yup. It worked as a photo-voltaic cell. Going to a plastic case fixed the integrator problem.

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.
 
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Reminds me of an ADC, I think the number was 8010 something, that National Semiconductor had put in a white ceramic encapsulation. Worked well when in the instrument box. But showed weird results when we opened the lid. Putting a lamp close to it made it even weirder. I think jimkirk is on to something. Is it a glass body - like an 1N4148?

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Yes itty bitty solar panels!

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 -
 
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Hi Smoked!

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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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28guicn.gif
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Keith Cress
kcress -
 
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Itty bitty solar panel indeed. Some black electrical tape made the incosistent readings go away. Thanks a lot!
 
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cmb042; Thanks for the come-back! We love to hear about solutions.

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 -
 
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I always love these weird and wacky stories. Reminds me of fault finding equipment which used the old OC81 transistors. The first thing you did was colour the outside of the transistor with a thick coating with a black marker pen.

The slightest pin-prick in the coating on the case and these things would act like a photo-transistor.
 
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Or OC71 - as I remember them?

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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That's for sure!

I am doing overtime - severe!

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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Aah - the Galena Crystal! Those were the days.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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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...
 
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It is simply a question of who is the oldest and fartest. You outfart me there, Harry!

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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