I have a 1500µF 200V cap just sitting on the bench. I know that when I test a cap using a DVM the cap will slowly charge because the DVM provides a minute current. I've seen that for many years. However - and here's the part I find hard to believe - I have this cap sitting on the bench. Not connected to a darn thing. Leads face up (away from the conductive ESD mat). I just connected my oscilloscope to the cap. Immediately the line diverged from zero to off screen. It came back to zero in about 3 seconds. Scope was set to DC on the 0.1 scale. Was definitely surprised to see this happen.
The probe I'm using is a video balun connector with a cat-5 twisted pair of wires. Not your normal test leads. In fact, the whole reason for touching this to the cap was to see what would happen. I've already connected this test lead rig to a coin cell battery sitting at 2.9 volts. The scope reading was 2.9 volts as well as the DVM. I'm actually wondering about using this balun device as a means for connecting to my breadboard to test circuits - to see how it compares to regular scope cables. I have channels A & B so I can set up a test where I feed the same signal into both and observe any changes - IF any. I'll let you know how that turns out later. For now I'm stumped as to understand how a cap can develop some sort of residual charge. The last time I put this cap in circuit and removed it I made sure it was shorted. It's sat on the bench for about 9 months untouched. So any residual charge SHOULD have dissipated by now. So I'm stumped! Anyone know something I don't - regarding this phenomena. I know many of you know things I don't.
Anyway - thanks.
[edit] Following up on my balun cable test: The coin cell shows 3.2 volts via the standard scope probe and 3 volts via the bodged together balun cable. Doesn't look like I can expect to see any kind of accuracy. However, as far as seeing a signal (amplitude not withstanding) it should be a fair reading. Maybe I should pull the balun apart and see what's inside it. Likely some sort of line matching transformer. But then why the steady reading when checking a battery?
The probe I'm using is a video balun connector with a cat-5 twisted pair of wires. Not your normal test leads. In fact, the whole reason for touching this to the cap was to see what would happen. I've already connected this test lead rig to a coin cell battery sitting at 2.9 volts. The scope reading was 2.9 volts as well as the DVM. I'm actually wondering about using this balun device as a means for connecting to my breadboard to test circuits - to see how it compares to regular scope cables. I have channels A & B so I can set up a test where I feed the same signal into both and observe any changes - IF any. I'll let you know how that turns out later. For now I'm stumped as to understand how a cap can develop some sort of residual charge. The last time I put this cap in circuit and removed it I made sure it was shorted. It's sat on the bench for about 9 months untouched. So any residual charge SHOULD have dissipated by now. So I'm stumped! Anyone know something I don't - regarding this phenomena. I know many of you know things I don't.
Anyway - thanks.
[edit] Following up on my balun cable test: The coin cell shows 3.2 volts via the standard scope probe and 3 volts via the bodged together balun cable. Doesn't look like I can expect to see any kind of accuracy. However, as far as seeing a signal (amplitude not withstanding) it should be a fair reading. Maybe I should pull the balun apart and see what's inside it. Likely some sort of line matching transformer. But then why the steady reading when checking a battery?
Last edited: