4
\$\begingroup\$

I discovered that while my Siglent SDS1052DL oscilloscope has about a megohm of input impedance, the ground of each channel is actually common.

I accidentally connected a 12 volt, 25 amp power supply across through the ground of the scopes channels. The smoke came out of the ground clip on the oscilloscope probes. Both got hot enough to melt the insulation off the wire. The cable that leads out to the probe was not even warm to the touch. The rest of the probe appears fine.

I tested the oscilloscope against its 1 khz source that is on the front and both channels seem to work still. I am assuming whatever current path it took through the inside of the scope is actually very large in terms of conductor size, so the scope is probably fine.

Can I just heatshrink the wires on the probe ground clip to repair them?enter image description here

|improve this question
\$\endgroup\$
  • \$\begingroup\$ Is the supply insulated wrt ground? \$\endgroup\$ – Vladimir Cravero May 22 '16 at 22:13
  • \$\begingroup\$ Yes, the supply is floating. \$\endgroup\$ – Eric Urban May 22 '16 at 22:15
  • \$\begingroup\$ The built in signal generator probably doesn't exercise the ground, so do verify, perhaps by measuring the DC voltage on the scope of an AA cell between the probe and the remains of the ground wire, that the ground path is still intact for each probe and input channel. \$\endgroup\$ – Chris Stratton May 22 '16 at 22:29
  • 1
    \$\begingroup\$ This was a very cheap lesson, and I bet that you will always remember this moment every time you connect the ground clips from various instruments. :) \$\endgroup\$ – pipe May 23 '16 at 2:18
  • 1
    \$\begingroup\$ @Eric Urban While the built-in signal generators do have ground connections, it's quite common for them to share the ground internally with the rest of the scope, with the GND connector being there only to provide integrity for high frequencies. \$\endgroup\$ – AndrejaKo May 23 '16 at 6:01
3
\$\begingroup\$

You should be safe repairing the wires and calling it a day.

When you connected the second ground alligator clip, you made a short at the supply output. The full 25A passed through your scope, but I doubt it was damaged, and I doubt the cables were damaged too.

Inside the scope there is at least a plane on the pcb dedicated to ground, it should be more than capable to carry that current for a limited period of time, at least.

Coax cables have a fairly thick shielding, which can carry some current without being damaged. I sometimes use them to power things with no problem. I am not sure of the current rating, I would guess less than 10A but not much less, so they can probably withstand 25A for a short time.

The weak point is the tiny cable that connects the probe to the alligator clip, and it fried.

You already tested your probes with the 1kHz source, if possible I would try some higher frequencies and I would check also that the measured voltage is ok, but I think it is very unlikely you made more damage than some molten plastic.

|improve this answer
\$\endgroup\$
  • 1
    \$\begingroup\$ Hmm... I would have recommended replacing the probe. You can get them for about 10 bucks. Your time repairing the old one is probably worth that much. \$\endgroup\$ – Robert Harvey May 23 '16 at 5:30
  • \$\begingroup\$ I would throw the little black wires out (Or at least into the back of a draw never to be seen again), nothing good comes of adding that much lead inductance to whatever you are probing. The ground spring pressed over the end of the probe is better in all ways, but that is perhaps another lesson best learned the hard way. \$\endgroup\$ – Dan Mills Dec 7 '17 at 11:25
5
\$\begingroup\$

The damage to the insulation is pretty much purely cosmetic. You can either leave it as is, or heatshrink it, it's up to you. Alternatively you could get a new ground clip (they are generally quite cheap), but functionally there will be little difference.

The moral of the story is that ground clips should always be connected to the same potential. If you want to measure two completely independent signals with different ground potentials, you need a differential probe.

You seem to have gotten quite lucky, it is very easy to completely destroy an oscilloscope doing this.

|improve this answer
\$\endgroup\$
  • \$\begingroup\$ You can also use two regular probes and subtract, or build your own differential amplifier. \$\endgroup\$ – Jay Keegan May 22 '16 at 22:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.