# Oscilloscope ground-probe causing test circuit to malfunction [duplicate]

Today I was trying to measure the rectified SMPS incoming power supply ( 315VDC) using a tektronix TDS 3932C oscilloscope with P6139A probe. Every time the ground-probe touches the ground of the supply, the power supply rectifier diode is getting damaged. I assumed it may be related to earth fault current, but both equipment supplies are properly earthed.

Eqpt :Tektronix TDS 3932C Digital Phosphor Oscilloscope 300 MHz Probe used: P6139A , 500MHz, 8 pF ,10 MOhms, 10x

Edited I want to check the pwm controller IC O/p waveform with the oscilloscope, the Vcc of the IC itself is tapped from the rectifier supply, and I am unable to use the oscilloscope.

## marked as duplicate by Andy aka, winny, Dmitry Grigoryev, Voltage Spike, DoxyLoverJul 6 '18 at 7:49

• @Tony why bother? You were able to field an answer of some sort, and another answer even solved the issue. – W5VO Jul 5 '18 at 2:41
• Do you really “some sort” is accurate?? everyone with a scope can do what I said. But few have the active probes HV \$& – Tony Stewart Sunnyskyguy EE75 Jul 5 '18 at 3:13

I assumed it may be related to earth fault current, but both equipment supplies are properly earthed.

You cannot connect oscilloscope ground to anything that is not at mains earth potential.

You will get the following short circuit through your oscilloscope:

simulate this circuit – Schematic created using CircuitLab

I recommend this EEVblog episode:
EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
(since you are lucky the diode failed first)

I must add that measuring on these live circuits is dangerous. You are supposed to use an active high voltage differential probe. A Tektronix P5200A for example.

There are also many other solutions to evade this problem, but since there are many safety caveats on these methods, I will not list them.

• While it is clear what you mean by "You cannot connect oscilloscope ground to anything that is not at mains earth potential", that sentence is nonetheless false in general. If a circuit is floating (like a portable battery operated circuit), I can connect the scope ground anywhere I want. Just nitpicking. – Sredni Vashtar Jul 4 '18 at 15:18
• @SredniVashtar excellent point. However, I’m not sure how to rephrase the sentence. – Jeroen3 Jul 4 '18 at 18:05
• I would add "when the DUT is connected to earth..." at the beginning of your sentence. As I said, it is clear from context that you were right, but some rookie might take that to face value and unnecessarily ground floating circuits. – Sredni Vashtar Jul 4 '18 at 18:11

It is best practice to use a plastic cap >> probe capacitance in series with earth grounded probe leads then use two probes in differential mode on non-isolated DC supplies using 100:1 or 10:1 probes and ensure output is less than probe Vmax rating (e.g. 250V).

This avoids the diode V- short circuit loop current from earth bonded neutral to diode to V- to scope probe earth bond to AC input or similar scenario of non-isolated DC outputs.

• Thanks. As mentioned I am using a capacitive probe 10x but the problem prevails. – seccpur Jul 4 '18 at 16:04
• cap location needed on ground – Tony Stewart Sunnyskyguy EE75 Jul 4 '18 at 16:16