I'm having a few problem with a digital analyzer. When a probe is connected to an unpowered circuit. I see no reading on my digital analyzer. But once the power supply is plugged even if the circuit isn't powered.

The digital analyzer will read some pulses at around 50 Hz. This renders the digital analyzer completely useless when powering devices from an AC-DC power supply. I tried with multiple power supply at home but none help.

It's enough to connect the digital analyzer probe to the ground node of the power supply while the power supply isn't powering any load.

My guess is that the power supply are letting some AC through and aren't 100% isolated from AC.

I was wondering if there was a way to filter those 50hz spikes without messing with the power supply. I was thinking of a filter that could be connected in between the power supply and the device I'm powering.

Not sure what wasn't clear from the question. I'm not designing a power supply and not designing a digital analyzer either. I'm just trying to use my digital analyzer on something that is powered to an AC-DC power supply (those cheap wart plug).


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Describe what you are trying to achieve. Not the tiny subconcept. The WHOLE concept. What are you trying to achieve with your digital analyzer? I can't tell from your writing if your goal is to design and make a power supply; if you are designing a digital analyzer; or if there is a 3rd circuit ("device") that is being powered by some power supply and examined with a digital analyzer whose output you want to understand. That's just how clueless your writing has left me. I believe this is about some device. But I don't know for sure. SO WRITE MORE!! Please, for everyone's sake. \$\endgroup\$
    – jonk
    Jan 17, 2018 at 19:17
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    \$\begingroup\$ Sounds like your average Y-cap divider-ground-debacle. Draw a schematic diagram how you have connected and grounded everything. \$\endgroup\$
    – winny
    Jan 17, 2018 at 19:25
  • \$\begingroup\$ Capacitors. Lots of them. \$\endgroup\$ Jan 17, 2018 at 20:25
  • \$\begingroup\$ @TomCarpenter I tried that but I guess I should increase the value. I had a 4.7uf connected to + and - in parallel with the circuit. \$\endgroup\$ Jan 17, 2018 at 20:27
  • \$\begingroup\$ Oh man, fun isn't it? Try an isolating transformer. Try a smaller measurement loop. Try on a battery powered laptop. Try earthing one point of the circuit. Try capacitors. Try swapping out each component to find the culprit. Welcome to electronics. \$\endgroup\$ Jan 17, 2018 at 20:33

1 Answer 1


When a bridge drives a cap with some decay voltage , just before each peak the diodes force the caps back up to peak voltage. This causes EMI by the narrow current spike from the loop area of the antenna of current pulses and the rise time of current.

Even though the voltage is floating thru the transformer coupling, there is still this magneto motive force which can create some voltage into logic probes which may be say 1Meg. , enough to cross the threshold voltage of unconnected leads that are open to the transient repetitive field being radiated.

Usually the most common solution is to earth ground everything, but the last clue was that this house has no earth ground. hmm?

In that case, bonding the logic analyzer frame to the power supply return is next best bet. This reduces the amplitude of stray CM field so that it is less likely to create a differential voltage. Logic signals tend to be low impedance so that once connected to the circuit, you would find the same result, even if it was not powered up.

Conclusion: False flag signals from EMI and lack of AC earth ground to logic analyzer.

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    \$\begingroup\$ That's pretty much the same conclusion I came too. I finally found a power supply that works fine. It has a big transformer in it. It should be good for most work. Thought, I could try the common ground. It's a bit tricky because the Digital analyzer is USB but feasible. \$\endgroup\$ Jan 19, 2018 at 14:30
  • \$\begingroup\$ Usually USB ground if goes back to mobile, can be earthed also on speaker jack or frame. \$\endgroup\$ Jan 19, 2018 at 17:52
  • \$\begingroup\$ Well that's interesting, it's definitely earthed to the jack. I'm definitely going to try connecting the ground pin of the USB to the jack if that helps. \$\endgroup\$ Jan 19, 2018 at 23:57

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