Power Supply Issue

[msobreira27]

Hi Everybody,

I assembled a simple 555 astable multivibrator (the design with the two diodes) which was powered by a power supply (in this case a tektronix PWS4323) @9V on a breadboard. The signal appeared modulated inside a sinusoidal wave at a frequency of 60Hz (The frequency of the mains in my country).   

The circuit is as follows:


I tried with different C1 capacitors and results were similar (when the values were close to the 60Hz you would get a distorted square wave, but once they differed significantly  a square wave pattern is embedded on a carrier 60Hz Sine Wave). The partern was the following:


When hooking to another generic power supply the result was like this, for the same circuit and same DDP applied (9V) the result was:


 
The envelope for the "GOOD" power supply had some variation:


I posted because the variation that I would classify as ripple, was in my view also coupled to the mains frequency.

When I tested the Tektronix Power supply with my fluke 179 and fluke 87V, there was no AC signal at all (A couple of millivolts to be precise, but nowhere near the signal seeing on the scope).

Anybody could explain why the power supply is behaving that way? The voltages and currents are spot on on the Tek Power supply and it passes self test. Probes on scope 1X, DC coupled (tried AC and same happened).

Thanks!

[jmelson]

You should always connect a ground lead to the ground of the circuit under test.  Your first supply has a floating output, the second one apparently is internally grounded.
Jon

[Feliciano]

Anybody could explain why the power supply is behaving that way? The voltages and currents are spot on on the Tek Power supply and it passes self test.

Maybe you have a ground issue.
Or maybe you already tried the following, but anyway I would say:
Put a big capacitor between Vcc and Gnd, ie. 8 and 1. for instance 470uF 16V (often it's also recommended a ceramic capacitor of 100nF in parallel).
Measure your Tektronix under load. Maybe an output capacitor has damaged.

[msobreira27]

You are right, thanks!  It seems that my mains ground is faulty - I appreciate if you could elaborate on why that leads to this kind of behavior.

[msobreira27]

It seems that the faulty ground was the issue, but I did the big test cap anyway - thanks - Very interesting that in this circuit placing a large capacitor and turning off the power supply, the discharge pattern will make the frequency change - it will fall as voltage drops - ant it collapses at 2.2V. Simple circuit - lots of learning!

[msobreira27]

L&G,

For the ones that cross with that type of problem. I just reviewed the issue and the ground is ok - it is actually stray capacitance inside the transformer and the coupling with the ground that is causing this. The entire problem is explained in this post and the excellent video from  FesZ electronics. I did the experiment as he did and the behavior was exactly the same.

The video is found in this link:


There is also a Hackday post:
https://hackaday.com/2020/02/04/solving-the-mysteries-of-grounding-while-improving-a-power-supply/



EDIT:
After tinkering a bit more: I connected the minus to the ground on the power supply. Solved the problem.


Amazing stuff!