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Power supply ripple
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JuanGg:
Hello everyone,
I have been "putting together" a lab power supply with one of those DPS3005 modules and a 24 V 10 A power supply that goes with the model number S-240-24. I wanted a floating one so I don“t blow anything up probing around with the oscilloscope. I tested it with the multimeter, all looked as expected. Wired it all up the best I could and made case for it (see attached photo).

Then I hooked it up to the scope to measure its noise and what was my surprise when I found 180 Vpp (volts, not milivolts) ripple on the output. I used the "poor-man's diferential probe" and I don't really know how to interpret the results (see screenshot) Is this normal on a floating output?

Later, I referenced the supply to mains earth and measured the noise, this time with a single probe, and I found almost 0.5 Vpp noise! I knew some was to be expected, it being a switching power supply, but half a volt seems like a lot. (see other screenshot). I tried with the low inductance springy thing, but it made no difference.

Is there any way I can fix this, maybe a filter of some sort? Or is it just a faulty power supply?

Thanks in advance!
RobertHolcombe:
Curious; have you tried measuring the output ripple with a load connected?
ArthurDent:
It sounds like from what you were saying that initially you were using the 24V/10A SMPS without the necessary ground (earth) connected, which is not safe, and that probably accounted for the 180 volt capacitor coupling from the mains to the output. If you had probed the SMPS metal chassis with the scope with SMPS earth disconnected you'd probably have gotten about the same reading. 

If you try measuring the ripple or noise with a handheld DMM set to the most sensitive A.C. range you will probably find that the output from either the SMPS or the P.S. module will be much lower than 0.5 volts. Most good DMMs will have a wide enough frequency response to read higher frequency noise as well as ripple. As suggested, see if there is a difference if you put a load on the supply. It is sometimes hard to prevent induced loop currents in scope probe ground leads from giving false indications when the scope and the supply are both using a common ground. That is not to suggest that you should run either the supply or scope ungrounded which could represent a safety hazard.     

Oh, nice job on making the case, but I prefer metal to help keep the noise at a minimum. A thin layer of metal foil glued and tacked to the inside of the wood case wouldn't hurt. Just a thought.
JuanGg:
First of all, thank you for the fast responses!


--- Quote from: RobertHolcombe on September 07, 2018, 02:05:57 am ---Curious; have you tried measuring the output ripple with a load connected?

--- End quote ---

Yes, I have tried with various resistors, and it did not seem to make much of a difference, which makes me think of noise pickup on the scope probes maybe?


--- Quote ---It sounds like from what you were saying that initially you were using the 24V/10A SMPS without the necessary ground (earth) connected, which is not safe, and that probably accounted for the 180 volt capacitor coupling from the mains to the output. If you had probed the SMPS metal chassis with the scope with SMPS earth disconnected you'd probably have gotten about the same reading. 

--- End quote ---

The SMPS earth was connected all the time, and its case was grounded (the multimeter reads a direct short between mains earth and the metal case, as expected) . Probing the case with the scope results in 200 mVpp noise around ground. Both the scope and the power supply are conected to the same power strip.

The 24 V output of the SMPS is suposed to be floating (no direct short to mains earth) and goes directly to the P.S. module. The output of the module is then suposed to be floating, so if I want it to be mains earth referenced, I would join the negative output of the module to mains earth (black binding post and green binding post). I cannot do that before the module as it has a low side current shunt.


--- Quote ---If you try measuring the ripple or noise with a handheld DMM set to the most sensitive A.C. range you will probably find that the output from either the SMPS or the P.S. module will be much lower than 0.5 volts. Most good DMMs will have a wide enough frequency response to read higher frequency noise as well as ripple. As suggested, see if there is a difference if you put a load on the supply. It is sometimes hard to prevent induced loop currents in scope probe ground leads from giving false indications when the scope and the supply are both using a common ground. That is not to suggest that you should run either the supply or scope ungrounded which could represent a safety hazard.
--- End quote ---

Measuring the ripple with my UniT 61E multimeter on the ac range, it reads about 10 mVpp on both the SMPS and the module's output. Maybe it is just those loop currents on the scope probes. Both the supply and the scope are conected to the same power strip. I understand the importance of the ground conection.


--- Quote ---Oh, nice job on making the case, but I prefer metal to help keep the noise at a minimum. A thin layer of metal foil glued and tacked to the inside of the wood case wouldn't hurt. Just a thought.
--- End quote ---

I know, wood was all I had at hand. I thought about using some copper tape or similar inside the case and ground it. I will try it as soon as I can.

I also found that putting a 100nF ceramic cap between earth and the negative output of the SMPS cuts the 180 V swings to about a volt and decreases the noise from 500 mVpp to about 40 mVpp. Is that a valid solution?

Thanks again and sorry for the lengthy post.
xavier60:
It looks to me that the probe's ground lead is open circuited somehow.
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