Measure power supply ripple with this technique - Page 1
Forum section: electronics / beginners

author: ptluis
author_rank: Frequent Contributor
timestamp: February 19, 2021, 04:27:10 pm
content: Today I was researching different ways to measure the ripple of a power supply, and to eliminate the noise in the signal as much as possible. I found this technique on youtube using 2 capacitors. In your opinion, is this a reliable technique with real indication of the measured value and the waveform displayed?

author: radiolistener
author_rank: Super Contributor
timestamp: February 19, 2021, 05:53:37 pm
content: Quote from: ptluis on February 19, 2021, 04:27:10 pmNo, remove parallel capacitor and use AC coupling mode on oscilloscope

author: pqass
author_rank: Super Contributor
timestamp: February 19, 2021, 11:11:58 pm
content: I was curious myself and found that another PS manufacturer suggests the same parallel caps.Additionally, to use 20MHz bandwidth limiiting on the scope and not to use the flying ground clip.But forgets to say the scope should be AC coupled. Oh well.See @2:56 mark:Of course Dave has a video about it too:

author: srb1954
author_rank: Super Contributor
timestamp: February 19, 2021, 11:54:16 pm
content: Quote from: ptluis on February 19, 2021, 04:27:10 pmGenerally OK. The parallel capacitors at the end of the cable simulate the effect of having some circuitry connected to the PSU, which is a little more realistic than just measuring the standalone PSU.Ideally they should have also tested it with a range of resistor loads at the end of the cable to simulate current drain from zero to full rated load to determine the point of maximum ripple.The 20MHz BW limit on the scope is from older test specifications and potentially hides some of the spiky, higher frequency ripple voltage components produced by modern switching PSUs. It is best to operate the scope at full BW although you may be frightened by the amount of ripple you see.

author: ptluis
author_rank: Frequent Contributor
timestamp: February 20, 2021, 11:40:24 pm
content: Quote from: pqass on February 19, 2021, 11:11:58 pmI saw this video and he also use a capacitor, but contributing to my initial question, today I did some tests on AC coupled and found that the method of the two capacitors eliminates, from the analysis on the oscilloscope, the switch noise and transient spikes of a smps power supply. Twisting the conductive wires also greatly reduces HF noise and other interference. I also noticed that the ripple is presented cleaner, but the vpp is slightly lower and seems more realistic, I don't know how much realistic is, but much more within specs of the power supplies I tested. BTW my oscilloscope is an old model from GW Instek without 50ohm impedance option.

author: ptluis
author_rank: Frequent Contributor
timestamp: February 21, 2021, 12:20:01 am
content: Quote from: srb1954 on February 19, 2021, 11:54:16 pmUnfortunately my scope is fixed 20Mhz BW limit, so can't use full BW, but I'm looking for ways to filter each noise component. Is there any way to filter just ripple and keep only switch noise and transient spikes?

author: ptluis
author_rank: Frequent Contributor
timestamp: February 21, 2021, 12:35:26 am
content: This video is also interesting, I solve identical problems the same way, including a cheap Electronic load DL24. That is the reason I'm looking for ways to get a realistic analysis of the ripple, noise and transients.

author: srb1954
author_rank: Super Contributor
timestamp: February 21, 2021, 01:24:59 am
content: Quote from: ptluis on February 21, 2021, 12:20:01 amNot really without some fancy equipment like a spectrum analyser or a high end digital scope that can apply additional mathematical operations to the captured waveforms to selectively filter out some signals.However, the switching spikes are often related to the ripple frequency but just extend over a broader bandwidth. Filtering one without affecting the other is nearly impossible since their occupied bandwidths overlap to a large extent.

author: ptluis
author_rank: Frequent Contributor
timestamp: February 21, 2021, 08:49:17 pm
content: Quote from: srb1954 on February 21, 2021, 01:24:59 amThanks for the answer. Got it. It will not make sense for me to invest in this type of equipment for amateur use, so I put my idea aside.