Measuring power quality of a 12V switching power supply using an Oscilloscope - Page 1
Forum section: electronics / beginners

author: echomancer
author_rank: Contributor
timestamp: February 24, 2012, 02:54:23 am
content: I will be receiving my Rigol scope tomorrow, and I was wondering how to use it to see the quality of the output voltage of a 12V switching power supply. Is it really as simple as plug a probe in to the red side and ground it to the black? Will I fry the scope? I know these scopes can handle up to 300V, and that they are very high impedance, but my mind thinks it's wrong to plug the delicate scope directly in to the output of my power supply...Along those lines, if you plugged the scope directly in to a household electrical receptacle, would you see the AC waveform of your mains or would that fry the scope?I feel embarrassed about asking these questions, but I'm new to all of this and looking for uses beyond looking at the output of pins on my microcontrollerThanks in advance!

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 24, 2012, 03:14:43 am
content: you feel embarassed yet you asked question we asked last time, since any equipment is just like a black box we dont know whats inside (we at a mercy of the designers decision). my answer is..the +ve side of the probe is high impedance input, so you can safely connect it directly to its rated voltage directly without putting high current into it, i've successfully connected it directly to 240V mains without any accident (but be careful not to short nearby metal), but the -ve side (ground probe) is low impedance to earth ground, you cannot simply connect it with anything other than earth ground node in a circuit, you will risk flowing large amount of current and volt through it.about checking your psu noise, my answer is yes, thats the best way we have ie connecting directly the probe to psu output (with or without any other load connected to it), but you have to make sure the -ve outlet of your psu is grounded or floating, or at worst doesnt directly connected to any high side circuit, which less likely is (stupid assumption).

author: IanB
author_rank: Super Contributor
timestamp: February 24, 2012, 03:47:11 am
content: The main problem with using a probe to look at the output of a switching power supply is you may be horrified at what you seeYou may never trust your power supply again.When I looked at a supply it wasn't too bad when under load, but with no load the output waveform was like a turbulent storm in the middle of the ocean! It really wasn't pretty.

author: echomancer
author_rank: Contributor
timestamp: February 24, 2012, 03:59:58 am
content: Thanks for alleviating my fears! I will test under load with a radio pulling an amp on receive.Thanks!

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 24, 2012, 04:22:59 am
content: Quote from: IanB on February 24, 2012, 03:47:11 ambad design i guess. have you look at the input (offline) stage? and get hassled everytime your elcb got tripped by connecting probe ground to any point in the circuit.

author: Unknown
author_rank: Unknown
timestamp: February 24, 2012, 10:05:01 pm
content: Quote from: echomancer on February 24, 2012, 02:54:23 amAs already mentioned, the main danger is connecting the ground lead to some other potential than ground. As long as the circuit is floating, you should be able to clip the ground lead anywhere, but as soon as you ground one side, for example via a USB cable or another probe, you should make sure to clip the ground lead only to parts of the circuit already at ground potential.One thing that can be an issue in switching power supplies is inductive pickup by the loop between the probe tip and the ground lead. This may show up as apparent noise that's not really there. One way to check is to connect the ground lead and probe tip to the same point. Anything you see is due to inductive pickup, not what's happening in your circuit.Quote from: echomancer on February 24, 2012, 02:54:23 amA good 10x probe should be OK for mains, but connect the ground lead to ground, not neutral. You'll be observing the voltage between live and ground, not the voltage between live and neutral that your power supply sees. In most installations the difference between those is fairly small, however. Be careful and don't do it in high energy circuits (eg. industrial installations), however, since many probes are something like CAT II 300 V at most, while even a $50 DMM will at least claim CAT II 600 V. The specs should be in the datasheet for the probe, and may even be printed on the probe.

author: echomancer
author_rank: Contributor
timestamp: February 25, 2012, 01:26:31 am
content: Got the scope, and have been having a ball looking at things.I want to make sure I'm 100% clear on the power supply. Probe tip to red +12v side is fine. Ground should NOT be connected to the black side, and instead should be connected to the ground lug on the case that presumably is connected to the ground that goes in to my wall receptacle.Also, not strictly related, but while observing the blink state of an LED on the Arduino uno (on 1 sec, off 1 sec), the scope showed a frequency of 500mhz, not 1hz. This is unexpected behavior. I had the probe tip connected to the LED leg and the ground clip connected to the circuit ground. Any thoughts on why the scope read 500.0mhz and not 1hz? The scope also had trouble auto zooming on the wave. Are these scopes not good at doing stuff this slow?

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2012, 01:34:46 am
content: Quote from: echomancer on February 25, 2012, 01:26:31 amNot necessarily. It depends if your power supply has a floating and isolated output (laboratory supplies do, ATX supplies don't). If it does (have a floating output), then you should connect the probe across the positive and negative output terminals.QuotePerhaps. You should move away from using the "auto" feature on the scope and get familiar with manual adjustment of the controls. You can adjust the timebase, the vertical scale and the trigger options to get the display just how you want it.But a 2 second on/off period is quite slow for a scope. You can do it, but scopes are normally used for much faster signals.

author: echomancer
author_rank: Contributor
timestamp: February 25, 2012, 01:55:39 am
content: The power supply is a Samlex 1223M. It's designed for amateur radio operators. How can I tell what type of ground it is?Also, what about things like the output of a solar charge controller to see what it's outputting to a SLA battery? The only ground in this system is what is in the system, nothing running to earth.

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2012, 02:40:57 am
content: Quote from: echomancer on February 25, 2012, 01:55:39 amWith the power supply unplugged from the mains, use a wire to discharge any internal capacitors by temporarily shorting the output terminals and the input terminals. Then use the continuity tester on your meter to check for any connection between the output terminals and the mains input or ground terminals. If you find no path between any of the outputs and any of the inputs then you can assume the output is isolated. If the output is isolated you can safely connect the scope ground lead to the black negative terminal of the power supply. If the output is not isolated you should connect the scope ground lead only to a point at the mains earth potential.QuoteIn that case you can safely assume the circuit is floating and you can connect the scope probe freely. But one point to mention that I forgot--until I was sharply reminded by a loudly complaining circuit--is that the ground clips on both scope probes are connected. If you are using both scope channels you must make sure you don't create a short circuit by connecting the two ground clips to different places.

author: echomancer
author_rank: Contributor
timestamp: February 25, 2012, 03:19:31 am
content: Thanks for the advice and input! This is really helpful.I don't think I'll have to worry about the short circuit, as I'm only going to be using one probe, but I will keep this in mind for the future!

author: lavo-1
author_rank: Regular Contributor
timestamp: February 25, 2012, 07:27:59 am
content: Just an observation I have found when checking my power supply (thandar thurlby) for AC ripple, is that it has to be done under load. With no load the display just shows a steady DC voltage present, with no ripple.I am sure this is down to the fact, that the ripple filter capacitors in the PSU are not discharging due to the high impedance of the scope probes, but I would like someone with better knowledge of DC PSU's than me, give their opinion.