Psu ripple and transients, oscilloscope question.  - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 27, 2019, 04:20:15 pm
content: Looking for some advice please, can a 20mhz crt scope be used to view both linear and smps output ripple and transients ? I have a crt 60mhz scope, but wandering if a 20mhz crt scope will be sufficient.Thanks for any help with this question.

author: bd139
author_rank: Super Contributor
timestamp: October 27, 2019, 06:33:47 pm
content: It can. Typical 5x the bandwidth of the signal you want to look at is about right to catch harmonic products so that’d limit you to about 4Mhz SMPS switching frequency. Rise time is usually more important though.50MHz is about as low as I’d go these days.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 27, 2019, 06:54:32 pm
content: Thanks for your help, ok i see. I've got a lot to learn about oscilloscopes and there use, also setting it up right for the task.I really wanted to see the ripple elements of IT power supplys that have been converted for bench use etc. And output of buck and boost converters, when used with repurposed power supplys from power adapters and server power supplys.Typically example would be 19.5 Volt 11 Amp hp power adapter, using a buck converter. For HF Radio use, as the power has to be reasonably clean and free from high levels of electrical noise at the output.

author: bd139
author_rank: Super Contributor
timestamp: October 27, 2019, 07:18:34 pm
content: Ah yes. Makes sense. I'm staying low power with my HF setup after arguing with a POS MyDEL supply which puked all over 20m. Allows you to keep it linear, cheap and small!

author: rstofer
author_rank: Super Contributor
timestamp: October 27, 2019, 08:08:08 pm
content: Anytime I see "SMPS" and "oscilloscope" in the same sentence I cringe and hunt for Dave's video on How Not To Blow Up Your Scope. I realize now that you are working at lower voltages and not mains powered SMPS designs but, nevertheless, here is the video:

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2019, 08:40:27 pm
content: Quote from: rstofer on October 27, 2019, 08:08:08 pmThen I only see PSU and wonder how you can properly characterize its performance with a CRO ?Power ON/channel ON performance ?Nope, you need a DSO with Single shot triggering to do this.

author: bd139
author_rank: Super Contributor
timestamp: October 27, 2019, 08:42:46 pm
content: Think it’s more noise evaluation. Usually if it’s RF best thing is try and tune it on a radio. I still have an AM radio here as a handy noise tracer. 20meg scope will let you find switching hash though.

author: tautech
author_rank: Super Contributor
timestamp: October 27, 2019, 08:46:17 pm
content: Quote from: bd139 on October 27, 2019, 08:42:46 pmPSU noise means nutthing when power ON behavior fries your circuit !

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 28, 2019, 12:08:22 am
content: I'm not repairing or trouble shooting circuits, i just want the electrical noise from different repurposed psu adaptors, or server psu's. With this info i can determine if its worth repurposing a psu Adapter or similar psu, or not bother as its to noisy for HF radio use.This kind of thing below. That psu is a 90 watt hp Adapter, the small box with the 40mm fan is a buck converter. I need to evaluate the output noise for a given load.

author: BravoV
author_rank: Super Contributor
timestamp: October 28, 2019, 12:50:52 am
content: Quote from: davelectronic on October 27, 2019, 04:20:15 pmAs Tautech pointed above, for transients, you need DSO to "freeze" the signal during that period, analog scope can not do that, especially during power on/off period of the PSU.But for the moment during loading or unloading the power supply when its on by the load, for this particular measurement, while still using analog scope, you could try to DIY this circuit to simulate how the PSU behaves with this artificial load ->Dynamic Electronic Load Project, and to see how good the PSU reacts.

author: davelectronic
author_rank: Frequent Contributor
timestamp: October 28, 2019, 03:44:12 pm
content: That's interesting, i only need a steady supply of voltage and current. Inrush spikes transients should be reasonably straight forward to get attenuated, sudden changes in demand for power might be an issue. A lot to do with linear HF amplifiers drawing very high power, almost instantly, this is an unknown to me at this stage. A supply that's rated above the amplifiers rating, must be capable of delivering this power.Most of the testing ive done is with out a scope, and has been halogen lamps for loading. I understand now the cold resistance of the lamp means the start up current from any psu tested this way, involves huge sudden current demands.Most repurposed server power supplys pass this test, but a few hp and dell power adapters don't, i think this is because the psu sees the load as a dead short, and a protection circuit in the psu sences this lamp load and wll not start up. A few non starters with a lamp load show a brief power led start up, but goes out just as quickly. I really need an electronic load, but every thing i do, i have to put stuff away as I've very little space.