What's going on with these PSU measurements?

[rubber_jonnie]

@rubber_jonnie are you maybe planning on recording before/after readings like me? I don't think anyone in the Amiga community has done that kind of thing yet.

Honestly, no as I'd already started the process when you first posted this, so that train has already left the station...

[Steve]

Oh my lord ... I tried all the recommended settings, then tried zooming out a bit to 1ms as BW said.. then I tried 10ms and I see a sine wave! Why am I seeing a sine wave on the 5v output of this PSU? Surely something is wrong...

Scary, I plugged the damn thing into my Amiga and it worked lol. Kind of regretting that now!

sinewave.jpg

[exxos]

Sinewave is the "ripple". Varying between five and six volts does seem a little extreme though. Generally inadequate output capacitance for something like that.

The "fuzzy bit" is the noise. Again capacitors can help alleviate that problem to a degree. Also those small output inductors generally are a bit rubbish and why I use much larger ones to get rid of the switching noise better.

But it can also be very difficult to take those measurements accurately. You have to really make sure your scope leads and ground lead are nowhere near anything like a transformer or inductors etc. Basically keep everything away as much as possible. Otherwise your scope probe itself can pick up interference from external sources.

Also things like LED lighting can cause noise, Among a million other things.. So in some cases you can actually be seeing a problem that may not really be there...

I think @Badwolf Mentioned earlier about turning on the bandwidth limit on your scope (I think it's in the channel 1 menu somewhere cannot remember offhand) . Putting it as something like 1MHz should help alleviate " false readings".

EDIT:

Found this article which may be interesting but there are plenty on the Internet about it all anyway..

ripple-noise-measurements.pdf

[Steve]

@exxos You sure about that just being the ripple? The scope says it's going from positive +5v to negative -5v, that's a full sine wave of AC voltage isn't it? The divisions are set to 5v and it's going up a whole division and then down a whole division.

[exxos]

oh sorry it says -5V, didn't notice that. I don't know how it could end up going from positive 6V to negative -5V. Are all the output diodes OK ?

[rubber_jonnie]

What settings do you have on your probes? Are you set to 10x?

I very rarely use 10x for voltage measurements, with my scope I usually find 1x with autoset covers a lot of bases.

[Steve]

Okay! Here is a twist! I decided to take a reading with the PSU turned off and disconnected from the mains... I get a sine wave! Is this the PSU picking up radio waves or something? It's so weird... I don't see how I can take proper readings when it has this strangeness going on. I also disconnected the probe from the 5v of the PSU and the sine wave does disappear, so it's definitely coming from the PSU.
(this is with Badwolf's recommended settings BW limit on and averaging off)

background noise.jpg

[exxos]

Try disconnecting the GND wire in your scope mains plug and see if that helps.

[rubber_jonnie]

You may want to add a measurement for frequency, see if you're picking up something from the mains at 50Hz.

Is this an A1200 PSU? If so I can try it on mine to see what it measures, but I'll need to know where you're connecting to on the PSU.

[JezC]

Does look to have a 20ms period so I'm sticking with it being pickup from the mains.