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Recognise this trace?

Started by Cursitor Doom July 9, 2017
Gentlemen,

The link below shows a scope trace of the output of a linear PSU (voltage 
4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in-
circuit under load from the boards it supplies. Normally I would assume a 
filter capacitor to be at fault here, but they all check out fine so 
something else is causing this ripple. Note there is a characteristic 
'knee' on the high peaks and I'm thinking this must be indicative of 
*something* trouble is, I don't know what. If anyone recognises this 
waveshape and knows what causes it, that'd be "awesome" - as our American 
friends describe everything. Check it out:


https://www.flickr.com/photos/128859641@N02/35684682551/in/dateposted-
public/
On Sunday, July 9, 2017 at 5:14:43 PM UTC+2, Cursitor Doom wrote:
> Gentlemen, > > The link below shows a scope trace of the output of a linear PSU (voltage > 4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in- > circuit under load from the boards it supplies. Normally I would assume a > filter capacitor to be at fault here, but they all check out fine so > something else is causing this ripple. Note there is a characteristic > 'knee' on the high peaks and I'm thinking this must be indicative of > *something* trouble is, I don't know what. If anyone recognises this > waveshape and knows what causes it, that'd be "awesome" - as our American > friends describe everything. Check it out: > > > https://www.flickr.com/photos/128859641@N02/35684682551/in/dateposted- > public/
What's odd about it? The filter capacitor is discharging for about 6msec (while the half-sine from the bridge rectifier is less than voltage across the capacitor), and charging for the 4msec when it is higher. The "knee" is mostly likely the internal resistance of the capacitor, which adds on top of the capacitor voltage when the capacitor is charging, and subtracts from it when the capacitor is discharging. If that was all it was you'd expect something similar on the low point of the rippe, but if you throw in a little leakage inductance in the transformer the turn-on is gradual and the corresponding "knee" gets smoothed out while the charging current at the turn-off hangs on for a bit. I can't be bothered plugging this into LSpice - it's a question from Cursitor Doom after all - but it feels plausible. -- Bill Sloman, Sydney
On Sun, 9 Jul 2017 15:10:46 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>Gentlemen,
Surely, you jest.
>The link below shows a scope trace of the output of a linear PSU (voltage >4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in- >circuit under load from the boards it supplies. Normally I would assume a >filter capacitor to be at fault here, but they all check out fine so >something else is causing this ripple. Note there is a characteristic >'knee' on the high peaks and I'm thinking this must be indicative of >*something* trouble is, I don't know what. If anyone recognises this >waveshape and knows what causes it, that'd be "awesome" - as our American >friends describe everything. Check it out: > >https://www.flickr.com/photos/128859641@N02/35684682551/in/dateposted-public/
Is the mystery load the hardware appearing in adjacent photos? EXIF says that they were taken 4 days earlier, which suggests a connection. These photos looks odd: <https://www.flickr.com/photos/128859641@N02/34892442244/in/photostream/> <https://www.flickr.com/photos/128859641@N02/35564536392/in/photostream/> because C17 (the 2nd blue electrolytic from the right) seems to have been soldered into the wrong PCB holes. The white silk screen circle makes a good target for installing capacitors. If my crystal ball is correct, this capacitor might be presenting a higher than normal current load to the power supply, which is reacting by producing excessive ripple that you're seeing on the scope. Is this the capacitor you removed from the board? <https://www.flickr.com/photos/128859641@N02/35050368241/in/photostream/> If so, you ripped out the plated through hole. 4 terminal radial caps are not common, so I guess you substituted something else. What did you put in its place? Did you test and fix the PCB traces? Oh, never mind. I found your rather ugly solder wick PCB fix: <https://www.flickr.com/photos/128859641@N02/35602405551/in/photostream/> You might want to double (or triple) check your soldering and PCB through hole connections. From this photo: <https://www.flickr.com/photos/128859641@N02/35564539582/in/photostream/> there's a tuning screw, semi-rigid coax cable, and cavity filter on the left side. Too bad the power supply board is missing. There are some gold connector pins in the photo. So, what is it? -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
If it's your signal generator thing, yes, it's very possible / likely to 
have dried out caps (excess ESR).

Yes, the nippley appearance is likely ESR.  But, if you say it's okay, then 
it must be okay.  Right?

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

"Cursitor Doom" <curd@notformail.com> wrote in message 
news:ojth1m$vqg$1@dont-email.me...
> Gentlemen, > > The link below shows a scope trace of the output of a linear PSU (voltage > 4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in- > circuit under load from the boards it supplies. Normally I would assume a > filter capacitor to be at fault here, but they all check out fine so > something else is causing this ripple. Note there is a characteristic > 'knee' on the high peaks and I'm thinking this must be indicative of > *something* trouble is, I don't know what. If anyone recognises this > waveshape and knows what causes it, that'd be "awesome" - as our American > friends describe everything. Check it out: > > > https://www.flickr.com/photos/128859641@N02/35684682551/in/dateposted- > public/
On Sun, 09 Jul 2017 10:20:47 -0700, Jeff Liebermann wrote:

> Is the mystery load the hardware appearing in adjacent photos? EXIF > says that they were taken 4 days earlier, which suggests a connection.
If the adjacent photos were of any relevance, Jeff, I'd have mentioned it and linked to them.
> Oh, never mind. I found your rather ugly solder wick PCB fix: > <https://www.flickr.com/photos/128859641@N02/35602405551/in/photostream/ > > You might want to double (or triple) check your soldering and PCB > through hole connections.
All I did was replace an electro with one of identical parameters; it just happened to be a bit smaller than the original (which is not at all uncommon).
> > From this photo: > <https://www.flickr.com/photos/128859641@N02/35564539582/in/photostream/ > > there's a tuning screw, semi-rigid coax cable, and cavity filter on the > left side. Too bad the power supply board is missing. There are some > gold connector pins in the photo. So, what is it?
You're complicating this unduly and massively, Jeff. I simply posed a question about an oscilloscope trace, that's all.
On 7/9/2017 8:10 AM, Cursitor Doom wrote:
> Gentlemen, > > The link below shows a scope trace of the output of a linear PSU (voltage > 4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in- > circuit under load from the boards it supplies. Normally I would assume a > filter capacitor to be at fault here, but they all check out fine so > something else is causing this ripple. Note there is a characteristic > 'knee' on the high peaks and I'm thinking this must be indicative of > *something* trouble is, I don't know what. If anyone recognises this > waveshape and knows what causes it, that'd be "awesome" - as our American > friends describe everything. Check it out: > > > https://www.flickr.com/photos/128859641@N02/35684682551/in/dateposted- > public/ >
You might get this when you have two caps separated by an inductor. First cap usually gets more peak charge current than the second one and develops high ESR first. Even if both caps are good, the inductor (or resistor) in between can cause this effect.
On Sun, 09 Jul 2017 12:34:51 -0700, mike wrote:

> You might get this when you have two caps separated by an inductor. > First cap usually gets more peak charge current than the second one and > develops high ESR first. Even if both caps are good, the inductor (or > resistor) in between can cause this effect.
Someone on SER said a non-linear load could explain the shape of that trace and my initial reaction was that sounded quite plausible. However, on reflection I think if that *were* the case, then the knee would span 600-700mV (when the semis start conducting) and that isn't what we see here. The obvious thing to do next is whip off the output plugs from the PSU one by one and see if that clears the problem and if so, which sub- circuit is responsible for the overload as it may be nothing to do with the PSU at all. -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.
Cursitor Doom wrote:
> Gentlemen, > > The link below shows a scope trace of the output of a linear PSU (voltage > 4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in- > circuit under load from the boards it supplies. Normally I would assume a > filter capacitor to be at fault here, but they all check out fine so > something else is causing this ripple. Note there is a characteristic > 'knee' on the high peaks and I'm thinking this must be indicative of > *something* trouble is, I don't know what. If anyone recognises this > waveshape and knows what causes it, that'd be "awesome" - as our American > friends describe everything. Check it out: > > > https://www.flickr.com/photos/128859641@N02/35684682551/in/dateposted- > public/ >
Cause: stored charge in the rectifiers.
On Mon, 10 Jul 2017 01:10:46 +1000, Cursitor Doom <curd@notformail.com>  
wrote:

> Gentlemen, > > The link below shows a scope trace of the output of a linear PSU (voltage > 4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in- > circuit under load from the boards it supplies. Normally I would assume a > filter capacitor to be at fault here, but they all check out fine so > something else is causing this ripple. Note there is a characteristic > 'knee' on the high peaks and I'm thinking this must be indicative of > *something* trouble is, I don't know what. If anyone recognises this > waveshape and knows what causes it, that'd be "awesome" - as our American > friends describe everything. Check it out: > > > https://www.flickr.com/photos/128859641@N02/35684682551/in/dateposted- > public/
a soft diode in the bridge rectifier? -- Using Opera's mail client: http://www.opera.com/mail/
Cursitor Doom wrote:

> Gentlemen, > > The link below shows a scope trace of the output of a linear PSU (voltage > 4.7VDC) with about 300mV of 100Hz ripple riding on it. This PSU is in- > circuit under load from the boards it supplies. >
** Normal waveform for a supply frequency, unregulated DC supply. Scope a few others and see how dumb you are being. .... Phil