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Recognise this trace?

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:

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Reply to
Cursitor Doom
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What's odd about it? The filter capacitor is discharging for about 6msec (w hile the half-sine from the bridge rectifier is less than voltage across th e capacitor), and charging for the 4msec when it is higher. The "knee" is m ostly likely the internal resistance of the capacitor, which adds on top of the capacitor voltage when the capacitor is charging, and subtracts from i t when the capacitor is discharging.

If that was all it was you'd expect something similar on the low point of t he rippe, but if you throw in a little leakage inductance in the transforme r the turn-on is gradual and the corresponding "knee" gets smoothed out whi le 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 Cursit or Doom after all - but it feels plausible.

--
Bill Sloman, Sydney
Reply to
bill.sloman

Surely, you jest.

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: 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? 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: You might want to double (or triple) check your soldering and PCB through hole connections.

From this photo: 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
Reply to
Jeff Liebermann

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:

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Reply to
Tim Williams

If the adjacent photos were of any relevance, Jeff, I'd have mentioned it and linked to them.

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).

You're complicating this unduly and massively, Jeff. I simply posed a question about an oscilloscope trace, that's all.

Reply to
Cursitor Doom

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.

Reply to
mike

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. 
--
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Reply to
Cursitor Doom

Cause: stored charge in the rectifiers.

Reply to
Robert Baer

a soft diode in the bridge rectifier?

--
Using Opera's mail client: http://www.opera.com/mail/
Reply to
David Eather

** Normal waveform for a supply frequency, unregulated DC supply.

Scope a few others and see how dumb you are being.

.... Phil

Reply to
Phil Allison

----------------------

** Bollocks.

..... Phil

Reply to
Phil Allison

The peak is definitely from the charge pulse from the rectifier. A capacitor-input PSU charges only 5 to 10 % of the half-cycle time, depending on the resistances and inductances in the charge path.

--

-TV
Reply to
Tauno Voipio
-

When I tested this psu out-of-circuit with a dummy load I got less than

5mV of ripple on that rail. Now I may have too lightly-loaded the output, I can't say for sure, but that does tend to indicate to me I should check for shorted components down the line from the psu before I do anything else.
Reply to
Cursitor Doom

Regulator is not working. It's broken, doesn't have enough headroom, something wrong with its feedback path, reference circuit is corrupted, something is broken.

Reply to
bloggs.fredbloggs.fred

What's the problem with it? Why do you think it ought to be behaving any different? It looks like a standard unregulated 50Hz PSU.

NT

Reply to
tabbypurr

Yes, and that's the problem! This is the trace at the final output of the psu, as it supplies the signal boards, *after* it should have been smoothed and regulated.

Reply to
Cursitor Doom

Yup, that should cover it.

Reply to
Cursitor Doom

It took me

Reply to
Tom Gardner

Way down at 50Hz any kind of regulator attenuates ripple by 80-120dB. You do the math to get 300mVpp on output.

Reply to
bloggs.fredbloggs.fred

Sure would have saved time if you'd told us to start with it was a regulated psu. Looks like the regulator pass transistor is shorted.

NT

Reply to
tabbypurr

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