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: public/

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

Recognise this trace?

Jul 09, 2017 28 Replies

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John S 9 years ago

Well, just "something is broken" would cover it.

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krw 9 years ago

Or broken as designed.

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Cursitor Doom 9 years ago

You achieved full-wave rectification using just the above parts? May we see this remarkable circuit, please?

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Tom Gardner 9 years ago

Half wave rectification is sufficient to show your waveform.

Why do you think full-wave rectification is important? Which

*significant* aspect of your waveform would it change?

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Cursitor Doom 9 years ago

I'd KF'd you previously due to your politics, but it seems your electronics is no better.

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Tim Williams 9 years ago

For the record, you didn't answer his question. I'm genuinely curious now, too.

Tim

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

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John S 9 years ago

Kind of looks like the outline of my ex-wife.

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Cursitor Doom 9 years ago

Do I detect a note of scepticism here, Tim? I've checked all the electrolytics with my ESR/Capacitance tester which is claimed to be able to test for ESR in-circuit so I didn't remove any of the caps in order to test them. I'm relying on the meter manufacturer's claims that it's not necessary to do so and in the absence of any evidence to the contrary, I don't see what more I can do.

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Phil Allison 9 years ago

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** Electro cap ESR meters can be relied on to work normally with the caps in circuit.

Diodes and transformer windings in parallel have no effect.

.... Phil

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