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Minimizing rectification spikes in transformer secondary?
Minimizing rectification spikes in transformer secondary?
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Old 15th February 2020, 01:48 AM   #1
leadbelly is offline leadbelly  Canada
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Default Minimizing rectification spikes in transformer secondary?

I am working on adding a DC supply to an existing AC circuit.

At first blush, a simple bridge of 4 Schottky diodes and a large filter cap will do the trick, maximizing the rectified DC voltage.

Thinking about it some more, I am worried about the effect of rectification spikes on the AC loop, as there is a component there that might be sensitive to spikes.

I would say as a design criteria minimizing rectification spikes on the AC side would be highest priority, with maximizing DC voltage next priority.

Would someone be so kind as to walk me through what basic additional elements would achieve this? For example, diode type, snubber capacitors, inductors, etc. Not wanting a walkthrough of a scope trace, more like a 1st principles, belt and suspenders answer.
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Old 15th February 2020, 01:59 AM   #2
rayma is offline rayma  United States
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Treat the filter capacitor connections as a Kelvin arrangement. The input traces/wires to the pins
go in the opposite direction from the output traces/wires from the pins, and share no traces/wires.
Excessive filter capacitance will increase further the magnitude of the charging current spikes.

Last edited by rayma; 15th February 2020 at 02:02 AM.
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Old 15th February 2020, 02:03 AM   #3
jhofland is offline jhofland  United States
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Minimizing rectification spikes in transformer secondary?
The typical transformer secondary and rectification form a resonant circuit that gets excited when the rectifiers turn off. Mark Johnson et al have several postings on the subject. The usual method of controlling it is a capacitor in parallel with a series RC across the AC side to damp out the resonance. Values I have successfully used is a capacitance of 10nF in parallel with a series RC of about 100nF and 150 to 200 ohms. Mark designed a fixture for using a scope to dial in the resistance value to an optimum number to damp out the response. But something even moderately close stops the ringing in a cycle or two.
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Old 15th February 2020, 08:15 AM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by leadbelly View Post
Would someone be so kind as to walk me through what basic additional elements would achieve this? For example, diode type, snubber capacitors, inductors, etc. Not wanting a walkthrough of a scope trace, more like a 1st principles, belt and suspenders answer.
Inductors or resistors will work, even small ones. Resistors look the simplest, but you probably don't want them, as they turn the excesses into heat.

Inductors spread the charging spike on a longer period, and don't waste heat.

They will reduce the peak voltage slightly, but not like a resistor because the lower peak current is compensated by a longer conduction time.

Different arrangements are possible: common, in the DC output, common in the AC input, individual for each diode or conduction path.

Note that by combining the right topology with the right values of filter cap and inductors, you can even boost the output voltage very slightly, thanks to a pseudo-resonance effect.
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Old 15th February 2020, 04:34 PM   #5
leadbelly is offline leadbelly  Canada
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Many thanks for your detailed replies.
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Old 15th February 2020, 05:48 PM   #6
Mark Johnson is offline Mark Johnson  United States
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Minimizing rectification spikes in transformer secondary?
An inrush current limiter in series with the primary (such as the CL-60 that Nelson Pass includes in his First Watt designs) will reduce spikes AND limit inrush current. If your transformer's primary current is not anywhere close to 5 amperes then you will want a different ICL than the CL-60; the datasheet offers many alternatives.

An inductor connected in series between the diodes and the filter capacitors, will reduce spikes even further. The free software PSUD2 can help you experiment with different inductor values to see what they do to the output voltage waveform and to the diode current waveform.

And a C+RC snubber, connected across the transformer secondary, will eliminate oscillatory ringing.

Last edited by Mark Johnson; 15th February 2020 at 05:51 PM.
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Old 15th February 2020, 05:49 PM   #7
tvi is offline tvi  Australia
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Simple-math-transformer-snubber-using-quasimodo-test-jig
Heaps of info
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Old 16th February 2020, 05:16 AM   #8
trobbins is offline trobbins  Australia
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Leadbelly, have you tried any DC heaters before, and been able to measure any spikes getting in to the audio chain?
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Old 16th February 2020, 06:12 AM   #9
leadbelly is offline leadbelly  Canada
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Leadbelly, have you tried any DC heaters before, and been able to measure any spikes getting in to the audio chain?
This is about as far from DC tube heating as you can get, it's about putting LED lights on my very old dirt bike.

To answer your question, no I haven't, but SY has IIRC, albeit for multiple secondaries, not my case which is single secondary (actually alternator winding).
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Last edited by leadbelly; 16th February 2020 at 06:19 AM.
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Old 16th February 2020, 06:51 AM   #10
trobbins is offline trobbins  Australia
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Ok so it isn't a transformer secondary winding (as would typically be the topic in diyaudio), it's a generator winding. Is it a bicycle generator or motorbike generator?
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