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Old 21st Sep 2019, 4:55 am   #21
trobbins
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Default Re: Diode switching noise ?

The reasons that equalising parts aren't needed across series connected diodes in this application are:
- that the diodes are likely to be quite similar in characteristics (by using parts from the same batch).
- their junction temperature can be quite similar (by soldering their leads together with only a short lead length).
- their junction temperature will be relatively low, as it uses a full-wave rectifier not a doubler, and the amp is unlikely to be very powerful with that level of filter capacitance.
- rectification is just of mains frequency.
- diode junction has effectively no reverse recovery (its a B+ supply from a rectified 50Hz waveform and the diodes can be UF4007).
- there are no large periodic transients (relative to PIV).
- only 2-3 diodes are needed in series and have a large safety margin before rated PIV is reached for each, let alone when one may reach the on-set of increased leakage at which time the others soak up more PIV (because any increase in leakage has to pass through all diodes).

The spikes in the scope plot exceed 400mVpp, and unless another measurement is made the Vpp is not known, but it won't be OPT transformer turns ratio reduced from the B+ level, so unless the vintage amp has a bad part, or is poorly constructed, the spike level relative to HV winding voltage is not going to be substantial back at the rectifier.

If one wanted to add equalising resistors to a string of UF4007, they need to be at least 1kV rated (so perhaps 3x 350Vdc resistor types in series for each diode). If one wanted to add equalising capacitance, then even 100pF will swamp the junction capacitance which will be well below 10pF nominal.

1kV diodes can be easily tested with a digital megohmmeter used for insulation resistance testing - but even at 1kVdc a heated up UF4007 or 1N4007 won't register on the 2,000 Meg scale as leakage is below 0.5uA, even though the spec sheet indicates 50uA at 100C (as that datasheet spec hasn't been changed since 1965).

This is not an industrial application with slow stud or hockey-puck diodes and related to large transformers with substantial leakage inductance and transient energy, or requiring 50-odd diodes in series for some electrostatic precipitator.
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Old 21st Sep 2019, 7:02 am   #22
trobbins
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Default Re: Diode switching noise ?

Quote:
Originally Posted by G8HQP Dave View Post
That spike seems to occur around the point where the diode switches on. Very unusual. Spikes usually come from diode switch off.
If the scope probe is mainly sensing a capacitive coupling of rectifier voltage waveform then the duration of diode conduction aligns with the lowest voltage of the waveform (ie. around zero crossing) as that is when dV/dt is lowest (the diode is clamping one winding to a near constant level, and the other winding is passing through its sine peak where again dV/dt is lowest). The spike is then consistent with being at diode turn off.

Below is a scope plot when the probe tip is unconnected (ie. just capacitance coupled waveform), but in the vicinity of the HT secondary winding leads of a B+ power supply using 2 parallel UF4007 diodes with a doubler rectifier in a 80W'ish amp. And like Tritone, I too made some plots back then of the speaker output signal at idle to look at the rectification noise getting through.
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Last edited by trobbins; 21st Sep 2019 at 7:08 am.
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Old 22nd Sep 2019, 8:16 pm   #23
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Default Re: Diode switching noise ?

Hiya Folks, many thank you’s for all of those great detailed in-depth responses from everyone, quite an informative response I may say,

I have included another couple of captures if you want to take a look.

In the first scope capture I have increased the voltage on the Y axis to 100mv to get full view top and bottom of those spikes so here you can see the spikes reaching about 220mv from the 0v line.

In the second capture I have expanded it for more detail.

And finally in the third capture below answers my own question at post #7 silly me, sorry about that folks,

After reading through your detailed replies everyone I’ll order up some components within the next couple of days, re-read this thread in detail again and get this issue sorted out.

Thank you everyone again for your replies, I will take this knowledge away with me and come back in a week or so with a detailed report for the desk.

OK, Cheers.
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