Forums

PSU capacitor choice

Started by Cursitor Doom April 5, 2020
Greetings, gentlemen,

I'm attaching a scan of the rectifier board from an HP8547A network 
analyser. I'm just wondering how the designer(s) arrived at the choice of 
value for the caps across the incoming supply from the mains transformer 
secondary windings (the caps immediately before the bridge rectifiers)? 
In case you can't make it out, the values are 0.047uF, 0.22uF, 0.47uF and 
0.047uF.

https://yandex.com/collections/card/5e89cf18a7623e722fc4fa7d/

Thanks!
Cursitor Doom wrote...
> > I'm attaching a scan of the rectifier board from an HP8547A network > analyser. I'm just wondering how the designer(s) arrived at the choice of > value for the caps across the incoming supply from the mains transformer > secondary windings (the caps immediately before the bridge rectifiers)? > In case you can't make it out, the values are 0.047uF, 0.22uF, 0.47uF and > 0.047uF. > >https://yandex.com/collections/card/5e89cf18a7623e722fc4fa7d/
Those caps are to suppress spikes arising from rectifier-diode reverse-recovery-time current snap-off, and the transformer's leakage inductance. See AoE x-Chapters, 9x.6, for discussion and scope traces. The values aren't critical, and are related to diode current and transformer L_sub-ell. -- Thanks, - Win
On Sun, 05 Apr 2020 06:19:44 -0700, Winfield Hill wrote:

> Those caps are to suppress spikes arising from rectifier-diode > reverse-recovery-time current snap-off, and the transformer's leakage > inductance. See AoE x-Chapters, 9x.6, for discussion and scope traces. > The values aren't critical, and are related to diode current and > transformer L_sub-ell.
I'm most grateful, your Majesty <tugging forelock respectfully here> I should mention the top cap in that diagram failed short circuit, blowing the fuse immediately preceding it. I had a spare cap of exactly the same value and type in my spares bin - and it was 600VDC rated as against the original's 400 so I'm guessing it shouldn't happen again. Fortunately, there's so much real estate available in these old boat anchors you can slot in physically larger replacements that would never fit into the footprint available in today's products. You can't beat vintage electronics.
On 2020-04-05 12:47, Cursitor Doom wrote:
> On Sun, 05 Apr 2020 06:19:44 -0700, Winfield Hill wrote: > >> Those caps are to suppress spikes arising from rectifier-diode >> reverse-recovery-time current snap-off, and the transformer's leakage >> inductance. See AoE x-Chapters, 9x.6, for discussion and scope traces. >> The values aren't critical, and are related to diode current and >> transformer L_sub-ell. > > I'm most grateful, your Majesty <tugging forelock respectfully here>
Pure swank. A lot of us don't have forelocks anymore. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On Sun, 05 Apr 2020 12:48:36 -0400, Phil Hobbs wrote:

> Pure swank. A lot of us don't have forelocks anymore.
It was a virtual forelock, Phil. I'm in the same boat in reality. :-D That 8566B of yours - have you ever had the not uncommon "YTO unlocked" error appear on the screen?
On 2020-04-05 13:03, Cursitor Doom wrote:
> On Sun, 05 Apr 2020 12:48:36 -0400, Phil Hobbs wrote: > >> Pure swank. A lot of us don't have forelocks anymore. > > It was a virtual forelock, Phil. I'm in the same boat in reality. :-D > That 8566B of yours - have you ever had the not uncommon "YTO unlocked" > error appear on the screen? >
On one unit, yes. It went away after a few minutes' warm-up and hasn't returned. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On Sun, 05 Apr 2020 13:08:28 -0400, Phil Hobbs wrote:

> On one unit, yes. It went away after a few minutes' warm-up and hasn't > returned.
Well, don't worry about it. It's generally fixable and there's a fair bit of info on the web about this issue.
Winfield Hill <winfieldhill@yahoo.com> wrote:

> Cursitor Doom wrote... >> >> I'm attaching a scan of the rectifier board from an HP8547A network >> analyser. I'm just wondering how the designer(s) arrived at the choice of >> value for the caps across the incoming supply from the mains transformer >> secondary windings (the caps immediately before the bridge rectifiers)? >> In case you can't make it out, the values are 0.047uF, 0.22uF, 0.47uF and >> 0.047uF. >> >>https://yandex.com/collections/card/5e89cf18a7623e722fc4fa7d/ > > Those caps are to suppress spikes arising > from rectifier-diode reverse-recovery-time > current snap-off, and the transformer's > leakage inductance. See AoE x-Chapters, > 9x.6, for discussion and scope traces. > The values aren't critical, and are related > to diode current and transformer L_sub-ell.
That is a bad idea. Once the spikes are generated, they go everywhere and are hard to filter. Best to stop the noise before it is generated. 0.1uF caps right across the diodes.
Steve Wilson wrote...
> >Winfield Hill <winfieldhill@yahoo.com> wrote: > >> Cursitor Doom wrote... >>> >>> I'm attaching a scan of the rectifier board from an HP8547A network >>> analyser. I'm just wondering how the designer(s) arrived at the choice of >>> value for the caps across the incoming supply from the mains transformer >>> secondary windings (the caps immediately before the bridge rectifiers)? >>> In case you can't make it out, the values are 0.047uF, 0.22uF, 0.47uF and >>> 0.047uF. >>> >>>https://yandex.com/collections/card/5e89cf18a7623e722fc4fa7d/ >> >> Those caps are to suppress spikes arising >> from rectifier-diode reverse-recovery-time >> current snap-off, and the transformer's >> leakage inductance. See AoE x-Chapters, >> 9x.6, for discussion and scope traces. >> The values aren't critical, and are related >> to diode current and transformer L_sub-ell. > > That is a bad idea. Once the spikes are generated, they go > everywhere and are hard to filter. Best to stop the noise > before it is generated. 0.1uF caps right across the diodes.
Given the huge filter caps on the other side of the diodes, the two connection approaches are largely equivalent. Longtime practice by HP engineers and others seems to be caps to ground. Dunno if they had a good reason. Maybe clamping high dV/dt at that node. Both methods are very effective. -- Thanks, - Win
Winfield Hill <winfieldhill@yahoo.com> wrote:

> Steve Wilson wrote... >> >>Winfield Hill <winfieldhill@yahoo.com> wrote: >>
>>>>https://yandex.com/collections/card/5e89cf18a7623e722fc4fa7d/
>>> Those caps are to suppress spikes arising >>> from rectifier-diode reverse-recovery-time >>> current snap-off, and the transformer's >>> leakage inductance. See AoE x-Chapters, 9x.6, for discussion and >>> scope traces. The values aren't critical, and are related to diode >>> current and transformer L_sub-ell.
>> That is a bad idea. Once the spikes are generated, they go >> everywhere and are hard to filter. Best to stop the noise before it is >> generated. 0.1uF caps right across the diodes.
> Given the huge filter caps on the other side of the > diodes, the two connection approaches are largely > equivalent. Longtime practice by HP engineers and > others seems to be caps to ground. Dunno if they > had a good reason. Maybe clamping high dV/dt at > that node. Both methods are very effective.
Output caps are irrelevant. The diodes are fed by transformers, which are inductors and have high impedance to spikes. Caps connected directly across the diodes prevent the snap-off by slowing the discharge current.