PSU capacitor choice

C

Cursitor Doom

Guest
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/
Click to expand...

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.
Click to expand...

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 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?
Click to expand...
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 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
Click to expand...

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 13:08:28 -0400, Phil Hobbs wrote:

On one unit, yes. It went away after a few minutes' warm-up and hasn't
returned.
Click to expand...

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.
Click to expand...

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.
Click to expand...

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.
Click to expand...

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.
 
Steve Wilson wrote...
Output caps are irrelevant. The diodes are fed by transformers,
which are inductors and have high impedance to spikes.
Click to expand...

Output caps? All of HP's diodes were at the
transformer-diode node. They're input caps.


--
Thanks,
- Win
 
On 4/5/2020 9:19 AM, Winfield Hill 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.
Click to expand...

There's a papah:

<http://www.hagtech.com/pdf/snubber.pdf>
 
Winfield Hill <winfieldhill@yahoo.com> wrote:

Steve Wilson wrote...

Output caps are irrelevant. The diodes are fed by transformers, which
are inductors and have high impedance to spikes.

Output caps? All of HP's diodes were at the
transformer-diode node. They're input caps.
Click to expand...

I meant the large caps fed by the diodes. We can call these filter caps.

Connecting the diode bypass caps to ground puts these caps in series with the
diodes and bypass caps. The large caps have internal resistance which reduces
the effectivness of the diode bypass caps.

The object is to slow the current transition through zero and prevent the
spikes.

Recall that step recovery diodes require a sharp transition to generate
harmonics. If the transition is slow, there is no step recovery and no
harmonics.
 
Cursitor Doom wrote:

------------------------
> Greetings, gentlemen,

** You are joking ?

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/
Click to expand...

** The cap values are proportional to the current flow in each case.

Their purpose is to suppress RF noise that might affect the analyser.

You see them is analogue TV sets, scopes and radio gear, but not much else.

Same nobody else answered your ACTUAL question.


...... Phil

...... Phil
 
Steve Wilson <no@spam.com> 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.
Click to expand...

A couple of power supplies shown in _ARRL Handbook_s from back in the
day when dissipative regulators reigned supreme, use 0.01ÎźF capacitors
as snubbers across each individual diode in a bridge.
One 2,000V beast uses a 1000 PIV, 3A diode, a 0.01ÎźF, 1kV capacitor,
and a 390kΊ, 1W resister connected in parallel as a basic building
block. The blocks are then serially stacked six high in each leg of the
bridge. The capacitors suppress transients while the resistors equalize.

Thank you,

--
Don Kuenz KB7RPU
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.
 
On Sun, 05 Apr 2020 19:30:00 +0000, 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.
Click to expand...

(I think) Win's approach was approximating an optimum R value for
appropriate damping to practically eliminate the effect to avoid
having a hard-C load directly on the transformer (which has its
own issues... see below).

Best to stop the noise before it is generated. 0.1uF caps right across
the diodes.
Click to expand...

The diode recovery delay induces the effect, but the spike originates
from the secondary leakage inductance so it makes sense to put the
snubber on the transformer secondary.. when the current abruptly
stops the resulting spike has somewhere to go. When the caps are
placed across the diodes, then the current spike has to travel
through the filter cap then through another cap (if a bridge) to
complete the path. But yeah, caps across the diodes works fine too.
Best to use ceramic caps if not including series R, they can handle
the current and extra voltage from high frequency line transients.

Some experiences I've had with snubbers or lack thereof...

Rivera tube amps usually include a film-type capacitor across
the transformer HV secondary. It almost always blows, sometimes
blowing itself off the PCB and "fixing" itself. I always remove
this cap and replace it with 4 0.01uF 500V ceramic caps placed
across the rectifier diodes. If I bother, never noticed any noise
if the caps are omitted but after reading about the "spike" I now
just add the caps.

I suspect that it's less stress on the caps when placed across
the diodes (the voltage doesn't reverse) than when placed across
the secondary. But more likely ceramic caps just take the heat
better than film caps, especially when there's no series R.

Old Fender tube amps (center-tapped secondary) have no snubber
at all but never noticed any noise issues, even when souped up
with super-high-gain overdrive preamps. Next time I'm at the
shop working on one (when the shop finally reopens! ugh..)
will hang a probe on the secondary to see if there's a spike.
Could be high-voltage transformers have enough winding
capacitance to naturally absorb the spike.
 
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