When building or rebuilding HV power supply rectifier banks, I try to match
the diodes to the circuit requirements. Fifty years ago my HV rectifier
banks were an ugly snarl of diodes, resistors and capacitors, with each
diode "protected" against problems that could theoretically be caused by
sample-to-sample manufacturing variations, or to a failure of the protection
component itself. Now manufacturing tolerances have improved, and for the
past twenty years, I've used nothing across each individual diode and have
never had a failure. A few years ago, I ran a test of a half dozen series
connected 1N4007s, with 1ADC running through them continuously for a week,
and had no failures. They got pretty warm, though.
I think one can "oversize" diodes, as well as overprotecting them. Here I'm
referring to the use of 1N5408s, or big fat 6A10s, (or, worse, ultra-fast
UF1007) when the circuit merely calls for common-as-dirt 1N4007 rectifiers.
A lot of PCBs have been damaged by trying to stuff oversized diodes into PCB
holes not designed for them. Also, 6A10s are stiff and hard to bend, and can
exert a lot of stress on adjacent fragile components.
Lately, I've violated my own design principles by using 1N5554s. I'd never
heard of these tiny beauties until I bought a bunch being surplused by Los
Alamos Nat. Lab, which is a few miles from my home. The hermetically sealed,
radiation-resistant, mil-spec, glass versions are rated at 1000V/5A and are
about the size of a 1N4007 or 1/4W resistor. The data sheet says "This
standard recovery rectifier diode is military-qualified and is ideal for
high-reliability applications where a failure cannot be tolerated." Coming
from Los Alamos, that probably means they're used in hydrogen bombs, which
is why they cost $10 each. Los Alamos no doubt surplused them (for $.25/ea)
because they managed to find some diodes that were even more expensive.
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
> Sent: Sunday, July 26, 2020 10:48 AM
> To: amps@contesting.com
> Subject: [Amps] Caps,snubber, mov across diodes ?
>
> Date: Sat, 25 Jul 2020 23:42:54 -0700
> From: "Jim Thomson" <jim.thom@telus.net>
> To: <amps@contesting.com>
> Subject: [Amps] Caps,snubber, mov across diodes ?
>
> ## I found this on wickipedia, under ..diode bridge. Im guessing the
leakage inductance
> and parasitic capacitance they refer to... will change depending if the
plate xfmr is a step
> down..or step up type.
> Big difference between a 10 vac sec...and a 2000 vac sec.
>
> ## How do I look for these..oscillations ? I do have a battery operated
fluke scope
> meter..and a fluke dvm..and also hv probe. Where do I measure ? Across
each diode...or
> B+ to B-..... or B+ to chassis, or
> some place else ?
>
> Jim VE7RF
>
>
> Snubber circuits[edit]
> Power-supply transformers have leakage inductance and parasitic
capacitance. When the
> diodes in a bridge rectifier switch OFF, these "non-ideal" elements form a
resonant circuit,
> which can oscillate at high frequency. This high-frequency oscillation can
then couple into
> the rest of the circuitry. Snubber circuits are used in an attempt to
mitigate this problem. A
> snubber circuit consists of either a very small capacitor or series
capacitor and resistor across
> a diode.
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