[Amps] Other HV rectifier diodes

[Manfred Mornhinweg]

Lee,

> If you have a 1000 volt power supply, what should the diode stack
> voltage rating be?

In principle, just a little more than those 1000V should be enough, as 
long as it's a bridge rectifier. That is because with a bridge there can 
never be higher voltage across any of the four diode groups, than the 
voltage on the filter capacitor. So, basically the diode voltage 
required is the peak voltage at the highest line voltage expected, times 
the transformer ratio, plus some safety margin. And the output filter 
cap will charge to that same level (without the safety margin, of 
course!), under those conditions.

But since diodes are cheap, most people use more diodes.

> I have heard it is good practice to make it 4
> times the voltage of the operating voltage. 

That can't hurt, and won't break the bank, but looks rather generous to 
me. In my NCL-2000 I used 5 1N5408 diodes in each string. That one works 
with about 2500V. So I have only 2 times the output voltage in the 
diodes, and that has worked well. This amp uses a voltage doubler, which 
poses twice the current load on the diodes, but the same voltage as a 
bridge rectifier.

If using a two-diode full wave rectifier with a center-tapped 
transformer (obsolete nowadays), I would use a higher diode string 
voltage, because due to the imperfect coupling between windings, 
transients on the diodes might exceed the filter cap voltage.

> Also, would there be a difference when a choke input is
> used VS capacitive input?

Yes. With a choke between the diodes and the capacitor, the clamping 
effect of the cap is lost, and any transients on the power line will 
appear in full across the diodes, as long as they are long enough not to 
be absorbed by the transformer's loss at higher frequencies. In this 
case, some sort of transient protection should be added - MOVs, Transil 
diodes or Tranzorbs. And enough diodes should be used to safely survive 
the highest voltage your protective devices might allow to show up, plus 
a healthy safety margin.

Now if you use beefy enough avalanche-rated diodes, the diodes 
themselves can assume this transient clamping function! A lot depends on 
the size of any transients you have to expect. Whether it's just the 
transients created by the fridge switching off, or that of a big 
industrial motor powered from a long line. And if you are expecting 
lightning hits in your supply line, better forget protection, and unplug 
the amplifier (and everything else) before the thunderstorm starts!

I remember a funny problem at the job: The circuit breaker feeding a 
rack full of control equipment was tripping, very regularly, once every 
half hour or so. The current actually consumed by the equipment was just 
one tenth of the breaker's rating. The guy in charge was scratching his 
head. I was asked to help him fix it. I also scratched my head. The 
breaker would trip out of the blue, after roughly half an hour. And 
again. And again.
After lots of hours observing, measuring, guessing, finally we 
discovered the problem: There was a rectifier/filter connected across 
the line, to provide power to a system that was off most of the time. 
There was no bleeder resistor, so there was zero current consumption. 
That 450V rated filter cap was found to start at about 320V, and over 
the course of minutes the voltage would grow, and grow, and grow, 
reaching about 600V after half an hour. At that point it arced over 
internally, almost killed us with the sudden scare (it sounded like a 
gunshot), and at the same time the breaker tripped!
This capacitor must have been slowly charging way beyond nominal 
voltage, just from little high voltage spikes periodically appearing on 
the line. A 220 kiloohm bleeder resistor across that capacitor fixed the 
problem. The capacitor had apparently not been damaged by the arcing and 
current pulses, but just to be safe, we replaced it and the diodes 
anyway. Neither my colleague nor I liked to be phoned out of bed at 3 AM 
to fix a sudden problem caused by an abused and degraded component.

Lesson: Don't forget your bleeders, if there is no other current drain!

Manfred

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