[Amps] Rectifier equilization?

[N1BUG]

This brings up something I have been wondering about. I agree the 
maximum surge current would be limited by resistance in transformer 
windings and primary wiring. But I believe the surge current rating 
on diodes is for a very short period of time. My memory may be wrong 
but I think it might be one half cycle at 60 Hz? Surely one half 
cycle won't fully charge the filter capacitor(s). How many half 
cycles would required to fully charge them? My guess is that the 
surge at each half cycle would be somewhat lower as the capacitor 
charges, but would take several cycles to taper to a steady state 
value.

Why is the diode surge rating for a single half cycle? If it can 
handle that surge once, why not more than once? Does heat become the 
limiting factor after only one or two half cycle surges? What would 
the diode surge handling capability be on the second... third... 
etc. half cycle? What would be the point at which this repetitive 
but decaying surge would harm the diode?

BTW, my 4000 volt supply has a step start that slowly ramps the 
voltage to about 3000 volts over 4 to 5 seconds before the step 
start relay actuates. There still must be a considerable surge to 
finish charging the 32 uF oil cap, because when that step start 
relay pulls in the house lights flicker!

Note: my step start is controlled by a ramped DC voltage. The relay 
coil has no voltage applied until the DC control voltage ramps up to 
a pre-determined point. When that rising DC voltage reaches the 
pre-determined trip point, the control circuit momentarily applies 
150% of rated coil voltage to the relay. This causes it to pull in 
quickly and cleanly, which may not be said of some step start 
circuits I have seen. This may (or may not) have some influence on 
the surge taking the cap from 3000 to 4000 volts. After the relay 
has pulled in its coil voltage is reduced to 50% of rated, which is 
more than enough to keep it solidly closed while limiting heating of 
the somewhat large coil.

73,
Paul N1BUG



On 09/14/2013 03:14 PM, Manfred Mornhinweg wrote:
> I don't mean to say anything against 6A10 diodes, they sure work fine.
> But they are HUGE overkill. Even if the filter capacitors had zero
> equivalent series resistance, and the power outlet where the amp is
> connected had zero impedance, and there were absolutely no inrush
> limiting system used, and the power were switched on in just the worst
> possible moment (peak of waveform), just the resistance of the HV
> transformer will limit the current to roughly 20 to 30A, or maybe 40A if
> the transformer is really beefy. And such a condition of course doesn't
> happen in real life - specially the line from which the amplifier is
> powered does have some resistance and also some inductance. In practical
> use, in a normal home, it would be really rare to see more than 15A
> inrush current.
>
> 1N5408 diode has a 200A surge rating, which is still big overkill. The
> humble 1N4007 with its 30A surge rating would be about the right part to
> choose for a cost-conscious builder! It's also comfortable with the
> average current. Rated at 1 ampere average, a bridge rectifier using
> strings of 1N4007 diodes is good for 2 amperes. That gives enough
> headroom for a legal limit amp.