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.
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