Dear all,
I wonder why the diode selection is so totally overblown here.
Let's assume a pretty big amplifer, solid legal limit, CCS, which is more than
any ham needs. The power supply might deliver 3500V at 0.8A. Each diode string
in a bridge rectifier will then see a peak voltage that might reach 4000V in the
event of line overvoltage, and an average current of 0.4A at full output. The
current will be very peaky, but rectifier diodes are rated to take that.
The cheapy 1N4007 diode is rated for 1000V reverse voltage, and 1A continuous
average forward current, when mounted in a normal way that will allow conducting
about 1W of heat away, through its leads. Current-wise this diode has all the
safety headroom you might need. Voltage-wise you need 4 of them in series, and
that's it. For a bridge rectifier you need 16 of those diodes, which cost $0.099
each at Digikey, if you buy just those 16. For $1.58 you get all the diodes you
need! Or be generous, use 5 in series in each leg, and spend two bucks on them.
I hear some of you cry "and the voltage transients?" Well, what transients? The
diode bridge sits right across a big capacitor! Any voltage transients will be
clamped to the capacitor voltage.
Of course some of you will now cry "and the current transients?" Sure, any
voltage transients on the power line will translate into current transients when
the filter capacitor clamps the voltage. In addition there will be a big inrush
current at switch-on, if no step start or slow start circuit is used. So there
will indeed be some current transients. But how large can they be? Small
transformers have so much resistance in their wire, that just the resistance
would limit the short circuit current to about ten times the nominal load
current. Larger transformers like those used in legal limit amps have relatively
smaller winding resistances, so that the resistance might limit the current to
20 times the nominal value. But then there is leakage inductance, which also
reduces the current transients. So I would not expect such a transformer to
produce an inrush current stronger than 10 times the nominal current, even
assuming a zero impedance power line, which none of us has! So, the inrush
current with no step start circuit might be around 11A, considering that a
transformer for 0.8A output from a capacitor-input filter needs to be rated for
about 1.1A.
And a cheapy 1N4007 has a 35A inrush current rating. More than three times what
we need.
And how much transient current could we see during lightning? This is harder to
tell, because such fast, extremely strong hits are hugely attenuated by line
impedance, and clamped by all sorts of electronic devices in the homes, so that
the current spike resulting in your amp's rectifier diodes depends a lot on
those impedance values. But if such a spike exceeds 35A, it would have to exceed
roughly 350A on the 240V power line feeding the amp, which means that in a
typical home it would need to far exceed a kiloampere at the service input. The
voltage at that point would need to be VERY high, causing flashovers everywhere
and thus limiting the transient current into the home. I would expect the final
transient in the amp's rectifier diodes to remain well below the 35A rating of
1N4007 diodes. You might see your house catch fire from the flash-overs and the
molten wire, before those diodes blow up.
Of course everyone is free to use 1N5408 or 6A10 diodes, at $0.278 and $0.368
respectively at Digikey. And if somebody wants to use 10 in series in each leg,
who am I to forbid that? But technically it's pointless. Strings of 4 to 5
1N4007 diodes in each leg, properly mounted for decent heat sinking, are
perfectly safe and sound.
In my National NCL2000 I use 1N5408 diodes, but that amplifier has a voltage
doubler, so the diodes work at twice the current, compared to a bridge
rectifier. While 1N4007 diodes would still have been operating within ratings,
the current headroom would have been rather tight. So I chose the bigger diodes.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|