From: "Gerry" 
Subject: RE: [drakelist] AC-4 Cap values


"Gerry"  made an utterence to the
drakelist gang
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Unfortunately, inserting series resistors in the B+ leads will, =
depending on
the value, adversely affect regulation. It may work fine for static =
loads
such as the +250 supply but is not advisable for HV supplies with =
dynamic
loads. Years ago I tried a home brew power supply constructed with mil
surplus components. What I found is that choke input supplies need a
critical value resistive load to maintain the voltage below allowable
limits. With no load on the +650 volts, it went almost to the rating of =
the
filter caps which I believe was 800 volts. Trying another approach with =
a
capacitive input filter and a choke meant I had to find another =
transformer.
When I did, the 50 Ohms resistance of the choke became somewhat of a =
problem
which all made sense later. At 450 mils, that would be a 22.5 volt drop =
on
top of whatever the drop was in the first place. The thump comes from =
the
transformer and is loudest when power is switched on at the peak of the
primary AC cycle. Sometimes you can hit it just right. When I built my
Heathkit SB-1000 linear, there was an alignment procedure which called =
for
removing the screws and sliding the case back about an inch or so. This =
was
done per the assembly instructions to align the input coils. When I =
turned
on the amp for the first time, there was a loud KABAM! After changing my
underwear, it became obvious what happened: there was a surge at turn on =
and
the steel cover acted like a sounding board. Nothing bad happened to the
amp, it was just one of those things that makes you tremble whenever you
reach for the ON switch. This brings up another point about the need for
slow-start on such devices. I don't use the linear often but it is on my
to-do list.

=20

-----Original Message-----
From: owner-drakelist{at}www.zerobeat.net
[mailto:owner-drakelist{at}www.zerobeat.net] On Behalf Of k5vkl{at}juno.com
Sent: Thursday, March 03, 2005 2:54 AM
To: drakelist{at}www.zerobeat.net
Subject: Re: [drakelist] AC-4 Cap values

=20

Carl,

=20

Now might be a good time to review some slow-start feature design =
reviews,

looking forward to incorporating one into the turn-on function of the =
power

supply.  When you have a "thump" from turning on a power supply, it=20

generally is indicative of too great of a surge current going into the
filter=20

capacitors and stressing them unnecessarily.   It doesn't help the
rectifiers,

either.  Also, I'm not sure that it is a "leaky" capacitor that causes =
the

"thump" you heard at power supply turn-on....I've always been led to=20

believe that the "thump" was from the large flow of current into an

uncharged capacitor, although I've never had it proven to me that this

was the actual cause.

You could, however, benefit the power supply considerably, by inserting =
a

resistor of the proper resistance and wattage, into the 650 volt output, =


between D2 and the connection to R1.  This would also serve to bring =
your=20

650 volt HV (now approx. 700 volts, as you indicated) back down nearer =
to=20

the correct 650 volt value.

Your could also do the similar action with a resistor added between D5 =
and

the connection to C3, and also reduce the 290 volts down to the original

design value of 250 volts.  =20

Possibly, the original design of this power was slighted somewhat, by =
not=20

including a choke in each of the two points I noted above.   That would=20

have produced a significant reduction in the capacitor surge current at=20

turn-on.   Chokes would also have been more effective in reducing the=20

ripple you originally saw on the outputs.  =20

I'm naturally assuming, of course, that you do intend to reduce power=20

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