Still working on the AC-3... The drakelist wisdom was the 20k load resistor
didn't present enough of a load, compared to the original 10k resistor, and
that was the reason the HV kept climbing. I ordered a 10k 10w resistor and
it recently arrived and I installed it in place. Note that this resistor
connects the HV+ with the medium voltage B+ (which seems odd). With this new
resistor, the HV now reads a reasonable and respectable 720v (no load).
That's good. But the medium voltage now reads 570v !!! I can't explain it.
What's different about the medium voltage supply is I replaced the two 100
uf caps with 330 uf caps and the 80 uf cap with a 120 uf. That's what I had
in my junk box. Why is that supply so over-voltage now? Perhaps I have a
wiring error, though I carefully checked for errors.
Thanks,
Tom n7tm
On 12/20/06, Tom Taylor [EMAIL PROTECTED] wrote:
I replaced the diodes and capacitors in the early AC-3 w/choke power
supply. I used what I had on hand. The HV caps, originally 350v 80 uf, are
now 450v 120 uf. The 10k HV load resistor was open. I replaced it with what
I had, a 20k 50w resistor. While testing the refurbished supply last
Thursday night, the Northwest was hit with a terrific wind storm and our
power was knocked out at that instant. We're on our eigth day without power
at home (have power and internet here at work). I've posted pictures of the
trees and power lines here:
http://www.advrider.com/forums/showthread.php?t=189994highlight=northwest+wind+storm
Unfortunately, the HV exhibits the behavior you mentioned in your reply.
While measuring the HV on power up, the voltage quickly climbs to 850v, then
continues to climb at a slower rate to over 900v. At 920v (past the voltage
limit of the two series caps), I pulled the plug. Is the fact that the
voltage is climbing so high not under load indicating that my load resistor
is not loading the HV enough? The old value was 10k, the new value is 20k.
With values that high, I figured it wouldn't matter much, but perhaps it
does.
I have a couple comments after converting several AC-3's and AC-4's to use
the AC-4R replacement board:
- the supplies really did change over the years. One example, my newest
AC-4 has a 120/220v switch and a 3 wire plug, but none of the others do
- I'm surprised by the very small gauge wires in the wire bundle supplying
the xmtr. That HV wire is so thinly insulated -- no more so than any of the
other wires.
Has anyone else converted an AC-3 (the non-choke model) to use the AC-4R
board? I mounted the board on standoffs on top of the chassis. I'd hate to
reach around the back of an MS-4 enclosure to adjust the bias pot (which is
on top of the AC-3 chassis) and touch the open AC-4R board. There's some
serious voltage -- and a lot of stored energy with the new caps -- right
there. Has anyone built a little cage around the board? Seems like a lot of
work, but perhaps worth it.
Tom n7tm
On 12/12/06, Gerry [EMAIL PROTECTED] wrote:
What you describe is typical of choke input power supplies. Text books
tell
us that choke input supplies are better at regulation but that is simply
not
true if you have a wide ranging dynamic load. Choke input power supplies
require a minimum load. There are formulas in the old handbooks to
calculate
the value of resistance to draw a proper load current. Without a minimum
load it would be as if no choke existed (except for its DC resistance)
and
the filter capacitors will charge to 1.414 times the value of AC rms. In
other words, if you have 750VAC secondary the capacitors will charge to
over
1,060 volts with no load! With a properly chosen resistor this would be
0.9
times 750 volts or 675 volts. Please note the HV filter capacitors are
only
rated to 350 volts for a total of only 700 volts. So as you can see, the
10k, 20w load resistor on the output of the HV side of the supply is
crucial. That resistor dissipates around 16 watts. Notice that the low
end
is referenced to +250V and not ground. It should be rated for twice the
actual power dissipation or 32 watts. Put it another way, without the
load
resistor, key up voltage would be around 1kV while key down would drop
to
about 675 volts. I should mention that the choke has DC resistance which
may
be a significant contributor to voltage drop. If you draw 450 mils and
your
DC choke has 50 Ohms of DC resistance, you will drop an additional 22.5
volts across the choke. Also look at the +250 volt side and see that the
filter caps are rated to only 300 volts. This is much too close for
comfort
and is really a very slim margin of safety. Typically the voltage rating
of
electrolytics is related to the maximum voltage it can withstand for a
given
numbers of hours at some temperature. Component manufacturers usually
try to
embellish the specifications and say their caps will take 1000 hours of
use
at room temperature. But in actual operation ambient temperature will be
higher than 25 degrees C and they
