[R-390] Fan Power Supply Voltage and RTTY Question

[Bill Hawkins]

A 20 mfd cap with a load of 0.1 amp will drop 5000 volts per
second or 40 volts per half cycle of the line. It might as
well not be there. So yes, 9 VDC is plausible.

A 1000 mfd cap will drop about 1 volt per half cycle.
OTOH, 9 volts causes the fan to run quietly.

Regards,
Bill Hawkins

-----Original Message-----
From: Scott, Barry (Clyde B)
Sent: Monday, March 04, 2002 9:25 AM

Power Supply question:
I built a simple power supply to drive a cooling fan for my R390A.  I used
an 18VCT in a "double half wave" rectifier configuration (the same way the
R390A HV power supply is done).  I slapped a cap across the output (20mfd as
I recall) and it was delivering a clean 14VDC no-load voltage on the scope.
Fine, I thought.  Good enough to drive some small fans.

Well, when I connect the fan, the voltage drops to about 9 volts (according
to the DVM).  The fan is drawing about 120ma.  What causes the sudden
voltage drop?  I need to look at the voltage on the scope, but I suspect the
cap is discharged through the load between half cycles and the DVM "sees"
9VDC when, in fact, it is a rippled DC signal.  Does this sound plausible?