[Milsurplus] WTB Manuals and Power Supply Question

Cletus W Whitaker whitaker at pa.net
Tue Sep 28 07:11:06 EDT 2004


de WB2CPN           2004.09.28.1100Z

The power supply load resistor is a safety bleeder,
but its primary purpose is to put a load on the power
supply in excess of what the receiver or transmitter
draws.  That stabilizes the power supply, particularly
those with capacitor input.  A rule of thumb was the
bleeder took at least 10 percent, more if feasible.  Where
safety was a concern the resistor across the capacitor
could be of a much higher value, but I forgot what the
time constant was supposed to be.  Only an idiot trusted
a bleeder to make a HV capacitor safe.  That's what
shorting sticks, a.k.a screwdrivers, were for.
Although AM transmitters needed to support a varying
demand for current, SSB was so much more so that the
size of the output capacitors got to be more important
than the size of the bleeder resistor.  If you want to
play you can put a scope on the output of the power
supply and look at the 120 ripple. Then vary the value
of the bleeder and watch what happens.  (Many times the
bleeder in receivers was used to develop bias.)

73  Clete in South Central Pennsylvania






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