. Here is the counterargument. First, there are two benefits of the resistor;
it
> limits the inrush current when the HV supply is switched on, since without it
> the rectifiers would see a dead short from the uncharged capacitor. (Of
> course a step-start circuit in the xfmr primary is another way of limiting the
> inrush current.)
### a resistor + timer module + contactor works better, for step start.
## I cooked up a 2nd method, using a small variac, and cam operated
microswitch + relay. Variac brings the B+ up to max, and then is effectively
switched out of the circuit.[2nd cam and microswitch #2] The problem with
the standard step start resistor, in some applications is... IF the magnetizing
current on the xfmr pri is high, you will still get a V drop across the step
start R.
You never end up with the full 240 vac applied to the pri. You end up with
a sec surge. The cam is just a safety feature... in case you forget to turn
the
variac down.. when finished for the night. Variac has to be rotated fully
CCW,
so cam [on variac shaft] engages the micro-switch, which activates a relay.
Relay contacts turn on main DPST contactor. In this case, no step start
SPST contactor / relay required.
Second, it protects the transformer and rectifiers
> from a shorted filter capacitor. Many amps use banks of electrolytics for the
> HV capacitor and a short circuit in one can lead to a cascade of failure
> along the entire bank.
## and a resistor won't provide any protection at all, from that failure mode.
I use enough caps in series such that B+ is no more than 75% of the entire
strings ratings.
> The second point is that the effect of the resistor on power supply
> voltage regulation is negligible, On PEP peaks, the current is supplied by
> the
> charged filter capacitor, which is not limited by the resistor.
### IMO, the typ value of filter caps used, will not supply enough
energy. If the total uf is increased, you can kill 3 x birds with one stone.
Lower ripple, better dynamic regulation, and slightly improved static
regulation.
. A 25 ohm resistor in front of a 50uF capacitor has a time constant of only
1.25 mSec, which is much faster than the pulsed DC from the diode rectifiers
that replenish charge on the filter capacitor.
## Model it on PSUD. Then temp short out the 25 ohm resistor, then
re-test.
later........... Jim VE7RF
>
> 73,
>
> Jim Garland W8ZR
>
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