[Amps] resistor b4 the HV filter-part 2

[Jim Thomson]

 . 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
>