Jim Thomson wrote:
> . 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.)
>>
Some commercial amps are moving to Triacs and ramping the voltage up
instead of a "step start" and I understand they are working quite well.
73
Roger (K8RI)
>
> ### 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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