[R-390] Power supply capacitors

[Renée Deeter]

I think you will find about 680uf will do the job nicely.
also why not just use the LM317 as  a series current regulator set at 
300 ma.......
Renée

kenny wrote:
> Gord Hayward wrote:
>>
>> It depends on the voltage droop (ripple) that you can tolerate.  
>> 16,666 uF will drop 1 volt in 1/60 sec at 1 amp (half wave 
>> rectification)
>> and 8,333 uF will drop 1 volt in 1/120 sec at 1 amp (full wave 
>> rectification).  You want the smallest capacitor that will give the 
>> minimum
>> voltage for the regulator to work.  A bigger capacitor gives a higher 
>> minimum voltage (less ripple) but the extra energy just shows up as
>> heat in the regulator.  Bigger capacity also increases the inrush 
>> current - the diodes only conduct for a short time so the instantaneous
>> transformer current  is a lot larger than the average DC load current.
>>
>> Cheers, Gord (VE3EOS)
>>
> That's really interesting what you say Gord about the extra energy 
> showing itself as heat in the regulator. That didn't even occur to me. 
> My particular circuit uses a 2200uF and LM317 (among other things) to 
> produce a stable and regulated 12.6 volts for the filaments of the BFO 
> and PTO tubes and the regulator DOES run hot...even with a good sized 
> heatsink it will heat up the heatsink quite nicely. I had a nagging 
> feeling 2200uF was too big....so I'm going to start playing with it 
> until I feel I have a good balance between acceptable performance and 
> long term reliability.
>
> Regards, Ken (ZL3AA)
>
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