[Amps] big capacitor in HV PS

[Roger]

John Lyles wrote:
> The stored energy in that capacitance at 3.6 KVDC is 8.1 kilojoules. 
> This is a tremendous amount to deal with for a power supply. Roughly 
> equivalent to the mechanical force of a part of a stick of dynamite when 
> it lets loose. I would not want to have to deal with the safety aspects 
> of this design. For instance, if there was an internal arc in the power 
> supply, before the series resistor (mandatory) to the load, it would do 
> a lot of damage, such as rupture capacitors, blow holes in metal. This 
> is just from the stored energy. An active crowbar might be needed if a series resistor 
> isn't enough to prevent more than a few Joules of energy to be dissipated 
> in a thin wire shorted across the power supply output, to simulate a tubes grid wires.
> Then there is the inrush current. Rectifiers would need to be designed for high peak currents, 500 amps or more.
> Step start would need to be carefully designed to prevent taking out the diodes. 
>
>   
Having that much stored energy in a small area just isn't safe if 
something goes wrong.

> Seems like a lot of trouble to get stiffer high voltage that isn't really needed for am amateur
> amplifier for SSB, CW, AM. I would not recommend going above 1/10 of that capacitance, unless prepared
> to deal with the effects mentioned above. I have a 250 uF capacitor at 28,000 volts at work, and 
> it took months to design the inrush and crowbar protection, to prevent blowing a 30 AWG wire when shorted across the output. 
> This bank of capacitance is in a steel vault with chicken-wire reinforced glass in the windows. Of course, this
> is far more stored energy than 8.1 k, about 10X this amount, needed to prevent more than a kilovolt of droop 
> during the pulses of an RF amplifier. 
>   
We used to use 50,000 (50 Kilojoules) MOVs across our power supplies.  
When they went it was spectacular from both the light flash and almost 
unbelievably loud noise. We used #6 wire to them and you'd find the two 
wires sticking straight out and I do mean straight! They were even 
stretched a bit.  If you were working in the cage next to the one 
containing a running PS when that MOV went it was certainly an adrenalin 
rush.<:-))

73

Roger (K8RI)


> 73
> John K5PRO
>
>   
>> I am curious. I see some 10,000uf 450volt electrolytic caps for sale on
>> Ebay. 8 of these would give you 3600volt capability at more or less
>> 1250uF total capacitance. Is this too much?. Are there any dangers using
>> this much C in a filter for a PS? In the past I have used eight 560uF
>> caps and other values similar to this. Just curious, thank you. Lane
>> Ku7i
>>     
>
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