[ARC5] Back to basics: Capacitor breakdown voltages

[WA5CAB at cs.com]

My calculation is that an RF amp with 1100.000 VDC on the plates operating 
Class C and 100% modulated will have 1905.256 volts peak across it.  That 
comes from SQRT(2) * Vdc = Vpeak when not modulated.  Then that times 
SQRT(1.5)  at the modulation peaks if never over modulated.  So a 2500 volt rating 
on the replacement capacitors would probably be adequate but 2000 would 
definitely be marginal.  All calculations to 8 places and rounded to three.  And 
obviously assumes no ripple on the 1100.000.

By the way, 1100 VDC is a tad high from a BD-77 or PE-73.  They are rated 
at 1000 VDC at 28 VDC and 0.350 ADC load current.

In a message dated 01/24/2016 23:25:07 PM Central Standard Time, 
1oldlens1 at ix.netcom.com writes: 
>     How much voltage actually exists at this point?  Make sure to include 
> ripple voltage.  In general the voltages add, i.e. 2 1KV caps in series 
> equal a 2KV cap. 
>     The 2KV caps may be sufficient if the voltage with ripple is well 
> below this. 
>     Usually, equalizing resistors are needed only for electrolytic caps, 
> not paper or plastic film caps. 
> 
> On 1/24/2016 6:14 PM, Jim Falls wrote:
> 
> >> OK, I can calculate series/parallel combos, but how does the breakdown 
>> voltage work? 
>> 
>> My BC -375 has a leaky trio of 1uF oil filled bypass caps that have 3kV 
>> ratings. Mouser has pricey ($20) 1uF and .47uF units rated at 2kV. How does 
>> that breakdown work?
>> 
>> One shorted, and the plate current meter went up like a sparkler. Other 2 
>> are making my PSU lug down, B+ dropping to 900 from the usual 1100 VDC it 
>> idles at. I get 800 while transmitting instead of 1kV like it usually 
>> does. 
>> 
>> Cheers!
>> Jim K6FWT
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>>  
> -- 
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
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