[ARC5] Back to basics: Capacitor breakdown voltages

[Dennis Monticelli]

Let's not forget that the voltage at the plate of the final can exceed
twice the HV rail under certain load load conditions (i.e. SWR).  This is
true whether the rail is steady (CW) or time varying (AM peaks).  This is
one the big reasons why arcing sometimes occurs in tubes, var caps, and
switches.

Dennis AE6C

On Sun, Jan 24, 2016 at 10:19 PM, WA5CAB--- via ARC5 <arc5 at mailman.qth.net>
wrote:

> 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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