[R-390] Caps

[Bob Camp]

Hi

If you look at line transient data, a 2X line voltage burp is a fairly common / wide pulse event. An 8X burp is a much less likely / narrower spike sort of thing. The 2X ones are common enough / high enough energy that a transient suppressor that triggers below that is headed for an early death. PC's are routinely designed to accommodate narrow 800 V line spikes. 

That all sounds pretty alarming, since we are not talking about putting in caps rated at 8X the supply. 

The key here is the width of the spike and the inherent suppression of the power supply / line filter. You aren't going to get 8X spikes past the transformer inductance and the rest of that stuff. The open question is weather you will see the 2X spikes. 

Better to avoid the 400V parts. Definitely consider major spikes if you are on the line side of the power transformer. 

Bob



On May 1, 2013, at 11:43 PM, Charles P. Steinmetz <charles_steinmetz at lavabit.com> wrote:

> Sam wrote:
> 
>> CDE still makes 3 versions of axial-lead film/foil capacitors. Series WMC, WMF and WPP.
> 
> 400 Vdc is the highest rating available in the WMC line, due to the thinner dielectric used to miniaturize them (and note that 0.001 uF is the only value Mouser stocks).  Although they are cataloged, I have not seen WPPs above 400 V stocked by anybody for years (decades?), and even the lower voltage ones are very scarce.  If you can find some 600/630 V WPPs, they should work fine.
> 
> Some might ask why I think 600 V caps are necessary.  I assume (i) that the entire power supply voltage of a radio could appear across a cap under failure conditions, and (ii) that unforeseen faults (or even foreseen events, like the turn-on surge in radios with solid-state rectifiers) could cause the actual power supply voltage to exceed the nominal value.  In the (unlikely) event that both were to happen simultaneously, a 400 V rating is marginal for most BAs.  Thus, my recommendation to use 600 V capacitors in BAs.
> 
> Best regards,
> 
> Charles
> 
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> 
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