[Test-Equipment] Question regarding film capacitor voltage rating

[Barry]

Hi Brian,

I'll have to try that.  I'll have to construct a suitable voltage divider to safely see that on the scope but that shouldn't be too difficult.  Hopefully a high-impedance divider won't upset the transformer's function and give me incorrect readings.  I'm still concerned that the smaller cap (630V rating) is way under rated but am wondering how it's holding up like it is if that's the case.

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "Brian Clarke" <brianclarke01 at optusnet.com.au>
> To: "Discussion of Electronic Test Equipment" <test-equipment at mailman.qth.net>
> Sent: Monday, June 21, 2021 8:40:41 PM
> Subject: Re: [Test-Equipment] Question regarding film capacitor voltage	rating
> 
> Hello Barry,
> 
> You are right that the value of the capacitor in a ferro-resonant circuit is
> critical. The waveform across that capacitor is anything but sinusoidal. The
> waveform and the Voltage may vary with the secondary load - the amount of
> variation will depend on the amount of magnetic shunting in your specific
> transformer. To estimate the appropriate Voltage rating, use an oscilloscope
> across the capacitor and vary the secondary load.
> 
> Cheers, Brian, VK2GCE
> 
> -----Original Message-----
> From: test-equipment-bounces at mailman.qth.net
> [mailto:test-equipment-bounces at mailman.qth.net] On Behalf Of Barry
> Sent: Tuesday, 22 June 2021 12:43 AM
> To: Discussion of Electronic Test Equipment
> Subject: [Test-Equipment] Question regarding film capacitor voltage rating
> 
> I have been working on SOLA constant voltage transformer and have a question
> regarding the ferroresonant capacitors in it.
> 
> The original capacitor was 0.5uF and rated at 660VAC (as stamped on the
> can).  I replaced it with a 0.47uF 1kV film cap and that got it working but
> the voltage was slight under the specified output of 6VAC.  I discovered
> that I could fine-tune the output with small adjustments to the capacitor
> using parallel capacitors and found that apparently 0.5uF is somewhat of a
> critical value.  Placing a 0.033uF across the 0.47uF got me the output I
> wanted.
> 
> My question is regarding the capacitor's ratings.  The AC value across the
> capacitors is right around 700VAC.  I think that waveform is not exactly
> sinusoidal and I think the meter I was checking that with is not a TRMS so
> there could be some variance there; however, the 6VAC side appears pretty
> close to sinusoidal on my scope so I think the 700VAC side would have the
> same waveform so it's not too distorted and is likely very close to 700VAC.
> The 0.47uF is rated at 1kV (an Illinois Capacitor 474MSR102K) and the
> 0.033uF is also an Illinois Capacitor rated at 630V (I don't have the I/C
> part number handy for that one).
> 
> According to the Illinois Capacitor website, the 474MSR102K (0.47uF) cap is
> a DC rating and I presume the 0.033uF cap is also a DC rating.
> 
> With ~700VAC, what DC rating is safe for those caps?  The transformer works
> just fine but I'm concerned about running one or both of those caps near the
> edges of their ratings.
> 
> Thanks,
> Barry - N4BUQ
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