[Test-Equipment] Question regarding film capacitor voltage rating

[Brian Clarke]

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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