On Jul 31, 2004, at 1:30 AM, Will Matney wrote:
I had started designing a resonant choke filter for a supply I'm
thinking about building and figured I would share the information
here. I was looking at using a 5 Henry choke by Hammond Mfg. and
needed to figure a capacitor for it. The formula is;
C = 1 / f^2 x 39.4786 x L
f = frequency in Hertz (Full Wave Rect = 120 Hz, 1/2 wave = 60 Hz)
L = inductance in Henrys
39.4786 = 2^2 x pi^2
So after calculating for C, I got 0.00000035 (3.5^-7) or 0.35 uF.
Looking for a suitable capacitor, I needed one above 500 Vdc.
Resonating capacitors are normally 3x the DC output V of the PS.
I figured on using a "metalized polyester" because they go up to
around a 16 Kv rating. I found that in the 2Kv range was the highest
that would have something close in value. 4Kv didn't have close enough
really. Under 2 Kv, there was only a 0.33 uF so I need another to
parallel which I found a 0.02 uf. Both together will give the required
0.35 uF. these caps sell for around $7 to $8 each from Allied Elect.
which is not bad. Keep in mind though that even if the choke
manufacturer says it's a 5 Henry, it may test to be something
different. So, using a scope to test the DC output, adjusting the
value of the capacitor(s) slightly may have to be done. If so, add or
subtract enough C until the ripple goes as low as it can.
Will Matney
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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