[Amps] Resonant choke filter

[Will Matney]

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. 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