[Amps] Resonant choke filter

[R.Measures]

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