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17th March 2015, 05:25 PM  #1 
diyAudio Member
Join Date: Nov 2010

Filter Choke Selection
Am Setting up power supply for a line stage preamp. I am using a 3500350 CT power tranny, 5ar4 as rectifier and planning on a CLC filter section. Will be using a .47uf 630v as first cap. DCR of choke will be in the 200250 Ohm range. My question is how to calculate for choke size. Circuit B+ will be 300VDC with current draw of approx 47ma.I have looked at several formulas with no luck. I would like a precise formula to go by.
Thanks Cags 
17th March 2015, 06:12 PM  #2  
diyAudio Member
Join Date: Jun 2010
Location: WS, NC

Quote:
20 

17th March 2015, 06:30 PM  #4 
diyAudio Member
Join Date: Nov 2010

Choke Selection
How would I calculate using a .47uf 630v cap ? I was told by using a .22uf or a .47uf cap That I could tweak B+ a little. In any case use any figure you like as a matter of fact and what would be a solid formula to use in determining choke size. That is what I am looking to find out. CHOKE size.
Thanks again Cags 
17th March 2015, 07:01 PM  #5  
diyAudio Member

Quote:
However you could also try a capacitor multiplier supply based on a MOSFET. 

17th March 2015, 07:52 PM  #6 
diyAudio Member
Join Date: Feb 2009
Location: Melbourne, Oz

Use Duncan tools PSUD2  you get to choose what component sizes and ripple you are happy with.

17th March 2015, 08:08 PM  #7 
diyAudio Member
Join Date: Dec 2010
Location: Nottingham, NH

F = 1/(2*PI()*SQRT(L*C))
The smaller the first cap the larger the inductor will have to be. Since you are using a 5AR4 you are limited by how large a cap to use so this means a fairly good size choke, 10H with a 47uF is 7Hz. 
17th March 2015, 09:24 PM  #8 
diyAudio Member
Join Date: Nov 2010

Choke Selection
Famousmockingbird,
Will use formula thanks . Now the 7hz I,m guessing is the resonant frequency. Now the lower the frequency the better circuit stability and a smoother DC ripple. Am I close? Cags 
17th March 2015, 10:05 PM  #9 
diyAudio Member
Join Date: Oct 2008

Your 0.47uF capacitor is fine. It should leave you with a choke input filter that wants a minimum inductance of (350/50  Volts / mA) or 7H. If you don't have at least this then your B+ will climb and you lose the tuning you'd get from the first capacitor, and the filtering will behave differently.
7Hz is also a good frequency to go with when you look at the choke together with the next capacitor. 
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