Choke input Power Supply
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 6th November 2002, 04:05 AM #1 NickC diyAudio Member   Join Date: Dec 2001 Location: Earth Choke input Power Supply Normally for tube circuits everyone raves about choke input supply how about trying it for solid state? any this advantage of using that configuration in solid state particularly for class a amps basically i have a 1000va 2* 40vac. I need only 25 vdc. The supply will be regulated hence i need a minimum of 32vdc for regulation. Can I used a choke and then the caps after the diodes. is the calculation of vdc 0.9*vac for this case L=15mh C=144mf current drawn will be from 2 amps to 4 amps
eLarson
diyAudio Member

Join Date: Aug 2002
Location: Gaithersburg, MD
Re: Choke input Power Supply

Quote:
 Originally posted by NickC Normally for tube circuits everyone raves about choke input supply how about trying it for solid state? any this advantage of using that configuration in solid state particularly for class a amps basically i have a 1000va 2* 40vac. I need only 25 vdc. The supply will be regulated hence i need a minimum of 32vdc for regulation. Can I used a choke and then the caps after the diodes. is the calculation of vdc 0.9*vac for this case L=15mh C=144mf current drawn will be from 2 amps to 4 amps
The rule of thumb for choke input supplies that I had seen
(Radio Amateur's Handbook from 1970) stated that the current
draw, and desired voltage would give you a formula for minimum
choke value:

L (henries) >= desired voltage (volts) / minimum current (in mA)

so for your desired voltage of 32 V and a minimum current of
2000 mA, you would need a 16 mH inductor. If you can
guarantee 2133 mA at a minimum, the 15 mH inductor would be
sufficient for your filter to act like a choke-input supply.
Inductance smaller than the minimum would mean that it would
still tend to act like a cap-input filter with a higher peak voltage.

(Unfortunately the ARRL didn't derive their equation, and I've
been too lazy to go back and do it myself.)

Something to be aware of: make sure that the resonance of your
RLC is damped down. The DCR of the choke may be sufficient (it
was for my much lower current preamp supply). If not, add some
small-value resistors in series with the choke.

Hope that helps,
Erik

 6th November 2002, 06:55 PM #3 Nelson Pass   The one and only     Join Date: Mar 2001 You want to know the C value to do this. My approach is to make C as large as reasonable, and then you get use lower inductance values, such as 2 mH
 6th November 2002, 07:51 PM #4 carlmart   diyAudio Member   Join Date: Mar 2001 Location: Brazil [QUOTE]Originally posted by Nelson Pass You want to know the C value to do this. My approach is to make C as large as reasonable, and then you get use lower inductance values, such as 2 mH ...Which in the end means that most valve power supplies ended up using chokes because they could not get higher voltage caps, so they went for the higher choke. BIG. It's confusing, when you look inside some valve amps, as you see so many parts inside looking like transformers, being just chokes. At least we solid state mortals can afford smaller inductors. Carlos
 6th November 2002, 08:41 PM #5 Nelson Pass   The one and only     Join Date: Mar 2001 Correct-a-mundo. Of course, mostly they're being cheap, as the caps don't cost that much, and on top of that there seems to be a (mis)conception that lots of parallel caps are SLOW. Well, maybe in certain ways that's what we want.

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