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Old 6th November 2002, 03:05 AM   #1
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Default 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
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Old 6th November 2002, 01:44 PM   #2
eLarson is offline eLarson  United States
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Default 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
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Old 6th November 2002, 05:55 PM   #3
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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

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Old 6th November 2002, 06:51 PM   #4
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[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
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Old 6th November 2002, 07:41 PM   #5
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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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