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Math Formula to calculate DC power supply capacitor value?

gary350

Well-Known Member
Thread starter #1
Is there a math formula to calculate the correct size of a DC power supply capacitor?

Or is it easier to keep making the capacitor larger until AC ripple is gone?

What is best circuit drawing some have capacitors only & some have a choke coil too?

I read bleeder resistor should put 10% load on the power supply? This seems like a lot of wasted power.

I usually use my AC volt meter to check AC ripple on a DC power supply if 20 DC checks 1 volt AC I know it needs more capacitors. I know this needs to be tested under full load.
 
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gary350

Well-Known Member
Thread starter #4
I found this circuit drawing online I can link it here as an example.

Using a 22 VDC power supply I already have as an example it can produce 75 amps but the circuit never loads the power supply more than 50 amps.

50 amps x 1000 uf = 50,000. uf

How to calculate 10% load resistor often circuit only pulls 2 to 20 amps some times as much as 35 amps.

I don't know how to calculate choke coil value if one is used? What is L with 2 dots Rx mean?

If I use choke does 1/2 the capacitors need to be on each side of choke?

 
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audioguru

Well-Known Member
Most Helpful Member
#7
What is the load that needs "ripple that is gone"?
A half decent audio amplifier (LM3886) has a Power Supply Rejection Ratio of 120dB so the output hum is one millionth the supply ripple.
 

JonSea

Well-Known Member
#8
If I recall correctly, the ARRL Handbook covers DC supply topology and the values of filter caps needed. At least it did years ago. It's a pretty good general reference.
 

JimB

Super Moderator
Most Helpful Member
#9
Is there a math formula to calculate the correct size of a DC power supply capacitor? Yes

Or is it easier to keep making the capacitor larger until AC ripple is gone? Ugh! No

I read bleeder resistor should put 10% load on the power supply? This seems like a lot of wasted power. It is. The value of the bleeder resistor would depend on how quickly you need to discharge the capacitors once the power supply is turned off.
until AC ripple is gone?
Define "gone".
From a purely theoretical point of view, you will never remove all the ripple with a simple (inductor) capacitor smoothed PSU.

This is why most PSUs use electronic regulation of some kind.

Consider a simple 12v PSU which uses a 7812 regulator IC.
As long as the input voltage to the 7812 is above 14v, the output will be a nice smooth 12v DC with the noise and ripple attenuated to a very low level, regardless of how much ripple is on the input of the IC, as long as the troughs of the ripple do not go below 14v.

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#10
I found this circuit drawing online I can link it here as an example.

Using a 22 VDC power supply I already have as an example it can produce 75 amps but the circuit never loads the power supply more than 50 amps.

50 amps x 1000 uf = 50,000. uf

How to calculate 10% load resistor often circuit only pulls 2 to 20 amps some times as much as 35 amps.

I don't know how to calculate choke coil value if one is used? What is L with 2 dots Rx mean?
If you use a choke it's normally for valve amplifiers, where voltage is high and current is low - the choke is useful then because you can't get high value electrolytics for such high voltages, and they would be HUGE if you could. The opposite is true for high current low voltage, where you can get high value capacitors, and at a decent size - and the choke would be HUGE, VERY expensive (if you could find even one?), and massively heavy.

You need to specify EXACTLY what you're trying to do, and what you're trying to feed - it's unlikely that something requiring 75A is going to be concerned about a little ripple.

As far as bleed resistors go, why do you think you need one?, and what do you expect it to do? - and I've no idea where you got the idea you need 10% of the current.
 

gary350

Well-Known Member
Thread starter #11
I am using a 22 vdc power supply that I already have as a learning example and I want to build other projects.

I have a 70 watt per channel stereo amplifier project that needs to be finished. It needs duel input 24 vdc - 0 - 24 vdc. I have a transformer with center tap it will need 2 full wave rectifiers plus 2 pi filters to get duel 24 vdc.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#12
I am using a 22 vdc power supply that I already have as a learning example and I want to build other projects.

I have a 70 watt per channel stereo amplifier project that needs to be finished. It needs duel input 24 vdc - 0 - 24 vdc. I have a transformer with center tap it will need 2 full wave rectifiers plus 2 pi filters to get duel 24 vdc.
Why do you imagine it would need 'PI filters', have you EVER seen them used in anything other than valve amplifiers?.
 

gary350

Well-Known Member
Thread starter #16
I found this formula in my college text book. Teacher skipped over the power supply chapter for some reason.

%Eripple = Eripple / Edc x 100

Book says, 1% ripple is exceptable

Shunt capacitor between plus & minus filters the voltage ripples.

Choke coil in series filters the current ripples.

Resistor in series instead of choke filters current ripples, it is cheaper, smaller, lighter weight.

L filter is most common, 2 L filters in series for better filter.
 

ChrisP58

Well-Known Member
#18
I found this formula in my college text book. Teacher skipped over the power supply chapter for some reason.
Either he didn't understand it well, or didn't want to take the time to go over all of the different conditional what ifs of good power supply design.

Book says, 1% ripple is exceptable
For what type of supply and under what conditions?

Resistor in series instead of choke filters current ripples, it is cheaper, smaller, lighter weight.
And ridiculously inefficient. Any resistance in the current path makes heat as per I^2*R

L filter is most common, 2 L filters in series for better filter.
Again, in supplies for what type of load equipment?

For reference, I have never seen a choke coil (or a resistor) in the DC power path of a linear audio power supply.

See correction in post #20 below
 
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