# Allowable Ripple Voltage

Thread Starter

#### gargrahul277

Joined Apr 5, 2016
15
Hi all : )

Im designing a 15 V DC regulated supply.
Im calculating filter capacitor value for it using following formula,
C= (0.7*I)/(100*Vr)
where, I is load current
Vr is ripple voltage
0.7 because capacitor supplies the current for 70% of cycle

i want to know how much ripple voltage should i take for my calculation ??

Last edited:

#### hp1729

Joined Nov 23, 2015
2,304
Hi all : )

Im designing a 15 V DC regulated supply.
Im calculating filter capacitor value for it using following formula,
C= (0.7*I)/(100*Vr)
where, I is load current
Vr is ripple voltage
0.7 because capacitor supplies the current for 70% of cycle

i want to know how much ripple voltage should i take for my calculation ??
You are looking for a precise calculation for a reservoir capacitor that probably has a -20%/+50% tolerance? Figure 1,000 uF per Amp. The regulator will smooth it out.
(I know. I'm a barbarian.)

Thread Starter

#### gargrahul277

Joined Apr 5, 2016
15
i want to know standard value of ripple voltage that one takes to calculate capacitor filter value. i have read that there is trade-off between capacitor value and ripple value. u cannot keep capacitor value high to minimise ripple value because large capacitors get heated quickly.
So one always keeps some ripple value to lower the capacitor value.

#### wayneh

Joined Sep 9, 2010
17,493
I want to know how much ripple voltage should i take for my calculation ??
That's specified by you the designer. Some applications require no filtering, some require more.

That cookie-cutter formula is imprecise, by the way, because it includes a number of assumptions to arrive at such a simple formula. It's probably fine if you're shooting for, say, a 5% ripple.

#### #12

Joined Nov 30, 2010
18,224
So much depends on what you are not telling us. The amount of voltage between the peak charge of the capacitor and the minimum required by the regulator is where I work this problem. We don't know the peak voltage, the type of regulator, the output voltage needed, the amount of current...and you want the answer.

Thread Starter

#### gargrahul277

Joined Apr 5, 2016
15
sorry for providing less inputs

i want 15V output so im using LM 7815.
maximum load current 2A.
i want to keep the minimum input voltage for regulator at 22 V.
maximum input voltage for regulator (hence ripple voltage) is what i want to know.

#### #12

Joined Nov 30, 2010
18,224
That still doesn't make sense. You want to keep the minimum voltage applied to the regulator at 22 volts and you want to know the maximum input voltage for the regulator.
The maximum voltage the chip is rated for is 40 volts. That's 18 volts of ripple.

Thread Starter

#### gargrahul277

Joined Apr 5, 2016
15
yes
i know the maximum input voltage for chip is rated at 40 volts.
i was expecting the ripple voltage from capacitor.

i guess with 5%ripple i can take Vr(Ripple voltage) as 1V.

#### crutschow

Joined Mar 14, 2008
33,969
Worst case you want to keep the ripple voltage below the maximum voltage the 7812 can tolerate and above the minimum it needs to regulate.
Anything in between is fine.

#### #12

Joined Nov 30, 2010
18,224
What is the peak voltage at the filter capacitor?
The 7815 chip needs about 3 volts of excess voltage.
Calculate your ripple to be the maximum capacitor voltage minus 18 volts.

#### kubeek

Joined Sep 20, 2005
5,793
sorry for providing less inputs

i want 15V output so im using LM 7815.
maximum load current 2A.
i want to keep the minimum input voltage for regulator at 22 V.
maximum input voltage for regulator (hence ripple voltage) is what i want to know.
Why the min of 22V? Even if you have zero ripple, you will be losing 14W in heat, which will be a problem even with a pretty large heatsink.

#### wayneh

Joined Sep 9, 2010
17,493
We still don't know what the input is, either. I think we're all assuming full-wave bridge rectified AC, but the TS has not said that.

#### hp1729

Joined Nov 23, 2015
2,304
i want to know standard value of ripple voltage that one takes to calculate capacitor filter value. i have read that there is trade-off between capacitor value and ripple value. u cannot keep capacitor value high to minimise ripple value because large capacitors get heated quickly.
So one always keeps some ripple value to lower the capacitor value.
Correct, for what it is worth. Let me know what your calculations come to.

#### recklessrog

Joined May 23, 2013
985
Depends too on the quality of the capacitor, last year I repaired a regulated wall wart stabilised psu for for a vocal effect unit. The ESR of the reservoir/smoothing capacitor had risen to the point that the ripple trough fell below the drop out voltage of the regulator at half the rated output current. A nice new low esr capacitor rated to work to a higher temperature has given it a new lease of life. One thing that I have noticed is that there seems to be a poor understanding of what a regulator is being used for. If you need 5v at 1amp, you are going to need a big heatsink and fan if you feed it with 45volts!!! That unwanted 40volts has to be lost somewhere! That's why manufacturers spec sheets are published for correct design and operation.

#### hp1729

Joined Nov 23, 2015
2,304
Hi all : )

Im designing a 15 V DC regulated supply.
Im calculating filter capacitor value for it using following formula,
C= (0.7*I)/(100*Vr)
where, I is load current
Vr is ripple voltage
0.7 because capacitor supplies the current for 70% of cycle

i want to know how much ripple voltage should i take for my calculation ??
Does your formula suggest for 1 V of ripple at 2 Amps you need 14,000 uF? Did I do something wrong? If you change that 100 to 1,000 you just about get 1,000 uF per amp.

Last edited:

#### recklessrog

Joined May 23, 2013
985
i want to know standard value of ripple voltage that one takes to calculate capacitor filter value. i have read that there is trade-off between capacitor value and ripple value. u cannot keep capacitor value high to minimise ripple value because large capacitors get heated quickly.
So one always keeps some ripple value to lower the capacitor value.
Don't forget that the esr of the capacitor is important when designing power supplies, and as they age, the esr tends to increase.

#### #12

Joined Nov 30, 2010
18,224
I'm getting the idea that this is 100 Hz for the ripple. Not as critical for ESR as a high speed switching supply is.
Just calculate what you need and boost it by 50%. You'll be OK at power line frequencies.

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