Power supply filter caps

[Bill_Marsden]

Two frequencies, 50Hz and 60Hz.

Two rectification schemes, half wave and full wave.

I need to explain in simple terms how to pick capacitor values for these 4 types of circuits.

Any thoughts or rules of thumb?

This is where I am at on the AAC power supply article.

[bertus]

Hello Bill,

Perhaps the attached PDF will help a bit.

I had the remark once that the calculations where not all correct.
unfortunately I can not remember wich one it was.

Greetings,
Bertus

[Bill_Marsden]

Thanks, I still have some info Studiot left me a long while back. I need to condense them down into one or two paragraphs, plus math.

[rjenkins]

I did this for someone a few days ago as it happens..

Reference: A 1 Farad capacitance with a discharge current of one amp discharges at 1V per second.

For a rough rule of thumb, divide 1 Farad by the ripple frequency and you will get a capacitor value that gives about 1V ripple per amp of load; eg. 10,000uF at 100Hz.

[MikeML]

To my dying day, I will remember "Civil Engineers are Queer" (strange!) i.e. C*ΔV = q (Coulombs)

Now i*t = q, so by substitution: i*t = C*ΔV.

Rearrange gives: C = i*t/ΔV.

So if you want a capacitor to deliver 0.75A for 0.0083ms (full-wave rectified 60Hz), say at the input of a LM317, and ΔV must be smaller than 5V to keep the filter minimum voltage from sagging below Vout + VdropOut, then

C = 0.75*0.0083/5 = 0.001245F = 1245uF

[cumesoftware]

For full wave rectification, I use the following formula. It is a good rule of thumb:

C = Imax / (2 x Freq x Vripple)

Or you can use this one:

C = Imax / (2 x Freq x Vpeak * ripple_factor)

being ripple factor a value from 0 to 1.

Note that the formula is an approximation, and works better for ripple factors inferior to 0.1 (or for small Vripples when relative to Vpeak).

For the expected DC voltage, check out this guide:
Rectifier guidelines.pdf

This guide is more precise for AC currents on transformers, though:
power_xfrmr_Design_guide.pdf

[The Electrician]

Originally Posted by bertus

Hello Bill,

Perhaps the attached PDF will help a bit.

I had the remark once that the calculations where not all correct.
unfortunately I can not remember wich one it was.

Greetings,
Bertus

There's a strange misconception on page 12-36. Apparently the author thinks that "If the charge time becomes too great, then the capacitor may never reach full charge from the incoming pulses from the rectifier". This is not true, of course.

[lmartinez]

Originally Posted by The Electrician

There's a strange misconception on page 12-36. Apparently the author thinks that "If the charge time becomes too great, then the capacitor may never reach full charge from the incoming pulses from the rectifier". This is not true, of course.

How is that so?

[S_lannan]

for a capacitor to instantaneously change voltage you would need infinite power.
In such circuits there is nothing restricting current flow so you can have infinite power to change the voltage to the peak as soon as the diode conducts.

Obviously any small resistance, i.e from the wire from the rectifier to the capacitor would prevent this from occuring in the real world...

[The Electrician]

Originally Posted by lmartinez

How is that so?

Look at the picture on page 12-35. It shows what happens as the capacitance increases. Now imagine that the filter cap was 1,000,000 microfarads instead of just 1000 microfarads. The slightly downward angle of the blue line would become a nearly horizontal line for 1,000,000 microfarads. It might take a while to reach steaedy state, but once it did, the pulse from the rectifier would keep the cap voltage very near the peak of the sine wave because with a cap of 1,000,000 microfarads the load would discharge it so little that the next rectifier pulse would easily replenish it.

As long as the transformer winding resistance plus the rectifier bulk resistance is substantially less than the load resistance, the DC voltage across the cap will be approximately equal to the sine peak voltage, no matter how large the capacitance.