# ps filtering and regulation

Discussion in 'Electronic Basics' started by Jon Slaughter, Apr 17, 2007.

1. ### Jon SlaughterGuest

suppose I have a ps that has about 35VDC after filtering. I'm trying to
determine what minimum filtering caps I can get away with and still have
good regulation for ic's.

I know that VR = I/f/C is my P2P voltage ripple size.

Now can I just take that and subtract it from my total voltage and then make
sure my regulator is a few volts below that or will it sill cause problems?

e.g., I have 35VDC non-filtered. If say I use 3300uf cap for smoothing that
gives me a ripple of about 5V at max 2A. This means I have a minimum voltage
of ~30VDC. If I keep my regulator below about 25 volts then will I be fine
and have good regulation? or do I really need to add the extra capacitance
to improve regulation signficantly?

I would imagine that the regulator can easily keep up with the ripple and I
just need to use enough filtering to keep the minimum above the max voltage
+ headroom that I need and that there is no need to waste the extra
capcaitance since it won't really be used? (obviously will probably double
it just in case)

e.g., it seems for the case above that if I have atleast 3300uf that I'll
have approximately the same regulation as anything greater than 3300uf?

Just trying to save some caps

Thanks,
Jon

2. ### Phil AllisonGuest

"Jon Slaughter"

** Means what ?

The average rectified DC value is 35 ??

That makes the peak voltage 54.

You GOTTA get this right.

Plus know what the regulation factor of the AC tranny is with diode and cap

......... Phil

3. ### Jon SlaughterGuest

No, not average. Don't worry about that because it doesn't matter. Its just
a number. As long as I'm consistant and don't mix then its ok. The numbers
are not whats important at this point.
Not really. Those numbers are not important. (I don't even remember what the
exact rms/peak voltage is. I think its a 10:1 transformer)

I didn't know what this means? You mean it might not be optimal and I'll
need to increase the filtering?

I'm not so concerned with the details as I have a lot of wiggle room with
the actual numbers. Since its a power supply I'm building for myself I can
pretty much do what I want with it within reason. I'd like to get atleast 2A
at 12V which I think I can do pretty easy. Hell, I might go for a split
supply.

Just wondering if the regulators have any issues with a badly filtered input
as long as the minimum is well above the regulated voltage(within reason of
course). I'll figure out the details once I know this and I actually sit
down to decide what I want. (busy doing a few other things right now)

Thanks,
Jon

4. ### Phil AllisonGuest

"Jon Slaughter"

** You do NOT get to decide what doesn't matter - fuckhead.

** You do NOT get to decide what doesn't matter - fuckhead.

** You do NOT get to decide what doesn't matter - fuckhead.

** You do NOT get to decide what doesn't matter - fuckhead.

** The actual numbers and ratios are the ONLY things that matter.

** No. The tranny has to be rated and sized to do the job.

** You do NOT get to decide what doesn't matter - fuckhead.

** The reg ICs need at least 2 volts more input than output - at every
instant in time and under all operating conditions - plus they must not
overheat.

The AC tranny is the crucial issue.

Not the damn filter caps.

......... Phil

5. ### Michael BlackGuest

"Jon Slaughter" () writes:

As long as the bottom of the peak is above the minimum voltage of the
regulator (ie the output voltage plus whatever the head room that the
regulator requires), it can't make a different.

Michael

6. ### John PopelishGuest

Linear regulators do have a frequency response curve. They
reject low frequencies a lot better than high frequencies.
Ripple with sharp corners will bleed through a bit more
bounce in the regulated output than ripple that consist of
less and lower harmonics of the line. Whether the
difference matters depends on the use of the regulated output.

7. ### John PopelishGuest

Try away. But electrolytics change capacitance over time
and temperature, so something that works one day mat become
marginal on another.

If you have excess voltage to get rid of, I think a better
way (than using the smallest capacitor that produces the
largest usable ripple) is to replace the capacitive filter
with an RC filter. It lowers the RMS load on the
transformer, for a given DC output current, lowers the
ripple peak to peak magnitude, as well as lowering the
higher harmonic content in the ripple, for a given size
capacitor. It also lowers the capacitor RMS current and so,
its internal temperature rise and cools the regulator by
shifting some of its heat to the resistor. If you have a
transformer that idles hot and this is all it is doing, it
works even better if you move the resistor upstream of the
transformer, because its drop reduces the core losses in the
transformer.

8. ### Jon SlaughterGuest

Sure it can. If we think of a regulator as an opamp then it has finite slew
and so it will make a difference. Ofcourse will it be significant? Thats
what I'm trying to find out but it seems not. I suppose the only drawback
is the extra heat generated in the regulator.

Jon

9. ### Jon SlaughterGuest

This is not a a ps that will be used in expensive equipment or anything and
the ps itself is not expensive so its not a big deal. There is a point
though when adding more caps does not add anything significant to the
regulation and I don't want to go to far over that point.
I was just reading on this but not sure if its a good idea for my circuit
because of the large current. Better to use an LC? I do have room for the
inductors.

Thanks,
Jon

10. ### John PopelishGuest

If you don't mind the cost and size of the inductor, it is
definitely the way to lower voltage efficiently, while
improving ripple.