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ps filtering and regulation

J

Jon Slaughter

Jan 1, 1970
0
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
 
P

Phil Allison

Jan 1, 1970
0
"Jon Slaughter"

e.g., I have 35VDC non-filtered.


** 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
load.



......... Phil
 
J

Jon Slaughter

Jan 1, 1970
0
Phil Allison said:
"Jon Slaughter"




** Means what ?

The average rectified DC value is 35 ??

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.
That makes the peak voltage 54.

You GOTTA get this right.

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


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
 
P

Phil Allison

Jan 1, 1970
0
"Jon Slaughter"
No, not average. Don't worry about that because it doesn't matter.


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

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.


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


Not really.


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

Those numbers are not important.


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


(I don't even remember what the exact rms/peak voltage is. I think its a
10:1 transformer)


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


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


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


I'm not so concerned with the details as I have a lot of wiggle room with
the actual numbers.


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


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)


** 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
 
M

Michael Black

Jan 1, 1970
0
"Jon Slaughter" ([email protected]) writes:

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)
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
 
J

John Popelish

Jan 1, 1970
0
Michael said:
"Jon Slaughter" ([email protected]) 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.

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.
 
J

John Popelish

Jan 1, 1970
0
Jon said:
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 ;)

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.
 
J

Jon Slaughter

Jan 1, 1970
0
Michael Black said:
"Jon Slaughter" ([email protected]) 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.

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
 
J

Jon Slaughter

Jan 1, 1970
0
John Popelish said:
Try away. But electrolytics change capacitance over time and temperature,
so something that works one day mat become marginal on another.

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.
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.

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
 
J

John Popelish

Jan 1, 1970
0
Jon said:
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.

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