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'[PICLIST] {EE}: Filtering Caps -- a complete myste'
2002\03\05@144306 by Colin Constant

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I was just wondering how people arrive at a choice of power supply filtering
capacitors.  I'm thinking  of a regulated 5V supply, around 1A (7805).

Looking at published plans, it seems that some people routinely specify 2 x
2200.  Others just put in a single 1000.  What's a guy to do?

Also, data sheets indicate 0.33 "or larger" on the input if an appreciable
distance from the filter caps, but published plans seem to use 0.1 or 0.22.
Anyone know why?

And, on the output, 0.1 seems to be standard.  Motorola says values less than
0.1 could cause instability.  Wouldn't this tempt you to use more than 0.1?

As I say, it's a mystery to me.  Any enlightenment on the Zen of capacitor
selection would be appreciated.

Thanks,
Colin

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2002\03\05@151156 by Spehro Pefhany

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At 11:41 AM 3/5/02 -0800, you wrote:
>I was just wondering how people arrive at a choice of power supply filtering
>capacitors.  I'm thinking  of a regulated 5V supply, around 1A (7805).
>
>Looking at published plans, it seems that some people routinely specify 2 x
>2200.  Others just put in a single 1000.  What's a guy to do?

Hi, Colin:-

Okay, the most important thing is to make sure that with the minimum
line voltage, maximum load and minimum capacitor and worst-case voltage-
regulator drop that the regulator never "drops out". The first part is a
function of the line voltage range you want to cover (-20%? -25% from nominal),
the actual transformer output voltage under fairly heavy loading (more on
that later) and the diode drop(s). The regulator dropout you get from the
data sheet. It is 2V at 1A 25'C typical, and 2.5V guaranteed over temperature.
Which one should you use? Right, 2.5V. Then you have to look at the ripple.
You can make the assumption that the capacitor is only charged in brief
pulses the peaks of the AC waveform. Thus, we have a peak-to-peak ripple
of Vr = (tx * I)/C  (time for one half cycle with full-wave rectification
or one cycle with half-wave, I is the regulator input current, C is the
capacitance). So, with a 4400uF cap, 60Hz, full-wave, you'll have
Vr = 1.9V (p-p). That means your average input voltage measured on a DVM
had better be higher than about 8.5V at minimum line input and maximum load,
because the "troughs" of the input waveform will be down at 7.6V, just
about the 7.5V minimum. Note that half-wave rectification means the caps
have to be doubled for the same ripple. 1000uF is not enough for any
reasonable 1A supply, but it's often fine if you only need 100mA or 200mA.

The actual transformer voltage will be a bit lower than you might expect
because the 1A is supplied in brief pulses of much higher current, depending
on the transformer winding resistance and other factors.

The other requirement is that the ripple current be less than the maximum
spec on the capacitor, and the more below that the better, because the
capacitor will last longer. Using two capacitors means that you can double
the ripple current spec unless your layout is really silly.

>Also, data sheets indicate 0.33 "or larger" on the input if an appreciable
>distance from the filter caps, but published plans seem to use 0.1 or 0.22.
>Anyone know why?

Maybe they are not far from the filter cap? I don't use any unless the
regulator is off-board, but there is a bit of a conflicting requirement-
the capacitor should be kept cool (away from regulator) but it has heavy
traces to, and should be close to the regulator. 1/2" or 1" is no problem.
If it's a through-hole electrolytic, 10uF/50V is about the smallest that
makes sense unless you're into the really small (7mm or less) heights.

>And, on the output, 0.1 seems to be standard.  Motorola says values less than
>0.1 could cause instability.  Wouldn't this tempt you to use more than 0.1?

7805's don't require an output capacitor for stability. The output cap reduces
the output impedance. What Motorola means is that you should not put a 0.01uF
capacitor on there, if you do use one (and you usually should) then make it or
them >= 0.1uF total. Usually there's more distributed capacitance around
the board
on  chip bypasses anyway. LDOs and negative voltage regulators do require
an output
cap and you should follow the data sheet recommendations.

Hope this helps,

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2002\03\05@151742 by Russell McMahon

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You can almost certainly find some design papers on the net.

Briefly -

> I was just wondering how people arrive at a choice of power supply
filtering
> capacitors.  I'm thinking  of a regulated 5V supply, around 1A (7805).
>
> Looking at published plans, it seems that some people routinely specify 2
x
> 2200.  Others just put in a single 1000.  What's a guy to do?

Depends largely on what peak ripple YOU wan't to tolerate and why.
Larger caps generally never hurt performance. Smaller caps might.
Also depends on "headroom" you ae providing the regulator. As this drops you
can tolerate less ripple before regulator dropout and will ned larger caps
in the limiting case.

Note - quite incidentally to this discussion, that a SMALL series R feeding
the main filter cap will often IMPROVE noise performance and will guive caps
a MUCH more gentle time as cap charging is "spread" across more of the AC
cycle. Many don't use this method. A little arcane (despite its apparent
simplicty) and you need to read more about it if doing it. Leave this until
you are happy with basic filtering. .


> Also, data sheets indicate 0.33 "or larger" on the input if an appreciable
> distance from the filter caps, but published plans seem to use 0.1 or
0.22.
> Anyone know why?

This is NOT a ripple spec per se but an impedance one related to stability
of the *AMPLIFIER* that also happens to be a regulator. (If you don't think
it's an amplifier, explain how it oscillates when you don't talk nicely to
it :-) ).

The 0.33 is probably design worst case and the 0.2 is someone knowing it
will work OK with a bit less and the 0.1 is people being lazy or assuming it
is NOT an appreciable distance from the filter caps.


> And, on the output, 0.1 seems to be standard.  Motorola says values less
than
> 0.1 could cause instability.  Wouldn't this tempt you to use more than
0.1?

Yes. 0.1 uF is the mother duck value. They are commkn and cheap and also
usually about OK.

Note that 0.1 uF was about the right value for general bypass capacitors in
older systems or new slower systems. It "just so happens" that for a typical
0.1 uF monolithic ceramic cap the series resonant combination of cap plus
lead inductance for a cap soldered with minimum lead length was good for
processors with clocks in the 1 to 10 MHz range which was good for most
systems. If you arenusing a 50 MHz clock (or 100 or 133 or 200 or ....) then
you want smaller bypass caps. The world has moved on buy many designs
haven't.

Much more on this subject but this should start things off.
As regards caps around actual regulator, until you are comfortable doing
otherwise, do what the data book says unless it's really hard to do so - and
then, do it anyway at first.
Note that some regulators have min and max cap size specs or spec in terms
of ESR rather than capacitance.

>  As I say, it's a mystery to me.  Any enlightenment on the Zen of
capacitor
{Quote hidden}

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2002\03\05@151947 by Thomas C. Sefranek

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Colin Constant wrote:

>I was just wondering how people arrive at a choice of power supply filtering
>capacitors.  I'm thinking  of a regulated 5V supply, around 1A (7805).
>
>Looking at published plans, it seems that some people routinely specify 2 x
>2200.  Others just put in a single 1000.  What's a guy to do?
>
T=RC
T=1/F
F= ?120Hz. (or 100 Hz , or 60 Hz or 50 Hz...)
(Full wave rectified transformer in the USA is 120 Hz.)
R = E/I
E = Raw unregulated voltage. (8 volts MIN. for your 7805)
I = 1 Amp.

>
>Also, data sheets indicate 0.33 "or larger" on the input if an appreciable
>distance from the filter caps, but published plans seem to use 0.1 or 0.22.
>Anyone know why?
>
I use a 1 Microfarad on the input.
a .1 is too small, and I have had oscillations.

{Quote hidden}

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2002\03\05@154244 by Rick C.
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True confessions of a circuit board layout man...........

There is no real magic in selecting filter or bypass capacitors for a given
circuit.

I have been in the business of laying out circuitboards since 1972. I have laid
out over 1000 circuitboards and designed many products with power supplies. I've
had engineers hand me a schematic (TTL, CMOS, analog) and have been told to
"scatter" bypass caps where ever I have the room.

True, there are many theoretical articles and references as to what the proper
capacitor values should be but this is not the time and place to to it.

There are two types of power supply capacitors. Filter and bypass. In specifying
the exact value of a filter cap, you would need to know the parameters of the DC
source, half or full wave rectifier, no load peak voltage, minimum and maximum
load current, temperature, and probably a few more things I can't remember. A lot
also depends on whether this is a one time project or a product that will go into
production for consumer, industrial/automotive applications, mil-spec, etc.

Obviously, the most important factor is the amount of ripple that can be
withstood under full load by your project. The more filtering capacitor, the
better....to an extent. Too much can be costly and bulky. Not enough, and your
project will become unstable under load.

Bypass capacitors in your circuit should be placed liberally. Usually as close to
the +5 and ground pins of the most sensitive devices you use. Not every chip has
to have them although a processor, PLL, counters, should be bypassed. The value
for most digital circuits is 0.1uf. From 1 to ~50 mhz, use 0.01uf. Above 50mhz
use a 0.001uf.

If you want to get into the theory and design of a power supply, a good reference
would be the ARRL handbook available from http://www.arrl.org
This is a good all around reference for most common electronic parts.

If you just want your project to work, like your post, (7805 1A) make sure your
DC input is no less than 3 volts over your regulated output, 470 to 1000uf is
adequate for filtering and NEVER omit a 0.1uf cap across the input of the 78xx
and ground pin, as close to the device as possible to keep it from oscillating.
Don't put too much capacitance on the output of the 78xx. A no load condition and
a drop in voltage on the input of the78xx will blow it. I have seen some circuits
with a diode placed across the 78xx, anode to the output, cathode to the input to
protect it.
Rick


Colin Constant wrote:

{Quote hidden}

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2002\03\05@162630 by Colin Constant

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Thanks guys ( Thomas, Russell, Spehro, and Rick).

That's just what I was looking for.

Ragards,
Colin

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2002\03\05@170426 by Tony Nixon

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"Rick C." wrote:

> A no load condition and a drop in voltage on the input
> of the78xx will blow it. I have seen some circuits with
> a diode placed across the 78xx, anode to the output, cathode
> to the input to protect it.

I've never seen this approach with 78xx regs - interesting. I've only
seen it with 317 types.

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Tony

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2002\03\05@172845 by Spehro Pefhany

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At 09:02 AM 3/6/02 +1100, you wrote:

>I've never seen this approach with 78xx regs - interesting. I've only
>seen it with 317 types.

It's valid- look at the internal schematic for the 7805, here's
a pointer to one, and analyze what happens for Vout > Vin.

http://www.onsemi.com/pub/Collateral/MC7800-D.PDF

It's only a concern under the following conditions:

Substantial capacitance on the output (but if you need it, you
need it)

AND either

A)      A very heavy load on the unregulated supply such that the
        current from the resulting dv/dt at switch-off could
        cause excessive current through the regulator. This is
        rather an unlikely scenario.
Or

B)      There is some possibility that the input could be short-
        circuited and there is no series diode. This is more
        likely where you'd use it. If there is a bridge or
        polarity protection diode, it's not a concern unless
        there is a possibility that it could be shorted on the
        *other* side of the diode.

Not all data sheets cover this issue, so it's a good point. Like
an input capacitor, it's wasteful to put it in there if it is
unnecessary.

(BTW, on the previous post, 470uF is about 10 times too small
for 50/60Hz mains filter for typical 1A supply, also the ripple
current rating will not likely be high enough to be reliable).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam.....interlog.com             Info for manufacturers: http://www.trexon.com
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2002\03\05@173257 by Dave King

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At 09:02 AM 3/6/02 +1100, you wrote:
>"Rick C." wrote:
>
> > A no load condition and a drop in voltage on the input
> > of the78xx will blow it. I have seen some circuits with
> > a diode placed across the 78xx, anode to the output, cathode
> > to the input to protect it.
>
>I've never seen this approach with 78xx regs - interesting. I've only
>seen it with 317 types.
>
>Tony

Just ran across this in a app note a few days ago. I think it was on the
Fairchild or the Phillips site.
It also suggested a couple of caps.

Tried both and didn't notice any difference.

Dave

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2002\03\05@175623 by Rick C.

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Yeah, 470 might be a little wimpy. I guess I usually keep my loads below 0.5a @
5v with the 7805 unless I want a combination power supply/space heater. ;-)

Another "condition" that can occur is a mutiple regulator power supply from one
source. +5, +12, +18 volt from the same rectifier (the +12 or +18 not short
circuit proof). Here, with a large cap on the output of the 5v reg, a heavy load
or short on another regulator could drop Vin under Vout.
Rick

Spehro Pefhany wrote:

{Quote hidden}

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2002\03\06@005440 by Robert A. LaBudde

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At 09:12 AM 3/6/02 +1300, Russell wrote:
><snip>
> > Looking at published plans, it seems that some people routinely specify 2
>x
> > 2200.  Others just put in a single 1000.  What's a guy to do?
>
>Depends largely on what peak ripple YOU wan't to tolerate and why.
>Larger caps generally never hurt performance. Smaller caps might.
><snip>

Too large a filter capacitor will result in too long a rise time on the
final regulated voltage.

PIC microprocessors have a maximum "come-up" time specification for
start-up reliability.

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