J
Jon Kirwan
At one time I had two reels of 5000 each of 1N4004 that I got surplus, but
I sold most of them. I also have a bag of about 1000 pieces of 1N4003. So I
have pretty much a lifetime supply. Either one is OK for 120 VAC mains and
perfect for lower voltage applications. But now my new designs are mostly
SMT. I was going to keep the thru holes and use the "free" parts I had, but
I figured that the labor cost of inserting, soldering, and clipping leads
on 6 diodes on 40 boards might be more than the $0.06 each for the S1G SMT
diodes. Once a commitment is made to SMT it is usually cost-effective to
use as many such parts as possible. I never fully analyzed it, though. I
figure about 2 minutes for the six diodes. At $60/hr, or $1/minute, I spend
$2/board for the leaded parts. The SMT assembly is probably $0.05 per part,
so I spend a total of $0.66 per board.
Some time ago I came up with a rule of thumb of 1000 uF per amp, and I
revised that to 2000 uF per amp. I used an RC time constant of 8 mSec
between peaks for a 37% discharge from peak which holds the approximate RMS
value, and for a typical 8 VDC power supply at 1 amp R=8 ohms. So C =
.008/8 = 1000 uF. But two time constants gives only 13% discharge so 2000
uF is much better. For a 16 VDC supply, 1000 uF is OK, and as the voltage
doubles the required capacitance is halved. So for most low voltage
applications, 1000 to 2000 uF per amp is reasonable, and easy to remember.
Okay.
Of course, if you enjoy mathematical analysis, you can spend time working
out effects of winding resistance and capacitor ESR and acceptable ripple.
I suppose what I don't enjoy is taking on faith "rules" or
"prepared charts" I'm handed. So I do the math once or
twice, just to verify and make sure I have a small sense of
understanding about the whys and wherefores. (And I enjoy
the math practice, from time to time.)
Or you can just use LTSpice.
Once I feel I grasp the theory I will use LTspice a lot and
not give it that much thought later on. But isn't it better
to make sure, at least once
But if I need a quick and dirty junkbox power
supply, 1000 uF/amp is good enough to grab and go.
And I think I understand the details why. (Unless someone
expresses an interest, I won't dump it out here.)
For example, using LTSpice, I find a 12.6 V transformer and I want to make
a 12 VDC power supply at 1 amp. Using a 1000 uF capacitor and a 12 ohm
load, my output is 13.3 V which has a peak of 16.1 V and drops to 10.4 V,
which is a 35% drop as predicted. With 2000 uF it drops to 12.6 VDC so my
output is high enough to provide the 12 VDC I wanted with a regulator. Of
course there are line variations and transformer regulation, but not bad
for a quick estimate.
As I just wrote, once I've done it in the "forward direction"
and feel I understand the details well enough, once or twice,
just selecting rules to follow after that make sense. If
something doesn't feel right in the simulation, you can
always return to the fundamentals on paper to double-check.
But I don't like using a tool in a fashion where I have no
clue whatsoever how to check the work on my own, should I
decide to do so. Doesn't feel right. (You are past that
point, of course, so no problem there.)
If I wanted 24 VDC, and I had a 25.2 V transformer, a 1000 uF capacitor
gives me a minimum of 26 VDC for a regulator with a little bit of headroom.
Now I actually add a simple emitter follower voltage regulator with a
2N3055 and two 12 V zeners and a diode in series, with 220 ohms and a 100
uF cap. I get an output of 24.18 VDC which varies from 23.99 VDC to 24.30
VDC. Adding the regulator improves the minimum voltage excursion on the
1000 uF main filter capacitor to 27.6 VDC.
Since I was originally designing for just such a regulated power supply, my
"grab-and-go" estimates for main filter capacitors seems to work out quite
well. And I found it more fun to build and test the circuit using LTSpice
rather than with math. Filter capacitors of this size are typically -20% /
+80% tolerance, so chances are the results will be even better than
expected.
Thanks. And I got it.
Jon