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Help Build Power Supply

R

Razzel

Jan 1, 1970
0
I am building a 5v regulated power supply so checked web for schematics
and hints.
1) Discovered that the pre regulator post-bridge capacitor should be
100, 220, 470, 1000, or 2200 uf......???huh?
2) Discovered that I should use a 0.1uf cap before and/or after the
regulator.....???huh?
3) Discovered that I should use either a 1N4007 or 1N4148 diode across
the input to output of the regulator on none???huh?
4) Discovered that i can use an LED across the DC output or never
there....again huh?
5) Some times one uses a 100 uf cap after the regulator or 220 uf, or
....???
I think (#2) the 0.1 cap is to leak any main frequency AC to ground.
I think (#1) that any of these would do but the bigger ones will smooth
better except I seem to recall something about its RC constant can limit
the smoothing.
I have no clue about 3, 4 and 5.
Any help please. Would like to know why the capacitors are used in
their particular location. I have forgotten a lot since grad school.
Ron
 
T

TP

Jan 1, 1970
0
I am building a 5v regulated power supply so checked web for schematics
and hints.
1) Discovered that the pre regulator post-bridge capacitor should be
100, 220, 470, 1000, or 2200 uf......???huh?

That sounds reasonable, depending on current and type of regulator.
2) Discovered that I should use a 0.1uf cap before and/or after the
regulator.....???huh?

That would be a ceramic type capacitor, which can charge/discharge
more rapidly than electrolytic type but has very little storage. It
helps with smoothing of high frequency signals or abrupt changes
in load. The parallel combination of ceramic and electrolytic
capacitors provides the best of both worlds.
3) Discovered that I should use either a 1N4007 or 1N4148 diode across
the input to output of the regulator on none???huh?

Not sure about this, but possibly to limit the voltage potential
across the regulator in the case where the input voltage is removed
and there is potential from another source at the output, such as
a battery or the charged output capacitor.
4) Discovered that i can use an LED across the DC output or never
there....again huh?

A power on indicator?
5) Some times one uses a 100 uf cap after the regulator or 220 uf, or
...???

Sounds good, depending on how much current flow, how much ripple
you can tolerate, and a few other things.

Check the regulator manufacturer's web site for a data sheet and
possibly application notes, either of which may help with component
choices.

TP
 
W

Walter Harley

Jan 1, 1970
0
Razzel said:
I am building a 5v regulated power supply so checked web for schematics
and hints. [...]

TP's specific answers were pretty good so I won't repeat them.

Big question: why are you building it in the first place? These days you
can buy a 5v regulated supply in any form factor and current rating you
want, for cheaper than you can build it yourself. E.g., try Jameco for many
options.

If you're building it yourself in order to learn something about
electronics, may I suggest that you'll do better with some structured
reading, rather than trying to decipher what you find piecemeal on the
Internet? There are many excellent books out there on power supply design,
both linear and switch-mode. I would heartily recommend the National
Semiconductor voltage regulator handbook, which is probably out of print now
but may be available used or online somewhere. Horowitz and Hill's "Art of
Electronics" has a good chapter on the topic. There are a number of good
"cookbooks" on linear power supply design. And manufacturer datasheets -
start by reading the datasheet on the LM117 - are a good source of
information, although they will only give you part of what you need to know.

There are quite a few issues involved in building good power supplies. You
touched on a couple, but there are others that will spell the success or
failure of your supply, such as appropriate heat-sinking and appropriate
sizing of the rectifier components. The circuits are simple, but because
power is involved, the schematics don't really tell you everything you need
to know.
 
R

Rich Grise

Jan 1, 1970
0
TP said:
Sounds good, depending on how much current flow, how much ripple
you can tolerate, and a few other things.

The ones that have a big cap at the output of the regulator are wrong.
The 0.1 uF is for stability, but that's about as much of a capacitive
load that a typical 3-terminal regulator is guaranteed to drive and
still meet the rest of the specs. If you need more mains filtering,
use the input cap, which is the one between the rectifier and the
regulator. Input refers to input to the regulator section, that is.
Check the regulator manufacturer's web site for a data sheet and
possibly application notes, either of which may help with component
choices.

OF course. RTFM is always right. :)

Cheers!
Rich
 
T

Tim Shoppa

Jan 1, 1970
0
Razzel said:
I am building a 5v regulated power supply so checked web for schematics
and hints.
1) Discovered that the pre regulator post-bridge capacitor should be
100, 220, 470, 1000, or 2200 uf......???huh?

The value of this depends on the no-load voltage, the current
draw, the rectifier output type
(full-wave, half-wave), AC frequency (most commonly 50 or 60Hz), and
transformer ratings (the bigger this cap, the bigger the current spikes
from the transformer, and the bigger the I^2*R losses in the transformer).

For a hobby project, just making sure the input to the regulator doesn't
droop below 7 or 8V at min AC volts and bottom of the ripple
is probably good enough.
3) Discovered that I should use either a 1N4007 or 1N4148 diode across
the input to output of the regulator on none???huh?

If there's any ever chance of power being applied to the output of the
regulator when the input is off, this prevents damage to the regulator.

Tim.
 
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