Power Supply Issues

media at mail1.nai.net media at mail1.nai.net
Tue Mar 24 04:12:50 CET 1998

I've been too busy to work on my theremin design, but I'd thought I'd take
some time to catch up with the list.

>> 	Why would a 7815/7915 power supply be good inadequate for a
>> modular??  Is there any reason why one should use a 723 or 317 regulator
>> instead??
>                                LM7815                  LM317(@15V)
>ripple rejection       :        70dB                    65dB

With the extra cap a 317 has a ripple rejection of 80db.  Although I
remember such a circuit, I do not know if ripple rejection can be improved
by adding cap to the ground pin of a fixed regulator.

>Assuming they are correct, one could say, that for the average analog
>circuit, with allmost constant load (only little output currents) and
>good psu with good passive ripple filtering a 7815 etc could well do the job.

This seems to be the current consensus.  Only one person said "they do not
provide sufficient noise rejection and repeatability of voltage (it varies
slightly each time you turn on, and drifts in use too). This applies to the
317 and all other 3-pin devices. The PSU's  built out of a 723 with a big
bypass tranny can be made so much more accurate it is foolish to use
anything else."

>So I'll go on using 78xx for the most stuff, and 317 only for "odd"
>voltages or
>demanding applications. Passive ripple/transient rejection components can be
>of much more importance than the type of regulator.

I agree, a 317 still seems to be the best way to get 9 volts.

Now on to a few more PSU issues:

1)   As we know, increasing the size of the capacitor between the rectifier
and the regulator  can decrease ripple and increase the available voltage.
I've built power supplies with 4700uF caps bypassing the output of the
rectifier to ground.

However, Horowitz and Hill says "When choosing filter capacitors don't get
carried away: An oversized capacitor not only wastes space but also
increases transformer heating (by reducing the conduction angle, hence
the ratio Irms/Iavg).  It also increases stress on the rectifiers."

That makes perfect sense, but I'm confused on how to calculate that maximum
value.  I would like to know how to do this because increasing that value
increases performance.  I've tried looking at as an RC but can't seem to
puzzle out a value for R.

There must be a mathematical method for determining the best value for the
capacitor.   Anyone have a formula??

2)    Several people have mentioned using a transformer with a secondary
rated for an RMS value lower than required input of the regulator.  On the
one hand we have brownouts at the mains and diode drops across the
rectifier, otoh, there is the peak voltage of the AC and the charge of the
capacitor.  Using too high an AC input into the regulator is going to cause
excess heat, to low a value will cause insufficient output.   Anyone have
formulas for this??

3)     Is there any reason NOT to use a bridge rectifier??   Is a full wave
(two diode) rectifier ever more preferable??

4)     If one were to desire a PSU better than a linear 7815/7915 supply,
are there any reasons to build one yourself (besides the fact that you're
on a diy list :) instead of buying one from say, Condor??


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