[sdiy] Problem with 723 PSU

[Jim Patchell]

    Without actually seeing the thing, I just making wild guesses, but here is something to check.

    The good old capacitor input power supply filter causes a very high current spike while charging the capacitor.  You can estimate what this spike is by looking at the ripple on the filter cap.  The diode is only conducting while
the voltage is going up, which means the diode could be conducting for say only about 1/10th of the time, for instance.  If you are drawing 1 amp from the power supply, this means the current flowing though the diode will be 10 amps
durring this short period.  By having long wires going back to the transformer, you might be creating a bad situation. (Short 10 amp current spikes).

    To keep this spike down in some of my applications, I have used either a small resistor, or better yet, a choke.  In either case, you might have to adjust the power supply voltage, because a capacitor input filter is basically a
"peak" detector.  A choke input filter will give you something closer to the average voltage.

    I don't know if any of this applies to your problem, but it is something to consider.

    -Jim
jorgen.bergfors at idg.se wrote:

> Hi all power supply gurus.
> I have built two power supplies from the Formant schematics.
> They each consist of two 723-based regulators, where the negative output of one is connected to the positive output of the other, to form -+15 volts and ground. So it is really two identical positive regulators, connected together.
> Anyway, this works fine as long as the transformer is connected directly to the units.
> But I wanted to put the transformer in a separate enclosure, to keep 230 volts out of the main unit.
> The cable from the floor unit is two metres long and this is evidently enough to cause some instability in the negative regulators.
> There is a small transient, with accompanying ringing. It is synchronized to the main frequency. It is only around 0,2 volts, but it is enough to cause a slight buzzing sound in some modules (both VCF).
> If I break the ground connection between the sections, the problem goes away.
> Loading the output reduces the ringing, but it doesn't go avay completely.
> I tried changing the freq comp capacitor and several other parts, but nothing seems to help.
> Any ideas?
>
> /Jorgen