Power supply mod

[MikeK]

After building my Elenco power supply kit I realized it has no reverse protection diodes on the regulators.  The series pass transistor at the 5V regulator confuses me, so I'm not sure where to put the diode.  Please comment (my changes are in red):

[Rerouter]

A, the regulator still gets evened out by the series resistor on the regulator,

the 100nF on the outputs are a nice touch, for low esr, but for the variable side, it would be better to have them on the inputs, and as close to the regulator as possible, even sharing pins if possible,

[MikeK]

I can add them on the inputs, but shouldn't I keep them on the outputs?  Don't the regs still produce ripple?

[Rerouter]

what the 100n capacitors you see on other supplies is to handle transients, ripple usually comes from the input side, by having a very low ESR capacitance at the input pins, you can help smooth it out a bit more than it would on the output,

low esr ceramics help couple the AC component to ground, and its always better to get rid of it on the input,

[MikeK]

Okay, thanks!

Now, if I wanted to have a switch for the outputs (so that the big caps don't continue to power my circuits when I switch off the power supply) can I just run the COM line through an SPST?

[Rerouter]

being its a dual rail supply, i would reccomend using a spst on each output, cutting ground still leaves anything powered across the rails running,

[SeanB]

Rather better will be to switch the outputs with a 4 pole switch ( use 1 pole to power a LED to tell you the outputs are actually on) and not switch the common connections. Use a switch rated for more than the regulators can deliver, or use a 4 pole relay with power coming from a separate bridge rectifier and small capacitor ( 100-470uF depending on relay current and voltage) fed from the transformer, this will drop out in around 2 cycles of switching the mains off.

[MikeK]

Or use MOSFETs in place of the relay?

[alm]

A relay gives you galvanic separation between coil and switch contacts for free.

[caroper]

Would it be feasible to use a couple of zener Diodes such that the positive input could offset the negative reference and visa verse to generate offset voltages to the loose ends of the two pots, so that regulation down to zero is available?

[Rerouter]

I believe that method is documented in the app-notes for an lm317, but yes, just remember to do an excessive drop, as a zeners voltage drop changes as its current does, e.g. when you turn the pot to near 0, it will allow more current through and change the drop

[SeanB]

Simple to do by using a pair of red LED's to provide an offset to the other rail, and supply them with 10mA via a suitable resistor. Connect the wiper end of the pot to the non common connection of the 2 LED's. The voltage drop across a standard LED is around 1.5V, and will be able to compensate for the 317 quite nicely. Will have a little dead space around 0V, and a little hard to set low voltages, but will go down to close to 0V.