Simple negative voltage generator, need some ideas

[RDL2004]

I am working on a power supply that will use probably an LM350T.

This will be my bench supply for a while.

I would like for it to be able to get down near zero volts, to do this I will need to reference the regulator to neg ~1.2 volts.

I was thinking of using LM385, so required current will not be much.

Problem is, I am trying to fit the whole circuit on a 3" x 3" pc board (75mm x 75mm) and I don't have much space to spare, maybe 0.5" x 0.75" (12mm x 20mm).

It seems that using a 555 is the easiest way to go, but I was wondering if anyone else had a better idea.

I hate to go to a lot of trouble just to get down to zero volts out, but I probably will. I guess I can re-do the pcb layout if I need to.

Some other questions pertaining to the LM385.

Will I need to regulate the input voltage to it?
If I use a 555 to generate the negative voltage, how should it be supplied?

The power supply wiill be running off a toroidal transformer with 2 x 20 volt @ 3 amp in parallel.

Thanks for any ideas.

 

[TheOne]

Look at this thread. You could use the idea to generate a -ve voltage.
https://www.electro-tech-online.com...-amp-zero-crossing-detector.13530/#post-72636

 

[Sebi]

I mean, when You have two 20V secondary use two way rectifier. The negative voltage from transformer also simpler as 555 solution...

 

[Nigel Goodwin]

I mean, when You have two 20V secondary use two way rectifier. The negative voltage from transormer also simpler as 555 solution...

You may as well add the fourth recifier, and make the negative supply fullwave as well - it should improve the smoothing on the negative rail, and allow a smaller electrolytic.

 

[RDL2004]

Doesn't this mean the secondaries have to be in series?

I was planning on parallel, since I won't need over 20 volts, but I may need 3 amps or more.

 

[Nigel Goodwin]

Doesn't this mean the secondaries have to be in series?

Yes it does!.