Adjustable negative voltage supply

[alexmoose]

Hello everybody,

I am making a High negative voltage supply for a differential stage in a tube amplifier I am constructing. I don't need more than -120v.

I stumbled across this gem http://www.national.com/an/LB/LB-47.pdf

Basically what I want to do is change the LM317 to an LM337. I was thinking the following steps would be needed

1. Switch NPN transistors to PNP
2. Reverse all Diodes
3. Reverse any electrolytic capacitors.

Am I forgetting anything?

thanks
-Moose

[AndrewT]

an isolated supply can have any output voltage, it is effectively floating.
Connect one side of the supply to a reference, let's use Zero Volts = Ground, then the other voltage is referred to that.
If you have an isolated +ve regulator with 120V between the two outputs, then either output can be connected to the reference.

Connect the more -ve output to Zero Volts and you have 0,120V supply
Connect the more +ve output to Zero Volts and you have 0, -120V supply.

This will work just as well as your proposal, without having to worry about whether it might work as modified.

[alexmoose]

I was thinking about that as well, Just connect the output to some -1xx voltage and use the regulator backwards? sounds even easier

Thanks

[AndrewT]

that's not quite what I said.

Connect one output pole to your voltage reference and the regulator sends the other output pole voltage at it's regulated voltage.

eg. build a 0,+120Vdc regulated supply.

Connect 0V output to Zero Volts and the other output will sit at +120Vdc.

Instead connect +120Vdc output to Zero Volts and the other output will sit at -120Vdc.

There is nothing fitted "backwards".

On the other hand you could have a 0, +60Vdc supply with 0 connected to Zero Volts. The other pole sits at +60Vdc. Now connect the 0 of your 120V supply to the existing +60Vdc and the other output of the 120V supply will sit at +180Vdc (+60V +120V = 180V)