Schmitt Trigger Oscillator

[FvM]

For a boost converter with negative output voltage, you need to change the polarity of the flyback circuit, NPN -> PNP switch.

 

[boylesg]

Why did you reverse the polarity of anything?

Replacing 0 and +12 with -12 and 0 doesn't change any relative polarities, it just shifts the 0 point and subtracts 12 from the numbers you previously saw.

Making that replacement and leaving everything else unchanged will change the outputs to -12 and +6 (relative to your input supply 0). That's still 18 volts.

If you want +18 use the lower output as output 0.

If you want -18 use the higher output as output 0.

Your meter shows +18. Swap the probes and there's your -18.

You haven't specified if you want your 0V output to be the same as your ATX 0V. If that's the case then yes, continue with your modifications. But if that's not the case, then you already have your -18.

If I am to power an opamp with this thing then simply swapping the outputs of the circuit on to the opamps Vcc and Vee pins wont do - it still amounts to a singly supply of 18V.

I have added a molex connector on my atx power supply that has -12V GND GND +12V. What I am aiming to do is boost +12V to +18V which the above circuit does nicely. But I also need to boost the -12V to -18V which I can't figure out how to do by modifying this circuit. Both must refer to the GND of the ATX power supply. My molex connector would then be -18V GND GND +18V.

I thought I might be able to simultaneously boost the +12V to +18V then invert using something like this: **broken link removed**. But this is not possible because you can only do one or the other.

I suppose another option is to convert the +12V and -12V to AC, pass it through a 12V to 18V transformer and then rectify it back to DC 18V. Bit painful though.

I find it difficult to believe that it is not possible to do with a negative voltage what you can do with a positive voltage. But searching for a negative boost converter has been fruitless.

Replace all this with a 3:4 voltage divider across your ATX -12 and +12, and there's another way to get + or - 18.

Huh? How? I thought voltage dividers can only reduce voltage. How can I get +18V from 12V with a voltage divider? Can you show me what you mean?

 

[jasonc2]

I wasn't paying attention again and missed your clear +-12 -> +- 18 comment. My comment was based on the assumption that you only wanted -18. It is clear to me now that you are intending to boost 24 to 36 and also keep the center 0V points the same.

Yes, a voltage divider can only decrease voltage. -12 to +12 = 24, the 3:4 divider would give 18 (-12 to +6 or -6 to +12 depending on the configuration).

It is not applicable for your requirements.

Apologies for confusion.