# Converting +15 Volts to -15 Volts

Discussion in 'The Projects Forum' started by mikeparker, Nov 1, 2011.

1. ### mikeparker Thread Starter New Member

Nov 1, 2011
2
0
I have an output of a NE555 timer. (0 to +15 volts)
I wish to convert it to a negative 15 volts.
I think I should be using a pnp transistor, except the base requires a negative voltage
Help me !!!

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
There are several good ways to do this. You did not mention your current requirements.

You could use a voltage doubler, and use that to reverse the polarity. I suggest a doubler because there will be losses.

You could hand wind a toroid transformer with a slight voltage boost, and drive that from the 555, then treat it like a conventional power supply circuit.

How much current do you need?

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Ahhh, did you mean that you want the output signal to go from -15v to 0v instead of 0v to 15v?

Do you have a -15v supply available, or do you also need to build a supply capable of generating -15v?

4. ### crutschow Expert

Mar 14, 2008
13,508
3,385
A PNP needs negative bias to operate, but it can not generate a negative voltage by itself.

If you just want a 0 to -15V output from the 555 you can use a clamp (DC restore) circuit. AC couple the 0 to +15V signal through a capacitor to a diode with cathode connected to ground. The diode will conduct on the positive transition which will generate a negative DC bias on the capacitor. After a few cycles, the capacitor will be charged to -15V (less the diode drop) and the signal will swing between about +0.7V and -14.3V.

That circuit works best if the output signal is continuous. It won't work well for one-shot applications.

The load you are driving and the frequency of the signal determines how large the cap must be to minimize droop on the bottom (negative part) of the square-wave.

You can reduce the offset to about 0.3V if you use a Schottky diode instead of a junction diode.

5. ### wayneh Expert

Sep 9, 2010
12,396
3,246
Please explain why you need this. Simply referencing the output to Vcc instead of ground will reverse the polarity of the output. Maybe you don't really need -15V?