Generating Negative voltages using a 555
I have found that I often want to generate a negative voltage for RS-232 type
interfaces when I only have a +5 or 12 volt rail available. I could use one of
the MAX232 chips, but they aren't the type of things I can scrounge from the
junk-box on on Sunday afternoons, or late at night. Also they are a tad
expensive.
To overcome this, I have built this negative voltage generator a few times.
There is nothing new about the design, I first saw it years ago, but it has
served me well. I built one and measured the output and set up this page so I
wouldn't keep losing the details. Also, who knows, you might find it
useful.
Note that the circuit can be used to generate 2 times the supply rail, by
reversing the polarity of the diodes and the output capacitor.
The circuit:
The figures:
| Load (Ohms) | I in (mA) | V out (V) | Iout(calc)
(mA) |
|---|
| infinite | 20 | 10.9 | 0 |
| 2200 | 30 | 9.1 | 4.1 |
| 446 | 60 | 8.5 | 19 |
| 248 | 95 | 4.3 | 17 |
| 240 | 100 | 8.2 | 34 |
| 96 | 175 | 6.2 | 54 |