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Generating -5VDC from +5VDC

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  • Nicolae Sfetcu
    Generating -5VDC from +5VDC (From Richard Friesen) If you happen to have the March 1984 issue of Radio-Electronics, turn to page 78. This issue has the very
    Message 1 of 2 , Nov 27, 2001
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      Generating -5VDC from +5VDC

      (From Richard Friesen)

      If you happen to have the March 1984 issue of Radio-Electronics, turn to page 78. This issue has the very first instalment of Robert Grossblatt's "Designer's Notebook" column. In it, he shows a simple circuit which will supply a negative voltage, given a positive voltage. It's basically a 555-based oscillator, and a voltage-doubling rectifier. He claims the negative-voltage output should be good for about 60ma. No-load voltage should be pretty close to the input voltage (but negative), although the voltage will drop a bit, depending on the load. If you put +5V into the circuit, it'll give you around -5V out. load. If you put +5V into the circuit, it'll give you around -5V out. If the load makes the voltage drop too low (-3V or -4V), you could always just feed the circuit with a higher voltage (like maybe 9V or 12V) and then just regulate the output down to -5V using a 7905 regulator. I've used this circuit a couple of times for powering op-amp's, and it works great!

      I'm not that great at ASCII-art, but I'll give it a shot. If the following schematic doesn't make sense, let me know, and I'll try it again...

                +V
                 ^
                 |
         +-------+---+
         |       |   |                  -V Output
        R1       |8  |4        +----+--->    
         |    7 -------        |    |            Parts List:
         +-----|       |      D2    |            IC1 = 555
         |  +--|       |  +    |    |             R1 = 1.5K
        R2  | 6|  IC1  |---C1--+    |             R2 = 10K
         |  |  |       |3      |    |             C1 = 10uF,16V
         +--+--|       |      D1   C2             C2 = 22uF,16V
         |    2|       |       |    |+            C3 = 1500pF
         |      -------        |    |          D1,D2 = 1N4001 diodes
        C3         |1          |    |
         |         |           |    |
         +---------+-----------+----+
                   |
                  ===
                 (GND)
      

      Note: In the above "diagram", both diodes point down (the anodes are at the top). Also, watch the polarity of C1 & C3.

      The circuit is set up to oscillate at about 45kHz, with a duty cycle pretty close to 50%. None of the values of any of the parts are terribly critical, so if the capacitors or resistors are "in the ballpark", it should still work okay.

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