Friends, I am very new to electronics and I am trying to design a simple switch with a transistor. Following is the circuit:

Switch circuit

The input is a square wave generator with positive and negative cycles but the output current is zero when the cycle is negative. Can anybody please help me redesign this to switch a negative current in the output when the input is negative? ie, the output current should be positive when the input is positive and the output should be negative when the input is negative.

  • \$\begingroup\$ Why don't you use a solid state relay (SSR)? \$\endgroup\$
    – Andy aka
    Aug 26, 2014 at 10:09
  • \$\begingroup\$ @Andyaka I don't see how an SSR makes his life easier? \$\endgroup\$
    – Dzarda
    Aug 26, 2014 at 10:18
  • \$\begingroup\$ For driving loads with reversible current, a H-bridge is needed. But to make use of negative input voltage, there must be a negative voltage source (at least a small one). \$\endgroup\$
    – venny
    Aug 26, 2014 at 10:45

2 Answers 2


After a lot of research and experiments with a software simulator, I could be able to redesign my circuit like this-

enter image description here

It works perfectly in the simulator.

  • \$\begingroup\$ Normally the PNP transistors connect to the positive supply and NPN to the negative. This way you don't have to raise/lower the bases so much in order to make them conduct. \$\endgroup\$ Nov 5, 2014 at 6:19
  • \$\begingroup\$ @IgnacioVazquez-Abrams - As shown the transistors are emitter followers, applying current gain and high drive capability, at the cost of the loss of x Vbe voltage drop at each end of the load. This is a valid and common way to provide power drive with an H bridge. The method you suggest is also useful but has different characteristics and different issues to be addressed. \$\endgroup\$
    – Russell McMahon
    Apr 5, 2015 at 16:16

Since your input only has two states (+ and -) you can use a simple H-bridge to do this.

The H-bridge must tolerate minus inputs and interpret it as a '0'.

For example:-


Driven by this:


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ The basics are fine here, but this circuit is going to have some make before break issues. \$\endgroup\$ Aug 26, 2014 at 11:41
  • \$\begingroup\$ @OlinLathrop Ah, at 40Hz, a little shoot-through shouldn't make much difference. \$\endgroup\$ Aug 26, 2014 at 12:08
  • 1
    \$\begingroup\$ I would suggest you download the free LTSpice and play around with the simulations. It is well worth getting to learn a SPICE package. \$\endgroup\$
    – user32885
    Aug 26, 2014 at 13:00

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