A soft latching power switch circuit can be made very simply, but many designs are complex. The concept of replacing the toggle switch with a simple push-on, push-off design is not new and there are endless ways to do it. This design has a couple of advantages compared to others. Some designs for this job are overly complex and most pull power in their off state. This soft latching power switch design pulls no power when it is off. It is a cute design suitable for various projects. There is a video describing it as the “World’s Simplest Soft Latching Power Switch Circuit” and I agree.
Two things make this circuit work well. Foremost is the MOSFET Q3. When off, the P channel MOSFET allows very little current to flow (typically on the order of one micro amp or less). Current does not flow through the gate of a MOSFET, so when R1 pulls the gate high to turn it off, there is no current path there either. As long as S1 is not pressed and Q2 is off, this circuit is very much off.
The second thing that makes the circuit work well is the node I marked as “A” in the schematic. Net node A provides the (magic) key to how the simple soft latching power switch circuit works. When the circuit is off (VOUT+ is low), net node A is at the VIN+ potential, ready to forward bias and turn on Q1 (and Q3) when S1 is pressed.
When the circuit is on (VOUT+ is high), after C1 is charged, R4 ensures Q2 stays on when the circuit is on. This causes net node A to drop to a near ground potential. This near ground potential is low enough to turn off Q1 when S1 is pressed again. Ah-ha! When the circuit is off, A is high and S1 turns Q1 on. When the circuit is on, A is low and S1 turns Q1 off. Brilliant!
The applications for this type circuit are numerous. You can use a soft latching power switch for battery or low voltage operated DC projects circuit like:
- LED Lighting
- Amateur Radio Equipment
- Cooling Fans for 3D Printers
- Solid State Relay Driver (for driving AC loads)
- Automotive electronics and actuators
- Device with an “AC brick” or “wall wart” that runs off DC
Should your circuit need need the ability to turn the soft latching power switch off (without you pressing the switch), there is an easy modification to do it. By connecting an N channel MOSFET or NPN transistor to the base of Q1, this circuit can even be turned off automatically. Once the new transistor is in place, the attached load sends a “high” signal to the new transistor. This pulls the base low and turns off Q1, turning off Q3, and the load.
I would like to try this circuit with Q1 and Q2 replaced by some small N channel MOSFETs, like a 2n7000 series. The MOSFETs do cost a little more, but the parts count will remain approximately the same. The advantage of switching to MOSFETs here would be a slightly lower power dissipation in Q1 and Q2 when the circuit is on. This might be more important for battery operated circuits using this type switch circuit as their front end.