#175 Soft-latch Switch

Test a soft-latching power switching circuit based on a p-channel MOSFET.

Build

Here’s a quick video of the circuit in action:

Notes

This is a neat circuit with low component count that provides an on/off toggle with a momentary push-button. It would be suitable for switching a power supply for a circuit.

Dave Jones explains it best in EEVblog #262 - World’s Simplest Soft Latching Power Switch Circuit, but the essence of the circuit is this:

When Q1 (power) MOSFET is off:

pressing the switch pulls up the base of Q2 NPN
Q2 turns on
Q2 in turn pulls down Q1 gate and Q1 turns on
Q2 kept on via R2

When Q1 (power) MOSFET is on:

Q3 NPN turns on once C1 is charged (per R4, C1 time constant)
with Q3 on, pressing the switch pulls down the base of Q2 NPN
Q2 turns off
Q1 gate pulls high via R1 and turns off

Component Selection

The R4/C1 time constant can be adjusted to debounce the switch and/or enforce a minimum “on time” before it is possible to turn the circuit off.

I have a BS250P p-channel MOSFET on hand; it’s not really for power applications but I’m using it here as Q1 just for demonstration purposes.

Performance

What this circuit does well:

when off, load current cust clean to 0mA (perhaps fractional µA at most)
the R/C network does effective button de-bouncing

A couple of things that could probably be improved:

the component values used here work OK at 5V, but not at higher input voltages like 9V (can’t turn off)
the capacitor holds its charge long after the circuit is powered down (slow to discharge through Q3 base)
the circuit starts up “ON”, though theoretically it shouldn’t. I suspect this is due to residual capacitance in C1.

So perhaps I’ll need to try some improvements or test some alternative circuits like this one from Talking Electronics

Construction

Breadboard

The Schematic