Test a soft-latching power switching circuit.
Here's a quick video of the circuit in action:
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
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.
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
- EEVblog #262 - World's Simplest Soft Latching Power Switch Circuit - original version of this circuit
- BS250 datasheet
- 2N3904 datasheet
- Push On - Push Off Transistor Switch - at a glance, I think a very similar circuit but with slight variations and higher parts count. Original circuit is from Talking Electronics