Hello all,
I have on my hands, a beard trimmer with partially dead electronics. It has a NiMh 3.7 volts cell and the charging circuit works ok.
The other part which has failed has electronics with ON/OFF and Speed Control.
I intend to wire up some basic electronics with Tactile Switches for ON and OFF and no speed control.
What I am wanting to know is whether the SCR can be Commutated by a MoSFeT as shown.
Your suggestions are welcome.

The Push Button on the Left is ON which triggers the SCR to conduction.
The Right is OFF which is to switch on the MoSFeT to full conduction and Commutate the SCR.

 

If you use a push OFF switch in series with SCR, maybe it will be easier and no MOSFET needed.

 

If you use a push OFF switch in series with SCR, maybe it will be easier and no MOSFET needed.

I used the same idea in a short circuit protection for a PSU.

 

This circuit from EEVblog will do the job without the 1volt or so loss over the SCR.

 

If you use a push OFF switch in series with SCR, maybe it will be easier and no MOSFET needed.

This was my first choice. But tactile switches "Push to Break" is not easy to come by ( here), and the permissible current is only about 30 mA. I need to handle about 250 mA.

 

This circuit from EEVblog will do the job without the 1volt or so loss over the SCR.

Another choice, Thanks.
Won't the 2N3904 on the Gate of the MoSFeT need a Pull Down resistor on its Base? To avoid any Collector leakage?
And how does the MoSFeT shut Off?

 

This circuit from EEVblog will do the job without the 1volt or so loss over the SCR.

Ooops.... I forgot the SCR loss - cannot afford it.
Dropping this idea

 

Use a SR or D latch to switch the MOSFET.
There is a post on that subject at the moment:
https://forum.allaboutcircuits.com/threads/momentary-switch-to-power-on-a-circuit.201680/

 

Below is the LTspice sim of an alternate action switch using one P-MOSFET and one BJT:
It draws only a small leakage current from the battery when OFF.