Big MOSFET slide switch, current draw by Q3 gate

Support Motor controllers/drivers and motors
1

Hello,

I am turning on/off the mosfet slide switch from a microcontroller, that is powered by a coin battery.

mosfet
mosfet695×408 19.5 KB

The problem in my setting is Q3, having built in bias resistors, provides a 20kohm escape route for the gate voltage to ground, which drains number of microamps, which upsets my scheme.

I have replaced the BJT transistor, (Q3) by yet another mosfet, BSS138. And it seems to work well, is able to switch the p-channel mosfets Q1,Q2, without drawing any continious current, since the gate of mosfet acts capacitive.

I am requesting your comments on the subject. What could go wrong with the replacement.

Best Regards,
C.

2

Hi.

Substituting that Q3 BJT with a MOSFET should generally work. My main concerns would be having some pull-down on the gate (can be much bigger than 10k for the MOSFET) to keep it from floating and making sure the voltages you apply to the gate are safe for the MOSFET you use. Since you are in control of that line with your microcontroller, you should be fine.

-Claire

3

Thank you very much claire. Indeed, this is what I have exactly done, but with a 10M resistor grounding the gate.

Best Regards,
Can