Mosfet driving circuit

[andrejsstrumfs]

Hello all!

I'm a newbie with electronics and i need a bit of reality check for my circuit. That's basically a n-mosfet low side switch driver. There's one thing - the low side power can go from 0 to -35 volts. I probably need to adjust the resistor values, but first i want to be sure that i got this on the right track. The switching is done manually, so no worries about large gate discharge resistor.

Thanks

[Wimberleytech]

Is R3 necessary?

[llkiwi2006]

At a worst case low side voltage of -35V, you will be pulling 30mA thorough your zener, and dissipating 0.9W in R4. Make sure to use a resistor capable of handling 1W for R4.

[fourtytwo42]

The switching is done manually, so no worries about large gate discharge resistor.

Well actually depending on the load current and nature the mosfet switching speed could be important and certainly in the present circuit the turn-off will be slow possibly leading to mosfet destruction.
Is there any reason the switch is in a 5V circuit ? why could it not switch the mosfet gate directly ? but perhaps you are intending to drive it from an mpu.
Your circuit shows both 24V & 0-35V, perhaps that is a nominal and a range ?
Finally is your load inductive ? if it is you will need a catch diode to protect the mosfet from avalanche.

[andrejsstrumfs]

Is R3 necessary?

You're right, not needed.

Make sure to use a resistor capable of handling 1W for R4.

Thanks. Larger R4 value would be better.

mosfet destruction.

Well, maximum current will be 3amps, i think should be ok.

Is there any reason the switch is in a 5V circuit ? why could it not switch the mosfet gate directly ? but perhaps you are intending to drive it from an mpu.
Your circuit shows both 24V & 0-35V, perhaps that is a nominal and a range ?
Finally is your load inductive ? if it is you will need a catch diode to protect the mosfet from avalanche.

Yes, i will drive the npn via 555 timer. Also that will allow turning on the mosfet when low side is all the way up to the ground?
The +24 is simply because Eagle cad had that label on component Bit misleading, sorry.
Yeah, catch diode is intended, just not shown here.
Thanks!