Hi all

Looking to switch point A to 48v/0v/Off
Point B and C from output pins on arduino

Wondering if the control for the P channel MOSFET will work

VGS +/-20v
Vgs(th) -2v to -4v


The P-Mosfet should work, however BC548 is overstressed C-E with the 48V when the mosfet is off.

Argh yes my mistake BC546B which had C-E limit of 65v

Thanks for your response


Switching losses of the p-channel mosfet will be high with <1mA gate drive.

Reduce values of resistors ? 10k down to 1K 47k down to 4K7

Depends on what you're switching (current), and how fast (PWM frequency). Leo..

Current <200mA
Its for a Relay coil I need to change polarities on, this circuit would form half of a H bridge


Was planning to apply PWM to N channel to reduce to 24v when needed

Slower switching <200mA seems ok for a TO-220 mosfet.

Not sure why you want to reverse polarity of a relay coil.
Another option could be to reverse coil polarity with a small 5volt DPDT relay, and switch to ground (PWM) with a single n-channel fet.

My oringnal design uterlised relays in place of the P channel Found it difficult to source a 5v relay that can switch 48v DC I crudely parraled up 2 relay contacts get gain larger contact area and seems ok although I'm unsure what life I will get out of the contacts I used the N channel to switch off the current before opening or closing relay contacts to help

So decided on redesign using mosfets

Also I'm assuming the internal mosfet diodes will cope and deal with the back emf from the coil ?

The small DPDT 8-pin 1Amp relays I used in a project were rated for 60volt DC standoff (when they don't have to switch any current). I think lifetime is >100.000 switch times. Use a relay with datasheet. A single n-channel fet needs a kickback diode between drain and 48volt (cathode to 48volt). A full h-bridge can utilise the integrated diodes of the mosfets. Leo..

wellyb00t: Reduce values of resistors ? 10k down to 1K 47k down to 4K7

That requires a >=0.5W 4k7 resistor or it will overheat.

I'd suggest replacing the source-gate resistor for the p-FET with a 12V zener diode, then you get full drive even if the supply is lower than it should be and benefit from zener protecting the FET from spikes.