Drive a relay with PIC micro using a high side n-channel MOSFET

Thread Starter

roxi60

Joined Sep 2, 2018
44
Hello.
I would like to drive a relay 12 V, with a pic micro 0-3.3V, using a N-channel high side mosfet.
I tried different values of R and put NPN transistor but I couldn't manage, since I can not find a solution to have a switching voltage on gate that is higher (I guess) then +V, in order to switch on and off the mosfet.

Any suggestion or example circuit will be very appreciated.

Many thanks
 

Attachments

ericgibbs

Joined Jan 29, 2010
8,722
hi roxi,
It is not possible to drive a high side NMOS with its Drain to 12V with a3v Gate voltage.
E
 

LesJones

Joined Jan 8, 2017
2,317
You will need the drive signal to go about 10 volts positive of you positive supply rail. If you use a logic level device about 5 volts above the + supply rail would work. Why do you want to use an N channel mosfet AND have the load on the low side ?

Les.
 

bertus

Joined Apr 5, 2008
20,044
Hello,

To drive a high side N gate mosfet, the gate voltage needs to be over the supply voltage of the circuit.
The circuit you posted witll not do this.
There are special high side drivers with a boost circuit to provide the higher voltage for the gate.
Here is one of the many examples:
https://www.analog.com/en/products/lt1910.html

Bertus
 

Thread Starter

roxi60

Joined Sep 2, 2018
44
Many thanks for your replies.
I would like to use this because I burned a high side P channel mosfet and I have now only n-channel mosfet, and I don't want to change the circuit, if possible.
Thank you.
 

LesJones

Joined Jan 8, 2017
2,317
It is not possible without changing the circuit. Even if you use a P channel device the drive signal must have a high level close to the supply voltage to switch it OFF.

Les.
 

Thread Starter

roxi60

Joined Sep 2, 2018
44
Thank Les.

I found in the forum such a circuit, but I don't think it is correct isn't it?Capture example driving a relay with high side nmosfet.PNG Anyway, I will change the circuit or the mosfet type.

Thank you.
 

ericgibbs

Joined Jan 29, 2010
8,722
hi Roxi,

The control signal from the MCU will be inverted,
MOS On when Ardunio pin is Low.
E
Corrected:
 

Thread Starter

roxi60

Joined Sep 2, 2018
44
Thanks Eric.
I saw the correction too (gate should be higher, as cited in Les #3rd)
 

ericgibbs

Joined Jan 29, 2010
8,722
hi,
When using NMOS I try to place the load in the Drain circuit, ie: low side drive.
It makes the circuit simpler and keeps the logic sense as Ard pin Hi = MOS On.
E
 

BobTPH

Joined Jun 5, 2013
2,021
If all you are doing is driving a relay, it is silly to use a high side switch.

Bob
 

dendad

Joined Feb 20, 2016
2,986
I made a Hula Hoop that has addressable LEDs in it. It is driven by an Arduino Mini. To turn it on, a push button shorts out an N channel FET on the high side. When the Arduino starts up, a 50% PWM signal generates the high side FET gate drive voltage via a diode/capacitor voltage multiplier. The number of stages in the multiplier needed depends on the processor volts and the needed gate volts. This works pretty well, but is only good for on/off control, not if you want to PWM the FET.
 

ScottWang

Joined Aug 23, 2012
6,820
There are some ways that you can pick one you like.
1. A low Vgs as 2.5V N ch MOSFET.

2. One stage NPN bjt on low side → P ch MOSFET

3. One stage NPN bjt on low side → one stage PNP bjt on high side → N ch MOSFET (Vgs±10V)

SQ2310ES -- Nch, 20V/6A, Vgs=4.5V/30mΩ, Vgs=2.5V/34mΩ, Vgs=1.5V/42mΩ.
Si2300DS -- Nch, 30V/3.6A, Vgs=4.5V/68mΩ, Vgs=2.5V/85mΩ.
 

Thread Starter

roxi60

Joined Sep 2, 2018
44
Thanks a lot Eric, Bob, Dendad, Scott, Bordodynov, for your kind replies!
 
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