N-channel mosfet turn constantly on after a while

Thread Starter

Amir Mohammad Soltanpour

Joined Sep 10, 2017
23
Untitled.png
Hi;
N-channel mosfet (Q4) work fine for couple of time and suddenly turn on constantly.
I changed Q4 multiple of times and each time it work for couple of time and suddenly turn on constantly.
the output of mosfet is connected to a 2.5 amper 12 volt car windshield washer pump and the reason of C11 and R33 is because of water pump noise that cause microcontroller to dont work normal.
What could be the reason of this problem ?
 

crutschow

Joined Mar 14, 2008
26,812
Try adding a diode (e.g. 1N400x) form the MOSFET drain to the +12V (cathode to +12V) to protect against inductive transients from the pump motor.
 

dl324

Joined Mar 30, 2015
12,685
Back EMF from the relay coil is shorting the source/drain of the MOSFET. Put a snubber relay diode across the relay coil pump.

EDIT: Corrected text.

What is the purpose of C11 and R33? Why do you have the two inverters?

Well drawn schematics don't have unnecessary jogs or whitespace. Ground always points down and power (positive) always points up. There's no reason for the pump input to be rotated. We don't draw multi-colored schematics either.
 
Last edited:

Thread Starter

Amir Mohammad Soltanpour

Joined Sep 10, 2017
23
C11 and R33 is the snubber circuit.

Back EMF from the relay coil is shorting the source/drain of the MOSFET. Put a snubber relay across the relay coil.

What is the purpose of C11 and R33? Why do you have the two inverters?

Well drawn schematics don't have unnecessary jogs or whitespace. Ground always points down and power (positive) always points up. There's no reason for the pump input to be rotated. We don't draw multi-colored schematics either.
 

hoppyest

Joined Nov 20, 2018
3
Hello
Most likely the MOSFET is destroyed. Replace it with a new one. Before applying power to the circuit, put a diode across the inductive load. Connect the diode cathode end to + (supply voltage). Connect the diode anode to the MOSFET Drain terminal. This is standard practice when switching inductive loads and is done to prevent inductive spikes destroying the device that is switching them. This is not a high-speed switching circuit so a conventional silicon diode will work.

Best regards

hoppyest
 

OldTech

Joined Jul 24, 2009
3
Hello
Most likely the MOSFET is destroyed. Replace it with a new one. Before applying power to the circuit, put a diode across the inductive load. Connect the diode cathode end to + (supply voltage). Connect the diode anode to the MOSFET Drain terminal. This is standard practice when switching inductive loads and is done to prevent inductive spikes destroying the device that is switching them. This is not a high-speed switching circuit so a conventional silicon diode will work.

Best regards

hoppyest
View attachment 222488
Hi;
N-channel mosfet (Q4) work fine for couple of time and suddenly turn on constantly.
I changed Q4 multiple of times and each time it work for couple of time and suddenly turn on constantly.
the output of mosfet is connected to a 2.5 amper 12 volt car windshield washer pump and the reason of C11 and R33 is because of water pump noise that cause microcontroller to dont work normal.
What could be the reason of this problem ?
How much current is the pump drawing? Is the MOSFET on a heatsink? If so, how hot is it getting when the pump is running?
Measure the starting current of the pump motor. How much does it draw when it first turns on (a storage scope will help measure thee startup current).
 

Picbuster

Joined Dec 2, 2013
1,026
View attachment 222488
Hi;
N-channel mosfet (Q4) work fine for couple of time and suddenly turn on constantly.
I changed Q4 multiple of times and each time it work for couple of time and suddenly turn on constantly.
the output of mosfet is connected to a 2.5 amper 12 volt car windshield washer pump and the reason of C11 and R33 is because of water pump noise that cause microcontroller to dont work normal.
What could be the reason of this problem ?
Hi,
remove q5 r7 r6
connect collector q6 to r11 and run a 10K resistor from collector q6 to +12V
add a fast diode between pin 3 and 2 from q4.
the relay could put a pulse on the 12V. a filter should put in place.
a simple solution are two caps 47uf and 100nF parallel plus a diode to clamp negative pulses at the gate and separate relay power from the other electronics.
This fast diode will clamp a negative pulse caused by the relay at the 12 v line.
( this pulse could easy reach +/- 50V)

Picbuster
 

Wolframore

Joined Jan 21, 2019
2,245
Or just get the right MOSFET.. like a IRL640 if you’re set on that one. Personally I would get one that’s better than that.
 
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