hello all,
please can you helpme with a simple opto isolated mosfet circuit, to use a low voltage to turn on a high voltage.
the attached circuit does not seem to work .
as a test i used a red led with 1k resistor.
the mosefet is always on ??? why is this it should be grounded throught he 10k resistor.
i have tried several mosfets with the same result.
utimately i want to use it to open a solenoid valve.
if you have better circuit please couldyou supply it.
the opto isolator is working , i checked with the led.
thank you
dave

 

The MOSFET is normally ON. To turn it OFF you need to supply a logic High to the input of the Opto Isolator!
To reverse the action, put R2 in the emitter of the Opto Isolator, ground the other end, and connect the Opto's collector to +12V. You won't need R4!

 

Your resistor network of R2, R3 and R4 are making a simple voltage divider, putting 10k/12.3k ratio of 12V on the gate (9.75V), until the opto is triggered.
If you use a P channel MOSFET, the circuit would likely work.
Or try something like this:
https://forum.arduino.cc/t/optocoupler-and-n-mosfet-schematic/1243536

 

The MOSFET is normally ON. To turn it OFF you need to supply a logic High to the input of the Opto Isolator!
To reverse the action, put R2 in the emitter of the Opto Isolator, ground the other end, and connect the Opto's collector to +12V. You won't need R4!

thanks, i'll give it a try
dave

 

thanks, i'll give it a try
dave

hi,
i am afraid that did not work either and i tried several mosfets. it always seems to be on without any input from the optoisolator ??

the original circuit came from
https://www.learnabout-electronics.org/Semiconductors/fet_06a.php

whats going on?????

 

Hi dave,
Have you got the +5V input permanently connected??
E

 

Hi dave,
Have you got the +5V input permanently connected??
E

no

 

whats going on?????

I'd say you have a faulty part or a miswire!!!!!

 

hi,
i am afraid that did not work either and i tried several mosfets. it always seems to be on without any input from the optoisolator ??
whats going on?????

Circuit should look like this:

 

Circuit should look like this:
View attachment 354999

ok,
i will give that a go and report back
dave

 

ok,
i will give that a go and report back
dave

THANK YOU SGHIOTO, IT NOW WORKS PERFECTLY
EXCELLENT !!! MANY THANKS
DAVE