PC PSU 12 volt + 4n35 Optocoupler


I was wondering if it is possible to put a 4n35 optocoupler at a psu line of 12v, 25Ampere, (at the black wire)?

Will the optocoupler burn? If so, what's a better way to control a powerline of an pc psu. Because I want to controll the fan which is connected with the 4 pins black/yellow 12volt line.

An opto coupler will not take anything like that sort of load. I assume that the power supply rating is 25Amps, rather than the fan. Either way you need a suitably rated FET between the opto isolator and the fan.

One more question. The fans connected to the psu, use only 0.07 A. Now I'm not sure, how much pc psu delivers. The fan is connected to the yellow cable. What normally is connected to a harddrive or cd-rom drive, with 4 pins red-black-black-yellow.

So if the fan can use only 0.07 A, why isn't it possible for the optocoupler?

So if the fan can use only 0.07 A, why isn't it possible for the optocoupler?

Well you were asking at first about switching 25A, but for a fan you still need a extra transistor because a typical opto coupler transistor can not supply 70mA. Check the data sheet for the specific type of opto isolator you want to use. For the 4N35 the data sheet says the maximum output current is 50mA.

Note also that the 70mA quoted is probably the running current, typically this is much lower than the start up current or the stall current.

I understand, is this a good plan?

12Volt+ > fan+ > fan- > mosfet > 12Volt-

mosfet is controlled by optocoupler

arduino > optocoupler > mosfet > optocoupler > arduino

Tonight I will try to Fritz this...

Worst scenario will be, that the mosfet will burn??

Note you also need a resistor between the arduino and the opto coupler. This is because it is really just and LED and you need a current limiter. You also need a pull up resistor on the gate of the FET to the +12V.

everthing works fine, except, it wont turn off. When a put the multimeter on the mosfet G and S. It goes off and on. Maybe it's the pull up resistor what i need?

It seems, that the voltage stays high when its put high. The mosfet acts like a switch, when high i disconnect the gate, it stays high, when low, i disconnect it stays low. So why doesn't it go low, but only high. I just started with circuits. I'm a programmer, sorry for the stupitty :roll_eyes:

Yeah, I needed a pull down resistor 2x330Kohm from mosfet G to 9v ground.

is the optocoupler isolated of amperage as drawn at the diagram? Can pin 6 be connected to the arduino 9v?

First off all, I want to thank Grumpy_Mike for his support.

Can anybody confirm if the ampere is isolated to the optocoupler with this diagram? Because the + of psu is connected to pin 6 of optocoupler.

mosfet optocoupler psu fan control with rpm.jpg

This pic is better.

mosfet optocoupler psu fan control with rpm.jpg

I think I see several wiring errors, but I refuse to try and make sense of such pictorial pictures, when what is really required is a proper schematic drawing. However hang on for others that might have a higher threshold of pain the I do.


That circuit is not right. There is a common ground on both sides of the circuit, this means it is not isolated. As lefty says it is very difficult to follow a physical layout and generate a schematic from that. In fact is is impossible for anything but the most trivial circuit. You must have a schematic because you made it (you can't pretend to make anything without a schematic) so could you post that. Even a photo of a hand drawn one would be fine. It is much easier generating a physical layout from the schematic. But you don't look like you have got it quite right yet. Also is the FET part number correct?

I'm just started learning electro. I will post a schematic tonight. The fet name is right I think, I have to double check maybe. So if I understand, common must be seperated. But the + also goes to the optocoupler, I think thats also not right.

But the + also goes to the optocoupler

The opto coupler has two sides, it is quite alright for the + and ground to go the opto providing that only fan and fet connections go to the output side and only arduino and arduino ground go to the input side.

I manage to remove the ground from arduino to the FET (IRFZ44N). I also generated a schematic. I hope we can avoid the + from psu to optocoupler.

mosfet optocoupler without rpm dc_schema.jpg

Yes it is all wrong on the fan side.

Don't connect anything to the base of the opto transistor. Connect the emitter to the gate and the collector to the +12V, then a pull down from the gate to ground. The fan should be connected to the +12v with the other end to the FET drain. Finally the gate pull down and the -12V to the FET source.

Finally the gate pull down and the -12V to the FET source.

This part I don’t understand.

Can the +12 to optocoupler collector be avoided? Because +12 has 30 A.

mosfet optocoupler without rpm dc_schema.jpg

By the way, is it save to solder the 4n35. Does the high temp destory the IC?

Yes that new circuit is what I meant. The gate pull down resistor (the resistor's end not connected to the gate) and the source and the -12V all to the same point just like you have.

Can the +12 to optocoupler collector be avoided? Because +12 has 30 A.

The fact that the 12V supply can provide 30A should be of no concern to you nor the transistor. The current drawn will only be in accordance with Ohm's law, and if that doesn't dictate 30A then 30A will not flow. The important thing is that the 12V supply is totally isolated from the arduino circuit.

is it save to solder the 4n35.


Does the high temp destory the IC?

Only if you take more than ten minutes with the soldering iron on it.