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Topic: PC PSU 12 volt + 4n35 Optocoupler (Read 6401 times) previous topic - next topic



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?


Feb 02, 2011, 12:21 pm Last Edit: Feb 02, 2011, 12:24 pm by Grumpy_Mike Reason: 1
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  :smiley-roll-blue:


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.



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.

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