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Optoisolator 4N25 understanding the datasheets

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I've difficulties understanding what the datasheets says ... hope someone can help to explain...

I want to use an Arduino microcontroller digital pins 5V to control a 12V PC fan.

I've a 220 ohm between pin 1 & 2 ( I just assume it is like driving a LED to limit the current ) from a digital pin of 5V to the ground.


Looking at the Absolute Max Ratings :-

Input :-
Reverse Voltage of 5V.
Forward current of 60mA.

Output :-
Collector emitter breakdown voltage, 70V
Emitter base breakdown voltage, 7V
Collector current, 100mA

The electrical characteristics states forward voltage of 1.5V max and reverse current of 100 microA.

What does it means by reverse voltage and forward current ?
Then there is something about forward voltage of reverse current...

What are breakdown voltage ? It hv something to do with the NPN transistor inside the optoisolator...

I'm totally confused about the forward voltage/reverse current...

I see a lot of these specs in datasheet...

Do I need a MOSFET to drive the 12V fan ? Any suggested common part number for the MOSFET.. ? I found a IRF630 N channel mosfet..

I also read the 4N35 but they looks the same as the 4N25...

Thanks  :)

The forward voltage of 1.5V max means that the led will drop < 1.5V when its running (probably around 1.2v).
Because they use IR leds in optos the drop is quite low. Red leds are a bit more, (2v) and other colours even more. its just a property of the type of led.

The forward current of 60mA is the max you can feed into the led without damage. The more current you feed into it the harder and faster it will turn on the transistor inside the opto. You dont need 60mA,  5 - 20mA is perfectly fine for what your doing.

You can pretty much ignore the reverse voltage, its a limit for voltage around the wrong way that will damage the diode. Since you're applying the voltage in the forward direction (to light the led) you dont need to worry about it.

The transistor breakdown voltage is when the junction inside starts to breakdown from too much voltage. You want to keep the voltages less than this. That wont be a problem for you as you're only switching 12V using the opto-transistors collector and emitter, a collector-emitter breakdown of 70V is far higher than 12v.
Most of the time when you use an opto you dont even connect the base of the opto-transistor, so the emitter-base breakdown voltage isn't important either. But if you were using it you'd want to make sure you didnt put more than 7V into it. Since transistors turn on at around 0.7V you really never need to put anything like 7V into their base.

The transistor inside the opto has a max collector current of 100mA, so if your fan takes less than 100mA you can wire it directly to the opto-transistor. If the fan needs more you can add an external transistor or fet to connect power to the fan and use the opto-transistor to turn off/on this transistor or fet.

One thing to remember is that as you connect transistors to other transistors/fets in this way you invert the logic.  If you had the fan directly on the opto's transistor and you fed some power into the opto's led the fan will spin as expected. But if you add a 2nd transistor in the normal way the opto's transistor will turn "OFF" the bigger transistor.
So opto transistor on = 2nd transistor off. and vice versa.
Meaning you will have to program your micro to work in reverse and set the pin high if you want the fan off. Its easy to do, just something to be aware of.


The datasheet said that it will only do 50ma constant or 100ma for less than 1.0ms. I would not use it at all. I would go with a darlington array like a http://focus.ti.com/lit/ds/symlink/uln2003a.pdf
500ma and will give you 8 outputs for other case mods like rgb led light's and other stuff.

Just remeber that the darlington switches the neg side of your power circuit so you would wire + to fan and neg of fan to 1-7C and Com to -

Arduino pin to 1-7b and arduino neg to e and you can tie both neg together if you like.

@PSI, thank for you the details explanations... yes, the reverse voltage was confusing me most of the time..
glad that you helped me clear the air...

What MOSFET part should I get for this purpose ?

@jimmy, thanks for the suggestions... I'll stick with optoisolator and MOSFET..

By the way, what is the best way to find common electronic parts numbers ?

For example, I want to find a MOSFET, I usually use element14 to do the search and narrow down... list all the possible parts and go downtown in local electronic stores ( there is a street that hv like 10+ electronic stores in the city ) and ask the availability of the parts... they usually carry common parts mainly for repair technicians to do part replacement...

It is much cheaper than ordering online but these ppl at the store would not be able to consult me on what it does or the datasheets...

Something like an FOD3180 would be more robust and could eliminate the need for the output transistor.  1 FOD3180 probably costs less than a power fet and 4N25 plus lowers part count.

I didn't look into this carefully but I use the FOD3180 as my junk box optoisolator because I got a bunch for cheap back in the day and they are easy to implement with large output current (2A).


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