# Electronics 4N25 Optoisolator Limits?

Discussion in 'Modding' started by Beefy, 6 Apr 2004.

1. ### BeefyNew Member

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Greeting fellow modders.

I'm building a multi-voltage fan circuit that requires the isolation of both sets of fans. I planned on using 4N25 Optoisolators from Fairchild Semi-conductor but I have can't seem to figure out what the max current I can pass through each 4N25. I figure the max current for any circuit will be about 1.5-2 amps (2 high flow fans).

What do you think boys? Will I have to consider using relays instead?

2. ### whypick1The über-Pick

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Definitely need to use relays. Max current the emitter can handle is 100mA. You'll be hard-pressed to find 80mm fans that only draw .1A, and it gets more and more impossible as the fan size increases.

3. ### DanielArdelianNew Member

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4N25 are very low power optocouplers. These can handle only a couple of milliamps output current (< 10 mA).

If you need to drive a relay, use 4N33 optocouplers, those have a darlington pair for the output transistor and can handle up to 100 mA continuous collector current. Do a google search for "4N25 datasheet" and "4N33 datasheet"...

The 4N25 has a "transfer current ratio" of about 20 %, which means that
100 * (IC / IF) = 20 => IC ~= 0.2 IF at best.

IC = Collector (output) current
IF = Forward (input) current

For IF = 10 mA (typical value) => IC = 2 mA

4. ### cpemmaEcky thump

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It's easy enough to boost a 4N25 with a small transistor, R1 above limits the opto Ic to about 2.3mA which will supply at least 230mA to any small fan or relay used as load R2. A relay will need an anti-back-EMF diode across the coil (band end to positive).

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