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 6th Apr 2004, 05:03 #1 Beefy What's a Dremel?   Join Date: Apr 2004 Location: Toronto, Ontario Posts: 2 4N25 Optoisolator Limits? 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?
 6th Apr 2004, 08:22 #2 whypick1 The über-Pick     Join Date: Oct 2002 Location: In front of my monitor Posts: 2,014 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. __________________ If you're trying to do electronics mods without a multimeter, GET ONE NOW! If you don't have enough money to buy one, STEAL ONE NOW!
 6th Apr 2004, 08:47 #3 DanielArdelian Minimodder   Join Date: Mar 2004 Location: Romania Posts: 45 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
 6th Apr 2004, 16:50 #4 cpemma Ecky thump     Join Date: Nov 2001 Location: S.Yorkshire, OK Posts: 12,328 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). __________________ Nobody's perfect. But being a Yorkshireman is as close as you can get.
 6th Apr 2004, 18:34 #5 Beefy What's a Dremel?   Join Date: Apr 2004 Location: Toronto, Ontario Posts: 2 Thanks guys! I'm actually in a position where it's either relays or Optoisolators so I geuss I'll take the relay route then. Too bad I can't find a company that will give me samples of some relays like I can get the ICs