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?
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.
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
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).
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
