1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Electronics 4N25 Optoisolator Limits?

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

  1. Beefy

    Beefy New Member

    Joined:
    6 Apr 2004
    Posts:
    2
    Likes Received:
    0
    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. whypick1

    whypick1 The über-Pick

    Joined:
    17 Oct 2002
    Posts:
    2,014
    Likes Received:
    2
    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. DanielArdelian

    DanielArdelian New Member

    Joined:
    17 Mar 2004
    Posts:
    45
    Likes Received:
    0
    4N25 are very low power optocouplers. These can handle only a couple of milliamps output current (< 10 mA). :nono:

    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"... :thumb:

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

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

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

    cpemma Ecky thump

    Joined:
    27 Nov 2001
    Posts:
    12,328
    Likes Received:
    55
    [​IMG]

    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). ;)
     
  5. Beefy

    Beefy New Member

    Joined:
    6 Apr 2004
    Posts:
    2
    Likes Received:
    0
    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 :(
     

Share This Page