bit-tech.net

Go Back   bit-tech.net Forums > Modding > Modding

Reply
 
Thread Tools View Mode
Old 6th Apr 2004, 05:03   #1
Beefy
What's a Dremel?
 
Join Date: Apr 2004
Location: Toronto, Ontario
Posts: 2
Beefy has yet to learn the way of the Dremel
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?
Beefy is offline   Reply With Quote
Old 6th Apr 2004, 08:22   #2
whypick1
The über-Pick
 
whypick1's Avatar
 
Join Date: Oct 2002
Location: In front of my monitor
Posts: 2,014
whypick1 has yet to learn the way of the Dremel
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!
whypick1 is offline   Reply With Quote
Old 6th Apr 2004, 08:47   #3
DanielArdelian
Minimodder
 
Join Date: Mar 2004
Location: Romania
Posts: 45
DanielArdelian has yet to learn the way of the Dremel
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
DanielArdelian is offline   Reply With Quote
Old 6th Apr 2004, 16:50   #4
cpemma
Ecky thump
 
cpemma's Avatar
 
Join Date: Nov 2001
Location: S.Yorkshire, OK
Posts: 12,328
cpemma can run Crysiscpemma can run Crysiscpemma can run Crysiscpemma can run Crysiscpemma can run Crysiscpemma can run Crysiscpemma can run Crysiscpemma can run Crysiscpemma can run Crysiscpemma can run Crysiscpemma can run Crysis


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.
cpemma is offline   Reply With Quote
Old 6th Apr 2004, 18:34   #5
Beefy
What's a Dremel?
 
Join Date: Apr 2004
Location: Toronto, Ontario
Posts: 2
Beefy has yet to learn the way of the Dremel
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
Beefy is offline   Reply With Quote
Reply

Thread Tools
View Mode

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +1. The time now is 00:36.
Powered by: vBulletin Version 3
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.