2

I am using a 4N25 (datasheet) in my project and just had a few questions about it. First what I do not understand is that pin 6 is a base pin, but whenever I look up a tutorial for how to use it online they just ignore it. I was wondering what the purpose of that is and how to use it (and if I need to use it). Secondly, if I do ignore it, will I be able to control 12V 70mA with an Arduino digital pin?

TLDR; What is the base pin for? Can I use an arduino to control 12V 70mA load?

Thanks in advance!

Jared Cohen
  • 248
  • 1
  • 12

1 Answers1

4

"Can I use an arduino to control 12V 70mA load?"

No, not without some additional circuitry around the opto.

With a CTR of 20%, and needing to sink 70 mA on the output transistor, you would have to pump 350 mA through the input diode. The absolute maximum forward current for the 4N25 is 60 mA. No can do.

enter image description here

SteveSh
  • 10,726
  • 2
  • 15
  • 31
  • Thank you for the answer! Do you mind explaining what the CTR means? One more thing I'm confused about is why the input current is higher than the output current with a transistor on the output? – Jared Cohen Apr 11 '20 at 23:58
  • Think of the CTR (Current Transfer Ratio) as similar to the beta of a normal bipolar transistor. Only in the case of this opto-coupler, the beta (CTR) is 0.2 (min). – SteveSh Apr 12 '20 at 01:00
  • what about when the transistor is in saturation? could it handle the 70mA then? – Jared Cohen Apr 12 '20 at 01:05
  • 1
    If CTR is 50% then the LED needs 140mA to get 70mA without saturation. In saturation the CTR is lower. The datasheet only shows LED current in saturation up to 60mA, and already CTR is less than 50% of nominal (ie. <30mA out when CTR is 50%). You won't get 70mA from this optocoupler. Use it to switch a power transistor, or use a PhotoMOS relay (which is like an optocoupler except it has photodiodes and a FET in it). – Bruce Abbott Apr 12 '20 at 02:41
  • No. Look at the Collector current row in the Abs Max Table. For DC, the max current is 50 mA. You want to back off that number for a robust design. – SteveSh Apr 12 '20 at 02:43