I am looking into using a 4N32 opto-isolator and am struggling to find information on its pin 6. This looks to be the base of the transistor (darlington). I see example circuits where they appear to leave this pin floating. I am curious to know if I should make a pulldown resistor connection. Anyone have any advise or insight as to what I should do. What pull-down resistance to use.

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    \$\begingroup\$ Although many circuits show this pin left un-connected, I would like to offer a word of caution about leaving the base floating. Humidity can cause a small conduction between collector (pin 5) and base (pin 6) causing a slight turn-on. In circuits using opto-isolators for MOSFET gate drivers, I have seen MOSFETs fail for this reason due to partial turn-on. You can get opto-isolators without the base being brought out to a pin, which IMO is a much better choice if not connecting the base. \$\endgroup\$ – Tut Jan 25 '16 at 18:33

Normally you would leave it floating. Adding a resistor to the emitter can speed up the isolator somewhat, at the expense of CTR.

You wouldn't (or shouldn't) be using a Darlington optoisolator if you cared about speed, but if you want to give it a try, something in the 100's of K ohms would be a starting point.

  • \$\begingroup\$ There have been previous queries in BJT circuits where a floating base leads to false turn on. Is it possible that a similar phenomenon occurs in opto circuits as well? \$\endgroup\$ – Stoic_beast Mar 3 '20 at 3:58
  • \$\begingroup\$ @Stoic_beast It's possible if there was enough DC leakage or EMI. It's not generally an issue. \$\endgroup\$ – Spehro Pefhany Mar 3 '20 at 4:19
  • \$\begingroup\$ Could you point out to any instances/references where this occurrence has been discussed \$\endgroup\$ – Stoic_beast Mar 3 '20 at 18:19

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