Quote Originally Posted by boldford View Post
I did mean R2. I'm sorry for not being clearer. Many thanks for the information. I'll try with a 120R in parallel using ugly construction and report my findings. If that works I might try find out if I can still solder SMT devices.

What would you recommend for the I/P side? I.e R1.
Whats in there already? and what version on the opto is it? and whats the isolated supply Vcc?

The reason being, when you reduce R2 to a lower resistance, you increase the collector current therefore you'll need to increase the drive current (If) to the LED. Current I'd guess R1 is 1K, which gives a forward current of (5 - 1.4)/1000 = 3.6mA. with the lowest spec'd opto the transfer ratio (CTR) is 50 which will give a collector current of 130mA but obviously this is limited by R2 to (assuming Vcc = 5v and R2 = 100R) (5 - 0.4)/100 = 46mA so R1 won't need to be changed. However you may find slew rate improves if you reduce it to increase If to, say, 10mA, which needs R1 = (5 - 1.4)/.01 = 360R, so if R1 = 1k try a 560R in parallel.

The fact that these are both 1k may well be an example of offsetting cost of manufacture v breadth of applicability. by making them all 1k it reduces inventory, handling and piece cost, but means there are performance limits that are probably OK for 95+% of cases... you haven't actually said if this is giving you issues, you only asked about the likely maximum speed.