AndyUK's Build - 1.2x1.0m Gantry

[AndyUK]

Andy,

I think you understand a fair bit on the old electronic side. I was casually browsing this thread and one thing is niggling me. Opto isolators typically have a low CTR - usually significantly less than 1, often less than 0.5. That does mean that cascading optos is going to end (eventually, depending on CTR and number of optos in chain) with the latter optos not switching effectively. Really, the opto should be switching a high-z logic device rather than the relatively high current LED input on the successive circuit.

You link a rating of the output at 100mA - that /could/ just be the Ice(max) for the PT in the opto, rather than an actual expected switching current (reading the 4n25 data sheet this is the case).

I'm not suggesting this is your problem, but bear it in mind and measure the switching voltages along the opto-cascade,

Hi Doddy,

Thanks - that is quite high praise, but I'm still very much stumbling my way around so don't assume too much!

I hadn't really considered the CTRs of the optoisolators, however is this relevant if I'm effectively connecting each drive in parallel with oneanother? My intuition is only really concerned about the fact that I have the Optoisolator from the drive, and then the one in the OSSD output which as I understand is effectively acting as a transistor to ground. Upstream of the drive is merely the current limiting resistors and the +24V rail. My ~40mA came from the sum of 10mA per drive based on the current limiting resistors, and if the CTR<1 then I shouldn't have damaged anything staying under 70mA.

I suspect I've totally missed the point here though :) Please educate me further!