opto-isolatorplc
I have an input of pulse width 5 ms and which is fed to a Buffer IC and then the output of the buffer is FED to an opto isolator whose collector is pulled with a pull up of 7.5k resistor with an 24 V source. When no load is connected the output is fine but when connected to a load (PLC input pin) The output drops to 10V.
We increased the pull up to 15K the output is the same.
The opto used here is a 4N35
Please suggest some points to TAP on and a solution for the same.
Thank you
It's not getting to zero because the Current Transfer Ratio of your optoisolator is not high enough. You need to have about 3-5x better to be safe. The lowest grade of 4N35 has a CTR of 40%, meaning you might be able to drive a 4-5K pullup safely. So your pullup is too low.
The reason your rise time is so sluggish is that the pullup is too high for clean square waves at 30kHz. Here (from a datasheet) is the rise time vs. load resistance:
Phototransistors are just not very good for this sort of application. Fortunately, there's a very similar (in ease of application) part that should work for you- the "Logic Output" optocoupler with Schmitt trigger action, for example the H11L1M.
This part is okay with a 12V supply (operating range to 15V), and will sink 16mA with less than 400mV drop, so your 12mA from the 1K pullup will be fine. Turn on and off times are 4usec maximum (1.0/1.2 usec typical) and the rise and fall times are 100nsec typical.
Your two configurations are equivalent in output drive, although obviously the output logic is inverted between them.
The decision therefore comes down to which polarity is more fail-safe and idle current issues. Config 1 fails to high and takes active current to be low, with config 2 the opposite. Use whatever works best for your system.
One reason people knee-jerk to config 1 is that a switch between a line and ground with a pullup is more common. It is often convenient to be able to ground one side of the switch. Far more things come with built-in pullup resistors, as apposed to built-in pulldown resistors. If this signal were going into a microcontroller, for example, then R1 could be internal to the micro and you'd use config 1. In that case logic polarity doesn't matter since micros can generally deal with low or high equivalently.
Best Answer
Your PLC may have an opto-coupler on its input which is connected to 'ground' (common terminal) and needs to be pulled up by your opto-coupler. If so then you should wire the Collecter to +24V, and the Emitter to the PLC input. A pull-down resistor may not be necessary.