High frequency 24V to TTL voltage drop

[andrew_c]

Hi all. I wish to use a 24V NPN PLC output to control a 3kHz 50% duty signal to a 5V (TTL) stepper motor controller. Unfortunately I need to control about 10 signals and those 24V DIN rail optocouplers are looking rather pricey.

Is there a nice simple (with as few components as possible) way to drop the voltage here? I thought about simply hooking a decent 10k resistor between the two, though I suspect fires would be started? Maybe a voltage divider - or would there be too much parasitic capacitance in the resistors?

Any help appreciate. Thanks in advance guys.

Andy

[Zero999]

3kHz is slow.

You could use a resistor but you'd need to connect a Schottky diode from each input to the +5V supply to clamp the input voltage to little over 5V. 10k might be too low, as the 5V power supply could rise due too the extra current being injected into it.

[qwaarjet]

I had a similar problem
Used a two 6k resistors as a divider and a zener diode  as a clamp works over a wide range of voltages with minimum power burn.

[andrew_c]

Hi guys,

Thanks a lot for the valuable responses. However, I was curious about using a ULN2002A darlington array? Sorry to second guess your ideas but would it work?

Thanks,
Andy

[qwaarjet]

it could work but you may need additional parts depending if the input to the drive is sourcing or sinking.

[andrew_c]

So, bad timing but I've just dug out the stepper motor controllers and it looks like they have built in opto-isolation. I'll have look at how much current they draw this afternoon



Andy

[andrew_c]

Okay, so I measured 16.3mA current draw for the diode @ 5V. Plonked a 1.2k resistor in there and so far everything seems to be working great! I did notice that at 1200Hz it drops to 4V - which I've compensated for, by bringing the duty cycle up from 50 to 75%. This is by far a nice, cheap, simple solution - though only because I already have the opto-isolation built into the controller.

Thanks again for the help guys.

Andy