Bar (Customer) asked a question.
Hello, I am measuring (perhaps incorrectly) the PWM signal from Y5 High Speed Output. I have the PWM output configuration set for Y5. PWMOUT instruction is set to 40% Duty Cycle, 100Hz, 5V Sourcing output.
I am using an oscilloscope probe to measure. I am seeing the fall time on the pulse of 0.8 ms, but the specification for the the output is 2μs from On to Off. The rise time of the pulse is measured as 2μs which corresponds with the spec. I have no load in this test circuit. Just measuring the BRX voltage from the output to 5V power supply COM . The probe's CH 1 and Ground is connected as shown below. Is this not the correct way to measure? Is this a measurement issue or an output issue? I want to confirm that the device been controlled is seeing 40% Duty cycle and not 44% due to the falling edge slope. Thanks!
You say you have no load on the circuit. You have only the input impedance of the oscilloscope. I'm guessing the oscilloscope input impedance is very high. Try connecting an appropriate load on the output. See if this allows the fall time to be more to the spec. The min output current for the BX output is 0.1 mA @ 24VDC.
That was it. I placed a 1k resistor as @Tinker (Customer) suggested and fall time was corrected. Thank you!
I'll second K Price's answer, your scope probe is likely 1 meg-ohm (X1) or 10 meg-ohm (if X10), the specs you posted a copy of show a minimum output current of 0.1mA for rated performance. At 5V the maximum load resistance to get that minimum current would be 50K-ohm, 1/20 or 1/200 of what you likely have. Since the output can handle up to 0.5A I'd probably add a 1K load resistor, though a 10K should be fine too, whatever you have handy. Or, maybe better, connect the actual device you will be driving if you can.
Thanks! A 1K Ohm Resistor worked.