6N137 Isolating Ground Issue

[dev.]

Hi All,

I am trying to control speed of a DC motor using PWM. To reduce coupling of motor noise onto my electronics, I am using a separate power supply for both(LED and Phototransistor) and am planning to interface PWM pin from µC to motor using 6N137. Speed of PWM is abount 20KHz as recommended by motor manufacturer.

When I try to test 6N137, when I use a single supply for both input and output, i.e LED and phototransistor everything works according to truth table listed in the data sheet. When I use two different supplies(with or without isolating ground) output is always high irrespective of the input. Can anybody point out my mistake for this behavior? Find attached the schematic and output waveform.

Regards,
Ani

 

[erikl]

You're sure to measure Vout between Vout and GND2, i.e. the neg. terminal of your right-side power supply?

 

[dev.]

Yes I am pretty sure about this. I double checked this before posting.

 

[erikl]

Some more silly question by myself: You did not (unintentionally) stimulate Vin between 330Ω-pin2 and GND2 ?

Bypass cap connected correctly (between pins 8 and 5)?

Measure result reversible (ok. with connected GNDs)? - Otherwise optocoupler defective.

 

[schmitt trigger]

Measure result reversible (ok. with connected GNDs)? - Otherwise optocoupler defective.

That is exactly what I would do.....connect a jumper between Gnd1 and Gnd2 and monitor your output with a scope. Then with the scope still monitoring the output, remove the jumper and see what happens. Be sure that you scope's ground probe is connected to Gnd2, and the signal generator to Gnd1.

 

[chuckey]

Pin 8 = Vcc, Pin 7 = Vstrobe = 0. See :- https://uk.farnell.com/webapp/wcs/s...atch|b|plid|&gclid=CKLYj4-lt8QCFZMZtAod0UMABQ

Frank

 

[erikl]

Pin 7 = Vstrobe = 0

No, I think pin 7 is the enable pin (positive logic), so pin 7 = Vcc should be correct.
Check the dataSheets you linked to.