PWM Voltage Converter

massynrt

Hi guys - I think I've read every comment and doc on this topic and I still feel stupid. I'm building a new CNC machine with a Duet 2 Ethernet I have. The only PWM to 10V converter I can get in South Africa is the blue one that is not recommended to be used. As a side no I'm a noob. I learn by blowing stuff up. hehe.. Not reallly. Please forgive my ignorance. I'm just trying to understand and work with what I can get.

Few questions -

The posts says that it is not recommended to use the small blue PWM converter board, because 1. It is difficult to drive from some duet boards and the 2. the shared ground issue. My question is then if it is possible to use it, although difficult, how would i go about it?
Lets say I use FAN2 as the PWM output. On the duet board - If I look on the schematic, one pin is FAN_Vin and the other pin, normally the ground pin, is the PWM pin. Does this mean the pwm signal is pulsed on the ground pin. One pin is always 5v (if Vin is set to int 5v) , the other pin (ground/pwm) pulses from 0 to 5v and thus create the voltage difference for the PWM signal with Vin as the reference. Like an inverter PWM signal. If that is the case then I will understand why the blue board wont work.. the pulsed 5v (Pwm/Ground) pin from the duet will always be pulled to ground through the converter board.

Any help in understanding is appreciated.

droftarts

@massynrt For info on the PWM to 10V boards, see https://docs.duet3d.com/en/User_manual/Machine_configuration/Configuration_CNC#connecting-a-spindle

There is a possible alternative mentioned in this thread: https://forum.duet3d.com/topic/34506/alternative-pwm-to-0-10v-converter-for-spindle-control

Yes, PWM pins on the Duet 2 are on the negative side, as MOSFETs (the 'switch') work best when placed after the load. It is possible to invert the PWM signal using a simple circuit, if that helps, with the addition of a 1kohm pullup resistor. See https://forum.duet3d.com/post/118283 (thanks again @DIY-O-Sphere ). The example controls a servo, but it can be used for any PWM output. I'm not sure if this is sufficient to overcome the problems with the blue board, though.

Ian

massynrt

Thank you for the reply- I tried it - Sort Of.

Maybe you can provide some ideas (or the math)..

Bascily I put a 1.3k ohm resistor (dont have 1kohm) between the FAN_Vin and the VAN_pwm(-). I then only connect the pwm signal. No ground to the PWM converter as the Duet and the PWM converter are powered by the same PSU, I suspect the grounds will technically be the same.

Results.. It works - Half way.

My FAN Vin is set to 24v (Input of the duet). And my pwm input is also set to 24v also on the PWM signal. If I measure the DC voltage over the PWM and ground (when at full speed) I get only 14v and not 24. The resulting Voltage after PWM conversion is only 5.5 or something, and not closer to 10.

Does the resistor cause a voltage drop or something? At least I'm starting to get some movement.