Sorry, probably I did a bit of confusion with the previous posts so I will try to be more explicit.
My doubts concern the following points:
- Thanks to Mark I knew I could handle the pins of solenoids with an pwm according to the scheme initially posted. What worries me about the scheme is that the anode of the zener returns on the basis of the transistor and not goes directly to the ground. This way I'm not sure if I can damage the microcontroller PWM pin. Can anyone confirm this thing? Perhaps it is better to connect the anode (through the resistor) directly to the ground, or is the same thing (since the darlington transistor internally connecting the base with the emitter through two resistors)?
That's done to amplify the zener so it doesn't have to be a massive device on a heatsink - it delays the
turn-off of the transistor. The transistor base cannot get more than a volt or two above the emitter voltage
without the transistor melting first. The circuit works, remember!
The second question concerns the replacement of certain components no longer in production. Specifically I would like to replace the original transistor (2SD2422) with a TIP142. Do You think it is a viable option?
I want to replace also the zeners. The original ones are glass and have a voltage of 91V. There is also a way to establish the value of the power dissipation? They are marked simply 91 B.
Dissipation is a function of the package. Most zeners are 0.4W or 1.3W if I remember rightly (prob not!)
-Finally a question regarding the power supply. I would use the internal system of the printer that already has all the components to convert 220 AC input and provide a connector with DC lines of 35V, 5V, and ground.
I would use the 35V lines to power the solenoids and stepper motors (as in the original system) and power externally the Arduino with another power supply. Do you think I can / I need to connect the ground of the two systems without any problems?
Is it an isolated supply? Is it floating at mains voltage? Is it grounded to mains earth? Three possibilities that
you will have to check. Yes you will need to connect grounds unless you use opto isolators, and if the
existing supply is live at mains voltage you definitely need opto-isolation.
I know I have many questions, but I think it's not all so trivial so I hope someone can help me.