Printhead solenoids circuit. Help

Hi all, I’m working on a new project and I need to drive the pins of a printhead with an Arduino…
I would like to use some elements of the printer’s electronics (like the power section) and try to make a new circuit in order to drive the solenoids inside the printhead.

Trying to follow the “signal path” on the pcb with a multimeter I found this scheme (see pdf).

Now, following the specs on the posted circuit I have some questions:

  • First of all, this circuit make sense for you?
    I mean I would expect a connection from the central pin of the mosfet also to ground but seems that this connection is absent.

  • Is this circuit enough safe for the Arduino pins?
    Could I add another diode (like 1n4004 or similar) before the 2 KΩ resistor in order to be more safe and without compromise the pwm speed?
    (Note: I need to drive the mosfet in an audio frequency range: ± from 30 Hz to 2 KHz).

  • Could someone recommend me a zener diode with the diagram specs?

Thank you in advance !!

Printer_Solenoid.pdf (352 KB)

After some more research I found that the switch is done with a darlington transistor and not a mosfet as I said previously.
The diagram of this component is in the attachment.

Obviously my previous questions remains…

I assume there is some connection between the motor and ground?

How do you know it is PWM drive?

A better circuit with more information may make it easier to see what is happening.

Weedpharma

I have read somewhere in the printer’s service_manual that the cpu calculate the “duty cycle” (so I assumed that it is a pwm control) and then pass the values to a gate_array witch send the “data” to the printhead.
As I said before I followed 24 signals (one for every dot_pin) from the gate array on the pcb and all the lines reproduces the scheme I posted before…(The µC pwm represent one single line of the gate_array)

The multi_connectors that goes from the pcbs to the printhead has some 35V lines that energizes the coils (24 in total but they are grouped together) and individual “data_pin” for every dot.
These data_pins are connected to the collector of 24 individual transistors that drive them individually, so I assume that the connection of the solenoids to ground is made by switching the transistors…

The Main Board look like this (see photo)

Its not PWM really, each solenoid pin is fired at the correct time for a short duration. It may be they drive a pin for longer to get a horizontal line segment rather than one dot, but that would be unusual and wear the pins out fast I think.

The zener diode in the feedback path causes rapid reduction of the solenoid field after firing without dissipating much heat in the zener (the darlington carries the bulk of the current).

The high voltages involved are to make the magnetic field rise and fall very rapidly to get useful printing speeds. Lots of power is also needed as a result.

The central pin is the collector and certainly should not be connected to ground! The centre pin of a power transistor is always collector or drain, since that's the back of the chip and the tab and centre pin are part of the metal plate it is mounted on for heatsinking.

Ok, thank you for the reply, but what I would like to do is drive the solenoids with pwm bypassing all of the logic from the mainboard, so probably my real question is:

Can I use a “standard” circuit (see photo, obviously with a solenoid instead of relay) to drive the solenoids individually in the printhead?

… And if yes, witch kind of components I need using the 35V as the source?

I have some IRF840/IRF740 and I would like to use a zener because I have read somewhere it is better for faster switching speeds…

Thank you again!

Well if you don't mind having to cool high power zeners you could do that. The existing topology is known to work and puts 100 times less current through the zeners so why not use it?. Yes you need the same supply voltage, again this is done to drive the pins fast enough (they have strong return springs).

As I said there's a fair bit of power involved, you have to provide enough cooling to the relevant parts (with existing circuit that means solenoids themselves and darlingtons only, not zeners).

The drivers have to cope with about 100V, if you realize that the zeners are there to limit the inductive spike to about that value. 150V rated transistors are probably about the right compromise.

And you must limit the pulse length and average duty cycle to prevent thermal overload too, note.

Yes, my first intention was to use the same topology as the original but I need to make this from scratch with new components because the original parts are no more in production so I need some new ones with the same characteristics in order to be able to replace them easily....

If you see the first (fritzing) schematic I posted before you can see the right components.. The original transistor is a 2SD2422 (no more in production) replaceable with a 2SD1415 (same as the other). The new TTD1415B is difficult to find for me....

The original zener diode is rated at 91V as in the schematic..

Do you think that I can use a TIP142 (easily to find) and a 91V zener and utilize the schematic of my first post?

In this case my previous questions about the "safe" behavior for Arduino pins remain open mainly because in this configuration when the zener "open" the current will flow back to the base of the transistor...

MarkT thank you for your replies and patience.

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)?

  • 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.

-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?

I know I have many questions, but I think it's not all so trivial so I hope someone can help me.

Thanks again!

If you provide links to the datasheets, people will be more inclined to help you - after all you must have done this already, why make us search around?

Sure, you are right…!!
The datasheet of the zener it is not the original but a zener that I can find easily.

Could you can tell me something about the questions one and four?

Thanks!

Datasheets.zip (442 KB)

Maybe the links are better than files:

  • Original Darlington Transistor, 2SD2422 (No more in production)

http://www.datasheets360.com/pdf/4404280062907539733

  • The Darlington Transistor that I would like to use as a replacement of the previous one, TIP142

http://www.st.com/web/en/resource/technical/document/datasheet/CD00000914.pdf

  • Zener Diode that I would like to use in the circuit, ZPY91 - ( I don't know if the power dissipation is ok)

http://pdf1.alldatasheet.com/datasheet-pdf/view/434654/DIOTEC/ZPY91.html

pjeve: 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.

Thanks again!