flyback diode parameters

Discussion in 'General Electronics Chat' started by scottyjr, Aug 14, 2011.

  1. scottyjr

    Thread Starter New Member

    Aug 12, 2011
    Hello All. Having latched onto the Arduino a few months back, I am in the process of building a coil winder. I'm using 42V steppers (5 wire unipolar) from a couple of Epson printers. I'm gonna use mosfets opto-isolated to the Arduino. Besides the mosfets I want to order flyback diodes. I read 5 ohms accross individual coils of the stepper. Is a shottky with a 100v, 5a rating overkill? Thanks - Scotty
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    What are you driving the coils with? If you're using the 42V then the coil current can be as large as the 42V / 5 ohms = 9 amps. That's a ton of current so perhaps a schematic is required here.
  3. scottyjr

    Thread Starter New Member

    Aug 12, 2011
    Sorry for the delay. Had to take a quick course in Eaglecad, Here is a schematic for one of the four coils of the stepper.


    MOSFET,IRLZ34N,TO-220AB, N-CHANNEL,55V is what I'm planning on using to drive the coils.

    You are certainly right about the current. Seems like a bit much for a printhead carriage motor. I have the service manual for the printer. They use a SLA7043 to drive the stepper. By the circuit description, i believe it has current regulation based on three bits it gets from a gate array.

    - Scotty
  4. SgtWookie


    Jul 17, 2007
    That will certainly succeed in keeping the MOSFETs turned off. They'll never turn ON, though.

    You've used a generic "optoiso" symbol and generic MOSFET, neither have part numbers. Lacking that sort of info, we can't tell you if you have made a grevious error in part selection.

    You should look at the driver circuit that is in the Epson printers; Epson usually has good designs for their products. That stepper driver was designed to last for years of hard use. You would be well advised to follow their design.

    You do need some sort of diode across the coils; however if they have a low Vf, it will take a long time for the current to stop circulating. You need more like a TVS diode.
  5. scottyjr

    Thread Starter New Member

    Aug 12, 2011
    Thank you both for responding. I would like to use the Epson circuitry but haven't been able to gather enough information. The only schematic within the service manual I have is for the power supply board so the gate array feeding signals to the SLA7043 driver is a mystery. I have been able to find some additional information from service manuals for other Epson printers using the same driver and am still seeking more.

    I did include the mosfet part number in my previous post. - Scotty