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DRV8805 Optional Zener Selection

Gary Preston
Prodigy 20 points
Other Parts Discussed in Thread: DRV8805, DRV8805EVM

I'm looking to use a DRV8805 controller with a hurst LSG35012F76P unipolar stepper motor. The datasheet for the 8805 mentions an optional Zener diode that can be connected between VM and VCLAMP to allow faster current decay. However, I'm at a bit of a loss as to the spec of diode I should be selecting or the rough range of safe values.

This site has the specs for the stepper motor beckwithelectronics.com/HURST/lds-lgs.htm the 12V LSD35012 D line. Which notes the motor is 12V with 65 ohm resistance per phase.

Can anyone offer any advice on how I can determine a suitable Zener for this usage?

  • 0
    TI__Guru** 114905 points
    Hi Gary,

    The Zener selection is dependent on several items.


    First is the VM voltage used.
    Second is maximum voltage desired at the Vclamp pin. This zener voltage plus VM should not exceed the recommended maximum voltage of the device. The zener voltage can be lowered to increase the decay time in the stepper.
    Finally, zener should be rated for the amount of current that could flow through the zener. Using a minimum of twice the current in one coil is recommended.
  • 0 in reply to Rick Duncan
    Prodigy 20 points
    VM: 12V
    VCLAMP: 12V

    I'm unable to find anything regarding the maximum safe voltage for this motor. I will likely have to assume ~12V. I take it this means the optional diode for fast decay is not usable unless I drop VM down a few volts such that VM+Zener voltage = 12V?

    Regarding current, all I can see in the specs for the motor is that it's 12V nominal supply, 65 Ohm per coil which would suggest (12/65) = 0.184A per coil. So a zener rated >= 0.4A should be used?

    > The zener voltage can be lowered to increase the decay time in the stepper.

    So for faster decay of the current for use with a unipolar stepper, I'd want a higher voltage zener but need to balance that with the VM+zener total being less than the devices max voltage?

    Thanks,

    Gary
  • 0 in reply to Gary Preston
    TI__Guru** 114905 points

    Hi Gary,

    Gary Preston said:
    I'm unable to find anything regarding the maximum safe voltage for this motor. I will likely have to assume ~12V. I take it this means the optional diode for fast decay is not usable unless I drop VM down a few volts such that VM+Zener voltage = 12V?

    Based on your second statement, you should be able to set VM to 12V, and use up to a 40V zener (depends on voltage tolerances). Another item to consider is the power through the zener. This can be large.

    In stepper motors, the key is current more than voltage. Many stepper motors are rated at much lower voltages than they are driven.

    Gary Preston said:
    Regarding current, all I can see in the specs for the motor is that it's 12V nominal supply, 65 Ohm per coil which would suggest (12/65) = 0.184A per coil. So a zener rated >= 0.4A should be used?

    A 400mA zener should be sufficient. Please note that you may not need the zener.

    Gary Preston said:
    > The zener voltage can be lowered to increase the decay time in the stepper.

    So for faster decay of the current for use with a unipolar stepper, I'd want a higher voltage zener but need to balance that with the VM+zener total being less than the devices max voltage?

    Yes, that is correct.

    FYI -- there is a DRV8805EVM available for evaluation. It has a zener on the board that can be bypassed or changed.