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DRV8825: Why the actual current is lower than the set-up current?

Part Number: DRV8825

I  configured the chip with 8microstep/step  and mixed decay mode.

I  set the Vref (pin12 and pin13) to  1.33V and make the motor rotate continuously.

The current sense resistor is 200mΩ.  So the setting current driving the motor is 1.33A.

When the motor is running,I measured the voltage of the current sense resistor using my multimeter,  which is 22mV. That's is only 0.1A driving current.

Why the actual current is lower than the set-up current?

I measured the voltage of the current sense resistor using my  oscilloscope, but waveform isn't a  sinusoidal wave.

What's the reason?

Here is the waveform of the current sense resistor:

  • Hi Feiyue,

    Why the actual current is lower than the set-up current?

    >>> The sense resistor does not indicate the total current through the winding. The sense resistor is used to indicate when the current has reached the desired level. The sense resistor voltage is valid when current is being driven through the coil. In slow decay the voltage across the sense resistor is 0.

    You can see the peak voltage is approximately 240mV to 280mV, which is an indication of 1.2 to 1.4A.

    I measured the voltage of the current sense resistor using my oscilloscope, but waveform isn't a sinusoidal wave.

    What's the reason?

    The waveform will not be sinusoidal, since the current does not flow through the resistor in slow decay. In mixed decay there will be a voltage across the resistor when current is being driven, an opposite voltage in fast decay, then 0V during slow decay.

    The waveform appears as though it is inverted. Please check your scope probe to see if it is inverted.
  • But the output current of 24V power supply is only 0.2A. According to Table 2 in the datasheet, the total current of the two coil should always larger than the full-scale chopping current(1.33A). Is it right?
  • Another question is :

    The datasheet says “current regulation is used to vary the current in the two windings in a semi-sinusoidal fashion
    to provide smooth motion”,
    what does “semi-sinusoidal” mean? the current should be only in positive half cycle or negative cycle?
    I say this because it seems the current in the coil is always negative according to the resistor's current waveform?
  • Hi Feiyue,

    But the output current of 24V power supply is only 0.2A. According to Table 2 in the datasheet, the total current of the two coil should always larger than the full-scale chopping current(1.33A). Is it right?

    The current in the power supply is the average current supplied to the board at 24V. Power is 4.8W.

    Power in the DRV8825 is ~.75W (RMS current (1.33A * .707)squared * (HS + LS Rdson) * 2 coils)

    Power in the sense resistors is ~.35W (RMS current (1.33A * .707)squared * .2Ohm sense resistor * 2 sense resistors)

    Power in the motor is  unknown but can be calculated the same way (RMS current (1.33A * .707)squared * Motor resistance/coil * 2 coils)

    There will be other power in the system but these are the largest.

    As the power supply increases, the average power supply current required to operate the stepper decreases. The overall power needed to operate the motor should remain approximately the same.

    The datasheet says “current regulation is used to vary the current in the two windings in a semi-sinusoidal fashion

    to provide smooth motion”,what does “semi-sinusoidal” mean?


    Semi-sinusoidal means the current is regulated in a series that simulates a sine wave. There are distinct steps in the current. Please refer to Figure 8 in the datasheet. When a step pulse is issued, the current increases or decreases to the next level. Once the current is reached, the motor driver regulates the current at that level.

    the current should be only in positive half cycle or negative cycle?

    Please refer to Figure 4 in the datasheet. The sense resistor voltage is representative of the current. During the drive time, the voltage across the sense resistor is positive and should reached the Itrip_level * Rsense, where Itrip_level is the full scale current * the indexer step.

    Once the current is reached, the device will enter slow decay or mixed decay. For slow decay, the current is recirculated through the low side FETs, and the voltage on the resistor is zero. For the fast decay portion of mixed decay, the current flows from ground back to VM as shown in path 3. During this time, the voltage should be negative. 

    I say this because it seems the current in the coil is always negative according to the resistor's current waveform?

    This is puzzling. If the scope capture looked like below, it would make sense. Positive only voltage is slow decay when the current is increasing. Positive and negative voltage is mixed decay when the current is decreasing.

     

  • what does "coil" mean in the calculation formula of Power in the motor ? What's the value of "coil"?

    I think I know what confused me.

    Current of the power supply is less than the set-up current. It's is right.

    And the waveform of the current resistor is also right.
  • Hi Feiyue,

    "Coil" is one phase of the stepper motor. This information should be provided by the stepper motor manufacturer.

    Current of the power supply is less than the set-up current.