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High Voltage DC PWM Motor Voltage Measurement using AD8420

I need to monitor the motor RPM to ensure that the driven device (propeller) is not over sped. I know some of the specifications for the drive motor and the propeller system:

Motor Max RPM = 18,720 @ 36V (520 RPM / Volt)
Prop Max RPM = 7,500
Drive Gear Ratio = 2.27:1
Desired Motor RPM = 17,025 or 32.75 Volts (Approx 21A = 688W (.90HP))

Since my max RPM is determined by the propeller, I need to monitor the RPM and adjust the throttle so that I do not exceed this design limit. Feedback to the shore based unit will display RPM to the operator.


So, looking at the AD8420 Instrument Amp, with gain = 1, per the data sheet R1 = open and R2 = short (0W).  The device will be powered from a regulated +5V source (+Vs, -Vs). 

The motor is powered with a PWM output from the Motor Controller.  Supply voltage is a 36V Li-Ion battery assembly with Max voltage of 42V and Min voltage of 30V.  Maximum desired voltage to the motor brushes is 32.75 volts.  The voltage divider circuit is a 1:10 ratio.

PWM average voltage range is -32.75 to +32.75 volts (the PWM controller is reversible).

Vin range is -3.275 to +3.275 volts

Vout should follow Vin but always be positive (0-5V).

The PWM output frequency can be set from 2.93 kHz to 12 MHz, I am considering 20kHz. The motor cannot respond fast enough to the 20kHz pulses, however the op-amps can.  Do I need to add an RC circuit to the inputs (+IN and –IN)?

Is this the correct device for an application like the one I am using?  Does it come in a Through-Hole version?  I do not have expertise in soldering surface mount components.

  • The motor drive voltage to RPM ratio will depend on the load condition of the motor and also some secondary effects such as coil heating, etc... So, I don't think your approach will be very accurate.

    Why don't you measure the RPM directly via a sensor on either the motor, the gear head or the propeller itself?


  • That was my first consideration.  However, I have not found an appropriate sensor.  I don't want to use Hall Effect sensor and magnet.  Attaching magnets to the shafts will make balancing the system difficult.  The pulleys I have called out for the belt drive are plastic.

    I could paint the pulleys black with white 90° or 45° stripes and use Eagle Tree Systems optical RPM sensor, but they do not have any specifics about the interface on their website as they designed it to directly interface with their Data Acquisition Unit.

    So, knowing the RPM / Volt for the motor gives me a close RPM estimation.

    I either need to convert RPM to a 0-5 analog input, or to a digital 255 bit serial word. 

  • Have a look at a SHARP GP2A231LRSA (available from DigiKey).

    Paint half your pulley black and the other half white. The sensor can go to about 500Hz pulse rate which will be fine for either pulley. Sensing distance is from 3 to 9 mm. It has a TTL output, so it's simply a matter of counting pulses.


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    EZ Admin