I have a DC motor, its specification states: "Electric DC motor manufactured by Buehler. Sinusoidal Encoder Output: 11.5 VAC - 870 Hz @ 12 VDC input. No load speed: 6800 RPM. Runs in either direction"

The motor is connected to an NMOS MOSFET amplifier with 12 V on the drain. The tachometer's two terminals are connected to a full-wave bridge rectifier which also has a smoothing circuit using a resistor and a capacitor.

My problem is that the output of the bridge rectifier does not change when I change the speed of the motor (I am measuring the output voltage of my smoothing circuit).

Does the tachometer output value vary with respect to its motor's speed?

Is it possible to convert a motor's tachometer output to DC using a bridge rectifier?

Should my circuit work?

  • \$\begingroup\$ The Tach output should be a fixed pulse per rev so the RC average Vdc = RPM, Just make sure the series R is not too low and C not too big. such that RC=>100ms If the Diodes are shorted out test them \$\endgroup\$ Commented Dec 30, 2018 at 22:16
  • \$\begingroup\$ Use a scope to monitor what the output of the encoder is. "Sinusoidal encoder ... 11.5 Vac ... 870 Hz" sounds a little strange to me. \$\endgroup\$ Commented Feb 18, 2020 at 10:27

2 Answers 2


If you have no load on the capacitor and you are monitoring the voltage with a high impedance (resistance) meter such as a digital multimeter with a 10 MΩ input then the capacitor will charge up and remain charged.

Try putting a discharge resistor in parallel with the capacitor. Pick a time constant that is suitable for your application - 1 s maybe will be quick enough for you to monitor with the meter. You can use the formula \$ \tau = RC \$ or, rearranging, \$ R = \frac {\tau}{C} \$ to calculate a suitable value. \$ \tau \$ is the time constant in seconds.

The above will only work if the generator is a simple permanent magnet type whose output voltage will vary with the speed.

If the tacho is excited by the 12 V DC supply then the output voltage will be constant value but only varying in frequency. If that's the case you need a frequency to voltage converter.


simulate this circuit – Schematic created using CircuitLab

Figure 1. An experimental setup.

Try the circuit of Figure 1. A 1 μF capacitor has an impedance of 183 Ω at 870 Hz rising to 1830 Ω at 87 Hz. The result should be a rising DC voltage on the output. I couldn't simulate it properly with the CircuitLab simulator.

  • \$\begingroup\$ I think my NMOS MOSFET amplifier only changes the current of the 12V power supply, therefore, I should use a frequency to voltage converter... Can a npn BJT transistor amplifier change the voltage going into the motor instead of the current ? \$\endgroup\$ Commented Dec 30, 2018 at 23:11
  • \$\begingroup\$ There isn't enough information in your question for me to answer the original question properly. You provided no part number, photo, pinout information or your wiring diagram so I've had to guess. The BJT seems to be a completely different question. \$\endgroup\$
    – Transistor
    Commented Dec 30, 2018 at 23:14

A very long time ago I worked on daisywheel printers. These used optical encoders that put out a pair of quadrature sine waves, from which position and velocity were derived.

The short summary is, the sines were converted to square waves, then the XOR of square waves were fed to a frequency-voltage converter. This produced the absolute-value velocity signal. Another circuit looked at the quadrature phase to find direction, and this was used to make a bipolar velocity signal.

In your case your outputs are already square waves (check your motor data) so you don't need that part. Then you just need an frequency to voltage converter. A very simple one could be an edge detect to make pulses, followed by an integrator.

There are also ICs for this purpose.


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