# Converting DC Motor's Tachometer Output to DC Using Bridge Rectifier

I have a DC motor that its specification states that "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 in a NMOS MOSFET amplifier with 12V 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 the output of the bridge rectifier does not change when I change speed of the motor (I am measuring the output voltage of my smoothing circuit)

Does tachometer output value vary 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 ?

• 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 Dec 30 '18 at 22:16
• 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. Feb 18 '20 at 10:27

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.

• 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 ? Dec 30 '18 at 23:11
• 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. Dec 30 '18 at 23:14