Converting DC Motor's Tachometer Output to DC Using Bridge Rectifier
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 M&Omega; 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 &ndash; Schematic created using CircuitLab

Figure 1. An experimental setup.

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