1
\$\begingroup\$

I need to calculate the speed of a little generator that produces a bipolar sinusoidal waveform with a peak to peak voltage value variable with the velocity. The produced signal also has a frequency proportional to the velocity. I prefer to process the frequency than the voltage, because I have a direct relation between revolution and speed. Four produce periods are corrispondent to one revolution of the generator drive shaft. My requirements are:

  • Max RMS output generated voltage = 45 volt, at the maximum velocity.
  • Max output generated frequency=50 kHz at the maximum velocity.
  • A square signal circuit output to be sent to the micro min = 0 V, max = 5 V.
  • Only unipolar voltage to 12 V or 5 V to supply the circuit.

So I thought to a Schmitt trigger op-amp comparator like this.

enter image description here

But I have two problems:

  • Due to the variable peak to peak signal I cannot fix the two comparator's threshold VH, VL.
  • I have to scale (I think) the voltage produced by the generator to avoid to damage the op-amp. Here could be sufficient a voltage divider to the non inverting pin, fixing the divider resistors to have a value that at the maximum RMS signal 40 V, produce for example Vcc.

Any idea how I could proceed?

update:I attached a wrong figure (a comparator), instead I would attach a schmitt trigger because from what I read, the only comparator (without positive feedback) could create false trigger in the output due to a noisy input.

Thanks a lot.

\$\endgroup\$
  • \$\begingroup\$ What is the smallest RMS voltage produced that you need to support at the minimum velocity? \$\endgroup\$ – jonk Mar 30 at 19:42
  • \$\begingroup\$ 80-100 mV that corresponds to more o less 1 revolution per second. I would evitate to measure the speed by a voltage (by the internal micro AD) to be independent to the specific alternator relation efm-revolutions that would need to be adjusted by a potentiometer during the tuning phase of the circuit. I would only set the number of poles as parameter in the code. \$\endgroup\$ – daigs Mar 31 at 15:03
  • \$\begingroup\$ You can do this with just a few bjts and no vref, as well. But I don't want to discourage your use of a comparator. \$\endgroup\$ – jonk Mar 31 at 15:16
0
\$\begingroup\$

First, use a comparator rather than an op-amp. They're designed for your application and don't latch up when the input gets close to the rail voltages.

Max RMS output generated voltage = 45 volt, at the maximum velocity.

A resistor divider will do the job. Add a diode between ground and the input (pointing up in your diagram) to prevent the voltage going negative. Make the VREF close to ground so you can detect low-amplitude signals accurately.

Max output generated frequency=50 kHz at the maximum velocity.

Shouldn't be a problem.

A square signal circuit output to be sent to the micro min = 0 V, max = 5 V.

Most comparators have an open-collector output so a pull-up resistor to the 5 V supply will give you the required square-wave signal.

Only unipolar voltage to 12 V or 5 V to supply the circuit. So I thought to a Schmitt trigger op-amp comparator like this.

So far, so good - but use a proper comparator.

Due to the variable peak to peak signal I cannot fix the two comparator's threshold VH, VL.

You only need one comparator. Since the voltage is alternating you can set the threshold very close to 0 V.

I have to scale (I think) the voltage produced by the generator to avoid to damage the op-amp. Here could be sufficient a voltage divider to the non inverting pin, fixing the divider resistors to have a value that at the maximum RMS signal 40 V, produce for example Vcc.

You've got it. Don't forget that peak will be \$ 40\sqrt 2 \$.

\$\endgroup\$
  • \$\begingroup\$ Thanks transistor! I updated the question. Using only one trigger point (as comparator do) could be a good idea but don't you think that the comparator (and not the schmitt trigger) could produce false trigger due to noisy input? Thanks. \$\endgroup\$ – daigs Mar 31 at 22:30
  • \$\begingroup\$ Yes you are correct, I would draw a schematic to show how to connect a comparator so it rejects noise, but for some reason I can't access editor. You will have to wait until someone else comes along. \$\endgroup\$ – EinarA Apr 1 at 2:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.