DC to frequency conversion

  • Thread starter Patrick Levinson
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Thread Starter

Patrick Levinson

Hello list members.

I´m looking for a DC to frequency converter that can convert the 4-20ma or 0-10VDC speed reference output of a VFD (or any similar analog signal, for the matter) to a pulse train of, for example, 0.1 to 10 Hz. This in order to monitor the VFD´s fequency by means of the digital inputs on my AB PLC-5, since I cannot install extra analog input modules (for cost, space, etc). Some of our VFDs do have pulse output signals, but most of them have only the analog signal.
I´ve looked at the Action AP7500 series, but would like to have other options.

Thanks in advance to all of you


Eric M. Klintworth

A couple of options to ponder:

The power measurement community does this sort of thing every day, e.g. integrate kW to kWh. An Ohio Semitronics VFC-070 takes 4-20mA input,
0-10000 pulses per hour out, about $260.
<http://www.ohiosemitronics.com/pdf/vfc.pdf> Simple, no adjustments.

Another option would be to use an Allen-Bradley Micrologix 1000 with analog inputs ($500-600) and write a little program to do the integration in it (I frequently integrate flow rate to get flow totals in PLCs). You just might have to tell the "suits" that it was a signal conditioner, not a PLC. A Micrologix 1000 analog has two current and
two voltage inputs, so you could easily use one Micrologix to integrate two current signals, or four if you used precision resistors on the
voltage inputs.

Hope that helps,
Eric M. Klintworth, PE
Columbus, Ohio

Curt Wuollet

Hi Pat There are many VCO's in both packaged and chip form available. A search for VCO should find many. you can also try national and signetics etal. for IC's. In a pinch a 50 cent 555 chip and a few components from radio shack will do admirably. This may be your best route at the very low frequencies you are seeking. I can probably find a circuit if you are interested. Curt W.