SN-WPC-C41

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• David A Cary asks:

  /techref/io/sensor/power.htm I am making a 300VAC voltmeter on PIC. I need circuit diagram for linear conversion from 0-300VAC to 0-5.00VDC FSD. Both the supplies are to be isolated. Optocoupler based circuit would be great. Have you considered using a transformer? 300 VAC is 424 V peak-to-peak (right ? please check this), so you want roughly 80:1 turns ratio on the transformer. Many suppliers list thousands of transformers that get pretty close to that (search http://digikey.com/ for "PC board transformer"). Let's say you pick a "115 VAC to 3 VAC" transformer. So you hook the 300 VAC to one side of the transformer, and you get roughly 8 VAC on the output (roughly 11 V peak-to-peak). Some people would attach 5 or so resistors to connect the output of the transformer and the +5V and Gnd power supply, to generate a 0 to +5V peak-to-peak wave (biased around 2.5V) and feed that directly into the analog input. (This requires more complicated software ... but it also lets you measure many things other than peak voltage -- frequency, 3rd harmonics, true RMS voltage, etc.). Other people would wire the output of the transformer to a bridge rectifier followed by a capacitor (to smooth out the voltage to a fairly DC level in the 0 to 11 V range), then 2 resistors for a voltage divider to feed into the analog input. You will want diodes to protect the PIC analog input from voltage spikes.

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