One wishes to measure the RMS current (12A max.) drawn by an AC motor using a PIC16F887 at intervals of 1 second. One cost effective solution I am considering is using a current transformer, a bridge rectifier and then a large-ish capacitor to filter the signal to be fed into the ADC pin. Like this perhaps:
Are these calculations correct?
For max. primary current: 12A RMS
Turns ratio for this CT: (1/70)
Secondary current = Primary Current * Turns Ratio = 12*(1/70) = 0.186A RMS
Burden Resistor: 33 ohms
Secondary Voltage = Secondary Current X Burden = 0.186 X 33 = 6.14 V RMS
Therefore Secondary peak-to-peak voltage = 6.14 X 1.414 = 8.68V
The DC voltage after the capacitor should then be close to 8.68V? Why does the above Proteus simulation show otherwise?
The diode D1 is supposed to avoid over voltage on the ADC pin due to inrush currents of about 40A that happen about every half an hour. Would that be adequate?
- Is there any reason this method should not be used at all/won't work?
Simulation with the burden before the bridge: