# Is this current measurement solution okay?

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:

1. 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?

2. 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?

3. Is there any reason this method should not be used at all/won't work?

Edit:
Simulation with the burden before the bridge:

• I haven't used current transformers before but it will be interesting to see a good answer. This might be wrong but as you're only simulating at the moment try putting the burden resistor before the bridge and don't forget the bridge will have a diode forward voltage drop (x2). Jul 23, 2014 at 12:37
• @PeterJ The way he has it means the diode drops don't introduce much error or nonlinearity since the output of a properly specified CT is (to first order) a constant output current for a given input current. Jul 23, 2014 at 13:02
• @SpehroPefhany, ahh yes that makes sense now I think about it further. Jul 23, 2014 at 13:08
• @PeterJ I was hoping current measurement and CTs would be more commonplace but this is turning out to be a less traveled path. I tried putting the burden before the bridge but still couldn't justify the voltage at the ADC pin (which has increased slightly) through any calculations. About the diodes, from what I understand about CTs and as points out, the CT pushes out the rated stepped down current independent of the burden. Jul 24, 2014 at 6:04