New DevHeads get a 320-point leaderboard boost when joining the DevHeads IoT Integration Community. In addition to learning and advising, active community leaders are rewarded with community recognition and free tech stuff. Start your Legendary Collaboration now!
Hello everyone, Good day. While working on my energy meter project, i noticed that the accuarcy of measurement ( AC and AC voltage) increases or decreases with a change in the feedback resistor R41 as seen in the images above. The image above is for AC reading, i used the ZMCT103 current transformer with a turns ratio of 1000:1, I observed the following:
1) While using frame 2 and R41 as 5.1k ohms, getting an accurate current reading for loads as small as 60 watts was difficult. For a 60-watt load, the current at 220v will be 0.2727A, flowing through the primary, and with a turns ratio of 1000:1, the current at the secondary will be 0.2727mA. With the burden resistor R35 set to 150 ohms, the voltage across will be 40.9mV. The gain of the amplifier will be gain = Rf(R41)/ Rin(R37) in this case will be 5.1/22 = 0.2318. Now Vin = Vout/ gain, which makes Vout = Vin * gain = 40.9mV * 0.2318 = 9.48mV. R40 which is 22k gives the voltage an offset of 3.3v, which now makes Vout = 3.3v + 9.48mV = 3.30948v at the output of the op-amp which is further divided by 2 since there is a voltage divider at the output which now gives my voltage going to my adc to be 1.65v.
2) While using frame 1 and R41 as 15k ohms, getting an accurate current reading for loads as small as 60 watts was really precise. For a 60-watt load, the current at 220v will be 0.2727A, flowing through the primary, and with a turns ratio of 1000:1, the current at the secondary will be 0.2727mA. With the burden resistor R35 set to 150 ohms, the voltage across will be 40.9mV. The gain of the amplifier will be gain = Rf(R41)/ Rin(R37) in this case will be 15/22 = 0.681. Now Vin = Vout/ gain, which now makes Vout = Vin * gain = 40.9mV * 0.681 = 0.027v. R40 which is 22k gives the voltage an offset of 3.3v, which now makes Vout = 3.3v + 0.027 = 3.327v at the output of the op-amp which is further divided by 2 since there is a voltage divider at the output which now gives my voltage going to my adc to be 1.66v.
The microcontroller Adc is 12bit which is 4096 meaning a step will be 3.3/4096 = 0.8mv. The output voltage of both is well above 0.8mV, but why is it that the frame 2 value was difficult to get an accurate reading, but frame 1 was really accurate.
Is it my calculation that is wrong? Is it the op-amp i used the issue? Will a rail-to-rail op-amp with lower offset voltage be better? I will like to hear our thoughts please
CONTRIBUTE TO THIS THREAD