Hello Forum,
I have an SMPS type Lead acid Battery charger ( 14.4V, 10A), it has Analog Voltmeter and Ammeter panels. The charger works fine and both the Meters work well. My aim is to replace the Analog (needle) type Panel meters with 4 Digit 7- segments LED displays.
I am using MSP430G2553 for measuring Voltage and current and the 7 segment displays are multiplexed, The voltage measurement is successful.
For measuring Charging current i am using a 0.01 Ohm shunt resistor and an Op-Amp (with gain of 10) to amplify the voltage across the shunt, the final voltage from the Op-amp is fed to one of the Analog pins on MSP430.
This is where i am facing problem, The displayed results are erratic - for example : i have connected the Analog Ammeter in series to the shunt and if the Ammeter reads 7 A, the voltage across the shunt is 70 mA and the voltage at the O/P of Op-amp is 700mA but the result displayed on 7 seg are random, like 9, 14,10 7, 5 etc.
i am using internal ref 1.5V, the O/P at the Op-Amp is smoothened with 10uF cap, i tried even 100uF cap but the results are still erratic. But the Multimeter shows steady voltages across the Shunt, O/P of Op-amp.
I know that the ADC takes snapshot of input signal at that point of time but i'd like to have result displayed like that on normal multimeter. I read something about damping but not sure about the concept, also read in other forums that we'll have to take 100 samples per second and then average the result.
Does the $10 el-cheapo Multimeter take 100 samples per sec to display a steady voltage ? If a cheap Multimeter can read the o/p of Op-amp and give steady result then why not my MSP430 ?
Any advice or help in this regard would be greatly appreciated.
Regards,
Nischay
