Current Sensing 101

Current Sensing 101

Now that starter project is done, it is time for real research. For the past week, I have been thinking of using current sense resistors for my research project. This blog post will introduce concepts of current sense resistors and how to use them.

Measuring current using current sense resistor, op amp, ADC, and MCU

The standard method of measuring current is to measure voltage drop over a resistor. The topology is shown in the above schematics. Ohm’s law can be applied to convert voltage measurement to current value using I = V/R. Current sense resistors are high precision low ohm resistors ranging from microohm to milliohms.

Consider a 12V power source with a variable load resistance from 250Ω to 1000Ω. This converts to a current draw range of 12mA to 48mA. The voltage drop over a 0.01Ω current sense resistor then ranges from 12uV to 48 uV. These are very small numbers and need to be amplified to the microcontroller voltage range of 0V to 3.3V. To amplify 48 uV to 3.3V, the gain required is 6875. This gain can be achieved using the equation Gain = R2/R1. The output then can be read using an internal 12-bit ADC of a microcontroller. Since the gain and value of the current sense resistor are given, the current can be calculated using Ohm’s law.

This concludes the basic setup of the current measurement circuit. This current sensing method has wide range of applications. For instance, battery protection circuit monitors current draw of a battery from mobile electronic devices and current draw from main’s power for charging. Lithium batteries are volatile so current draw and voltage need to be constantly measured for safety. For next few weeks, I plan to experiment with current sensing circuits to investigate vulnerability of these devices to external attacks.


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