Effortless Current Measurement with Specialized ICs
Measuring current in the positive lead of a power supply, like a battery charger, can be a challenging task. Thankfully, recent years have seen the development of specialized ICs tailored for this purpose, such as the Burr-Brown INA138 and INA168. These ICs feature unique internal circuitry, enabling direct connection of their inputs to either end of a shunt resistor placed in the lead where the current needs to be gauged. The shunt resistor, essentially a low-value resistor, exhibits a voltage drop whenever current flows through it. The IC translates this voltage into an output current, Io, simplifying the measurement process.
Versatile Current Conversion Options
The generated current can be utilized directly, or it can be transformed into a voltage using a load resistor (RL). In the latter scenario, the ‘floating’ measurement voltage across the shunt is converted into a voltage referenced to earth, simplifying its practical application. The choice of RL determines the gain: 5 kΩ provides 1×, 10 kΩ yields 2×, and 15 kΩ delivers 3×, and so forth. The operation is akin to any opamp; the IC endeavors to maintain equal potential on its internal plus and minus inputs.
Intricate IC Functionality: Maintaining Voltage Balance
The minus input is linked to the shunt resistor’s left-hand end through a 5-kΩ resistor. When current flows through the shunt, the voltage here is lower than that on the plus side. To equalize this, a small supplementary current passes through T1, reducing the voltage on the plus input. The IC ensures T1 conducts just enough to attain the necessary lower voltage. The required current equals Vshunt/5 kΩ, leaving the IC via the output connected to RL. If RL is 5 kΩ, the resultant voltage mirrors Vshunt precisely.
IC Variants and Operational Range
The IC is available in two versions: INA138, which handles voltages between 2.7 and 36 V, and INA168, which is operational up to 60 V. Pin 5’s supply voltage can range anywhere within these limits, irrespective of the inputs’ voltage. This implies measurements can be made with inputs up to 60 V, even with a 5 V supply voltage. However, in most instances, connecting pin 5 directly to the voltage on pin 3 simplifies the setup. It’s crucial to note that the supply voltage determines the maximum output voltage. Additionally, account for the internal base-emitter junction voltage of T1 (0.7 V) and the voltage drop across the shunt in your calculations.