How to Design a Current Sensing Circuit: Schematic Examples and Key Steps

A current sensing circuit typically uses a low-value resistor (0.1 to 0.5 ohms) placed in series with the load to develop a small voltage drop proportional to the load current. This voltage drop can be measured directly with a voltmeter or used as an input to protection circuits that disable the load when current exceeds a set threshold. Design steps begin with defining specifications such as current limit, voltage burden, AC or DC current type, response speed, accuracy, shutdown method, power supply constraints, and cost targets. Non-contact sensing methods using a step-down transformer and full-wave rectifier feeding an operational amplifier (e.g., CA3140) can monitor high currents without direct electrical connection. For DC current sensing and protection in a 9V voltage regulator project with a 1A threshold, a current sense resistor combined with a dedicated current-sense amplifier like the Texas Instruments INA139 is recommended. The INA139 output can be connected to a comparator with an RC filter to handle transient spikes. Alternative solutions using op-amps and comparators require careful input voltage scaling to avoid errors. Overall, the design involves selecting an appropriate sense resistor, defining threshold voltages, and implementing a control or shutdown mechanism based on sensed current.
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