Ken Yang EDN Power meters provide an early warning of thermal overload by monitoring power consumption in highreliability systems. Power monitoring is especially suitable for motor controllers, industrial heating systems, and other systems in which the load voltage and current are both variable. The power meter/controller in Figure 1 uses the principle that power is the product of voltage and current. The typical accuracy of the circuit is better than ±1%. A current sensor, IC_{2}, measures output current, and a fourquadrant analogvoltage multiplier, IC_{1 }and IC_{3}, generates the product of output voltage and current. An optional unitygain inverter, IC_{4}, inverts the inverted multiplier output. This power meter is most accurate for multiplier inputs (J_{1} and J_{2}) of 3 to 15 V. Select the currentsense resistor as follows: R_{SENSE} = 1/P, where R_{SENSE} is in ohms, and P is the output power in watts. If power delivery to the load is 10 W, for example, you would choose R_{SENSE} = 0.1 Ω.
The circuit in Figure 1 has a unitygain transfer function, in which the output voltage is proportional to load power. For instance, the output voltage is 10 V when the load power is 10 W. To change the transferfunction gain, change the sense resistor as follows: For the circuit in Figure 1, Figure 2 compares powermeasurement error with load power. Note that accuracy is better than ±1% for load power of 3 to 14 W. For proper operation, you must first calibrate the analog multiplier according to the following procedure. Remover jumpers J_{1} (X input) and J_{2} (Y input) before calibration.
Repeat the preceding steps as necessary. Materials on the topic 

