Power meters provide an early warning of thermal overload by monitoring power consumption in high-reliability 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, IC₂, measures output current, and a four-quadrant analog-voltage multiplier, IC₁ and IC₃, generates the product of output voltage and current. An optional unity-gain inverter, IC₄, inverts the inverted multiplier output. This power meter is most accurate for multiplier inputs (J₁ and J₂) of 3 to 15V. Select the current-sense 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 10W, for example, you would choose R_SENSE=0.1Ω.

The circuit in Figure 1 has a unity-gain transfer function, in which the output voltage is proportional to load power. For instance, the output voltage is 10V when the load power is 10W. To change the transfer-function gain, change the sense resistor as follows: Gain=10R_SENSE. For the circuit in Figure 1, Figure 2 compares power-measurement error with load power. Note that accuracy is better than ±1% for load power of 3 to 14W. For proper operation, you must first calibrate the analog multiplier according to the following procedure. Remover jumpers J₁ (X input) and J₂ (Y input) before calibration.

• X-input offset adjustment: Connect a 1-kHz, 5V p-p sinusoidal signal to the Y input, and connect the X input to ground. Using an oscilloscope to monitor the output, adjust R_X for an ac null (zero amplitude) in the sinusoidal signal.
• Y-input offset adjustment: Connect a 1-kHz, 5V p-p sinusoidal signal to the X input, and connect the Y input to ground. Using an oscilloscope to monitor the output, adjust R_Y for an ac null (zero amplitude) in the sinusoidal signal.
• Output-offset adjustment: Connect both X and Y inputs to ground. Adjust R_OUT until the dc output voltage is 0V.
• Scale factor (gain): Connect both X and Y inputs to 10V dc. Adjust R_SCALE until the output voltage is 10V dc.
• Repeat the preceding steps as necessary.

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