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05-31-2017

Power meter is ±1% accurate

Maxim » MAX427, MAX4372

Ken Yang

EDN

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 15 V. Select the current-sense resistor as follows: RSENSE = 1/P, where RSENSE 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 RSENSE = 0.1 Ω.

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 10 V when the load power is 10 W. To change the transfer-function gain, change the sense resistor as follows: 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 14 W. 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.

 Figure 1. This power meter, whose output voltage is proportional to load power, achieves ±1% accuracy.
Connect a 1-kHz, 5 V p-p sinusoidal signal to the Y input, and connect the X input to ground. Using an oscilloscope to monitor the output, adjust RX for an ac null (zero amplitude) in the sinusoidal signal.

Connect a 1-kHz, 5 V p-p sinusoidal signal to the X input, and connect the Y input to ground. Using an oscilloscope to monitor the output, adjust RY for an ac null (zero amplitude) in the sinusoidal signal.

Connect both X and Y inputs to ground. Adjust ROUT until the dc output voltage is 0 V.

• Scale factor (gain):
Connect both X and Y inputs to 10 V dc. Adjust RSCALE until the output voltage is 10 V dc.
 Figure 2. The measured power is better than ±1% accurate for power levels of 3 to 14 W.

Repeat the preceding steps as necessary.

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