*Jeff Kotowski*

*EDN*

A power-meter circuit typically requires either an analog or a digital multiplying circuit. These circuits can be complex, finicky, or expensive. A simpler way to achieve the multiplication first converts the current to a duty cycle proportional to the current. You can use this duty cycle to gate the input voltage; the gating effectively provides a multiplication function. A lowpass filter then provides an output voltage proportional to power. The method sounds convoluted, but the implementation is simple.

Figure 1. |
A low-cost duty-cycle IC forms the heart of a power-meter circuit. |

Figure 1 shows the complete power-meter circuit. The LM3812M-7.0 IC senses current and delivers a duty cycle proportional to the current. The current relates to the duty-cycle (D) output as follows:

The desired output is a voltage equal to the input power divided by 10. The divide-by-10 operation keeps the voltage within a manageable range. The output is, then:

To subtract the ½V_{IN} term in the equation, the first op amp inverts the power signal. The second op amp adds in the offset and again inverts the signal. Capacitors C_{1} and C_{2} provide filtering. These capacitors connect back-to-back to achieve nonpolar operation. Tests with input currents of –7 to +7 A and with a source voltage of 2 to 5.25 V showed better than 3% accuracy over the range of 0 to 25 W.

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