Digital Panel Meters (DPMs) are voltage measuring devices which can be used to measure current using a shunt resistor or hall effect current sensor. When a display of Power (watts) is required, the voltage and current values must be multiplied to produce a voltage that is proportional to the product of these two quantities.
Power (watts) = Voltage X Current
This application note will describe a method to accurately display DC Power values on a DPM.
There are several methods that can be used to calculate power based on voltage and current values. Most methods require a micro-controller to perform the calculation and then display the power level on an LCD or similar indicating device. While this can be an extremely accurate way to display Power, it requires that the analog values of voltage and current be converted to digital values that can be processed by the micro-controller. Furthermore, a firmware program must be written to make it functional. The method described here, uses an analog multiplier IC to perform the calculation directly on the two analog quantities which then produces a third analog value suitable for display on a DPM. This is entirely a hardware solution and requires no software program.
An Analog Devices AD633 device was chosen as the analog multiplier IC due to its excellent accuracy and ease of use. Since it is a 4 quadrant multiplier it functions equally well with any voltage polarity and produces an output voltage that is proportional to the product of the X and Y input voltages. This is the DC Power level that can now be displayed on a DPM.
This application note has shown how a Digital Panel Meter may be used to measure and display DC Power values based on a DC voltage and a DC current applied to the inputs of the circuit. The connection diagram is shown below.