An inexpensive switching regulator can provide all the active circuitry needed to implement a dc wattmeter and allow the power dissipated in a resistive load, RX, to be displayed on an analog panel meter.
The wattmeter’s operation is based on the condition that the average current flowing through the meter is proportional to the product of the applied voltage, Vin, and the duty cycle of the transistor switch at pin 8 of the TL494. PWM circuitry in the TL494 produces a output duty cycle that’s linearly proportional to the voltage across the current-sensing resistor Rsense. The average current passing through the meter is therefore proportional to the power dissipated in the load.
Component values depend on the required full-scale current and voltage ranges of the wattmeter as well as the value of the current-sense resistor and the current sensitivity of the meter selected. However, the following guidelines are noted:
- Oscillator frequency isn’t critical, but a relatively low frequency (as determined by C1 and R5) will minimize any effects the meter inductance might introduce.
- The value of the current-sense resistor should be significantly lower than the values of both the load resistor RX and R3.
- Choose the R3/R4 ratio so that op amp A1 produces a swing of just less than 3 V for the full-scale swing across the current-sensing resistor.
- Adjust R1 for 0% duty cycle with zero current flowing through Rsense.
- Select R6 and R7 to produce rated full-scale meter current based on an 85% duty cycle at maximum applied voltage Vin.
Applied voltage (Vin) should be limited to 40 V due to the voltage rating of the TL494. If the range of interest of Vin is never less than 10 V or so, pin 12 of the TL494 may be directly connected to Vin, resulting in a self-powered wattmeter.
