July 1998, Issue 96

A PIC-Based AC Power Meter

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HARDWARE DESIGN

Figure 1 shows the four subsystems of the power meter. The power-supply subsystem supplies DC power to the other subsystems. The analog interface contains Woodward’s circuit [1].

The microcontroller and A/D subsystem acquire data and compute power readings that are then displayed by the LCD subsystem. Figure 2 shows how the subsystems link together.

A 9-V battery connects to J6 that feeds a 78L05 regulator. The 5.1-kW 1% resistors in a voltage divider network provide the 2.5-V bias voltage for the analog section. They also provide the 9-V sense voltage to the microcontroller for low-battery detection.

A resistor divider network is selected over a 2.5-V reference diode because of cost to provide the 2.5-VDC reference used in the analog section. I chose 5.1-kW resistors since I’m already using this value in the analog interface.

Woodward’s original circuit used ±15-VDC power supplies and an OP27 precision op-amp. My goal was to modify his circuit using a +5-VDC single supply and a common LM358 op-amp.

The redesign for single-supply operation is straightforward: swap 2.5-V DC bias for ground, ground for –15 V, and +5 V for +15 V. Woodward’s circuit indicates power delivered to and from the load, with power delivered to the load being below the 2.5-V bias.

Power readings range between ground and +2.5 V, effectively reducing the ADC resolution to seven bits. Therefore, 128 discrete output values are possible between no load and full scale.

A pot, P1, calibrates the full-scale value to approximately 1200 W nominal. By adjusting the pot to give a full-scale reading of 1280 W, the actual power (in watts) is obtained by:

So, if ADvalue equals 0x76, the power is 100 W.

I initially chose the PIC16C71 because of its low cost and onboard four-channel eight-bit ADC. However, you can save even more by replacing the ’16C71 ($12.30) with a separate ADC, National’s ADC0831 ($3.29), in combination with the ADC-less PIC16C61 ($6.15).You save $2.86, but you need space for one more 8-pin DIP.

The National ADC0831 is a low-cost 8-bit ADC that has a simple three-wire serial interface (chip select, clock, and data) and a 32-ms conversion time.

Note there are no pull-up resistors on the two momentary switches. Pullups are provided internally by the PIC16C61. Again, you see the 5.1-kW 1% resistor used as the MCLR pullup, which means the MCLR pin can be shorted to ground without shorting out the +5-V supply.

This component could be replaced with a 0-W jumper for production. Currently, a crystal is used as the microcontroller clock, but you could replace it with a ceramic resonator for more cost savings.

The display subsystem uses a nonmultiplexed Varitronix LCD, with a Microchip AY0438 LCD driver. The AY0438 operates up to 32 segments of an LCD, providing a simple three-wire serial interface (data, clock, and load), and it’s capable of generating the AC waveforms required to illuminate LCD segments.

For a LCD segment to be on, there must be voltage differential between the segment pin and the backplane. However, this voltage cannot be static (non-time-varying) or the display gets permanently damaged. To drive the LCD correctly, a low-frequency (100 Hz) square wave is applied to the backplane pin.

For a segment to be on, a segment pin must have the inverted backplane signal applied. For a segment to be off, the segment pin must have the in-phase backplane signal applied.

The AY0438 generates this backplane waveform and the correct segment waveforms without processor involvement. Just hang a capacitor on the LCDf pin to control the onboard oscillator.

Initially, I used the onboard oscillator of the AY0438 to generate the AC waveforms needed and to get the display up and running. However, the AY0438 can only drive 32 segments.

I needed 33 segments to use the arrow segment and all four digits, three decimals, and the colon. With a couple of unused pins on the PIC, I figured it couldn’t be that hard to drive the LCD directly.

And, driving the LCD is easy. Just remove the cap from the LCDf pin, then drive this pin (backplane) and the thirty-third segment directly from the PIC. Remember that you can’t drive the LCD segment pin alone because the AY0438 oscillator (backplane) would be asynchronous with respect to the PIC.

The software must toggle the backplane and the thirty-third segment every 10 ms. Note that the AY0438 still manages the other 32 segments and provides a serial interface. The AY0438 just gets its backplane frequency reference from the PIC. This is a case where a watchdog timer should be used because display damage can result if the backplane doesn’t toggle at least every 10–100 ms.

Register definitions of external interfaces to the PIC are shown in Figure 3. The PIC16C61 is an 18-pin DIP and only has 13 I/O pins. Obviously, when you’re using an I/O-limited micro like the PIC, it’s important to choose peripherals with low pin-count interfaces.

Five pins are used for the LCD subsystem. It could have been three if I didn’t need the thirty-third segment. Three pins are used for the ADC and two for the mode switches.