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A Digital Ampere-hour Meter
Mark A. Peterson
©1992 Mark A. Peterson
In designing any electronic instrument a few fundamental principles should be kept in mind throughout the creative process. Economy of design is perhaps foremost. This implies that the complexity, cost, size, and concessions be minimized. Rule 1: Make your ideas into "recyclable" building blocks. You get spin-off technology that way. Rule 2: Don't reinvent the wheel. Take advantage of existing devices. In the design of my digital ampere-hour meter I used an IC (the AD654 voltage-to-frequency converter) which took care of most of the engineering problems associated with the cost, accuracy, and complexity constraints. After all, this is supposed to be a practical project.
Amp-hour Meters: General Theory
Ampere-hour meters can be engineered in many different ways. A Fluke 87 multimeter can be used to collect data and provide accurate results over a 36 hour period. The Homebrew in Home Power #26 featured an amp-hour meter with an architecture termed "charge-balance" in electronics jargon. It uses entirely analog components (op-amps) except for the counter/LCD display. My meter uses a precision voltage-to-frequency converter (VCF) which converts the millivolt analog input signal to a digital data stream. The digital information is then processed, stored and displayed in ampere-hours (counts up to
9999.9 Ampere-hours). Finding an accurate VCF capable of sensing millivolt inputs and needing only a single 5 Volt supply was a non-trivial task. Single-supply circuits like this one can be battery operated, important for hand-held instruments.
U1—LM 7805 five Volt regulator U2—AD654 voltage-to-frequency converter (VFC)
U3-U7—CD4017 decade divider U8—CD4066 Quad analog switch RS—0.01 W precision shunt 1% 2 Watt Dale type LVR-2 (for 20 Amperes maximum current) or 0.001 W precision shunt (for 200 Amperes maximum current) R1—1.5 kW wire wound, low temp coefficient (100 ppm)
R2—10 turn quality potentiometer (PC mount)
C1—10,000 mF 35V electrolytic
C2—0.01 mF ceramic
C4—1000 pF polystyrene or mylar
SW1—momentary on-off switch (normally off) Circuit Operation
The meter was designed to produce 100 counts-per-hour with a 100mV input across the precision shunt RS. This is equivalent to 10 Amperes flowing through a 0.01 Ohm shunt or 100 Amperes through a 0.001 Ohm shunt. The 5 digit display should therefore have a decimal point placed before its last digit. The voltage drop across the shunt controls the VCF's squarewave output frequency (CMOS compatible). This frequency is directly proportional to the magnitude of the input voltage. The resultant data stream (digital information) requires scaling because the VCF produces a kHz range output (necessary for accuracy and dynamic range at low level signals) and the LCD display/counter requires a very low frequency input (well under 1 Hz). Information scaling is achieved by sending the data stream through a cascaded series of five decade dividers (U3 through U7) to derive a data stream of frequency 1x10-5 (1/100,000th) of the VCF's output. The resultant low frequency squarewave is ready to be counted. The divider chain output is fed to an analog switch (U8) which acts as a relay, shorting the appropriate pins of the LCD counter to advance the count. An external reset button provides a CLEAR DISPLAY function.
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