When measuring AC/DC current, the current needs to be
processed and handled as a voltage. That’s why we need shunt
resistors to do the conversion. When current flows through a
shunt resistor, a voltage drop presents on the shunt and the
current value can be calculated by having the voltage drop
across the shunt divided by the resistance of that shunt resistor.
Shunt resistors R301~R303, R305 and R308 as well as an
additional 0.01Ω resistor (attached in series with the 20A input
terminal -no part number given) are used for current
measurement. K307~K310 are the relays used to select the
proper shunt resistors for a given current measurement range.
An additional microcontroller, U202 is used to drive the
seven-segment displays, receive input from the keyboard
matrix and actuate the signal switching relays in the signal
conditioning circuit. U206 (TPS3824,) a processor supervisory
circuit which is also used as a watchdog timer, provides circuit
initialization and timing supervision for the processor-based
system. U201, (AT93C56) is a 2K EEPROM that is used to store
temporary variables and readings when measuring. All the
data which is received from the keyboard matrix or transmitted
to the seven-segment LED display on the front panel are
buffered by the octal D-type transparent latches (74HC373).
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