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Current Measurement Circuit

IP.com Disclosure Number: IPCOM000102197D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 2 page(s) / 48K

Publishing Venue

IBM

Related People

Lochner, DL: AUTHOR [+2]

Abstract

Described is a circuit and control technique which enables a computer system to measure current drawn by a load from a DC supply. The system includes an analog-to-digital converter (ADC) and some number of digital output (DO) ports. An ADC input and a DO bit are used by the circuit, as shown in the figure.

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This is the abbreviated version, containing approximately 74% of the total text.

Current Measurement Circuit

       Described is a circuit and control technique which
enables a computer system to measure current drawn by a load from a
DC supply.  The system includes an analog-to-digital converter (ADC)
and some number of digital output (DO) ports.  An ADC input and a DO
bit are used by the circuit, as shown in the figure.

      DC supply voltage 1 is applied to load 3 through sense resistor
2, which has a value sufficient to produce a measurable voltage drop
while not lowering the voltage at load 3 by an unacceptable amount.
The current drawn by load 3 can be calculated by Ohm's law using the
known resistance of resistor 2 and the measured voltage drop across
it.

      The DC voltage at each end of resistor 2 is greater than the
input limit of the system's ADC.  Zener diode 7 reduces the voltage
sensed at the ends of resistor 2 by a fixed amount, passing the
result to the circuit output 4, which connects to an ADC input.  FET
multiplexer 5 operates as a solid-state SPDT switch, connecting the
zener network to one end or the other of resistor 2.

      In operation, the controlling computer places DO bit 6 in such
a state that MUX 5 connects zener diode 7 to one end of resistor 2.
The computer takes a reading from the ADC, then inverts DO bit 6 to
cause MUX 5 to connect zener diode 7 to the other end of resistor 2,
and take another ADC reading.  The difference in readings is the
voltage drop across resistor 2, from which the compu...