A5 Ohms PCA

Advance Technical Repair of Laptops Motherboard

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The following discussion covers the theory of operations for the A5 Ohms PCA circuitry. The A5 Ohms PCA sources 1x and 1.9x fixed value resistances, provides compensation, and generates an active guard. The A5 Ohms PCA can source ohms in one of several ways: two-wire, two-wire with compensation, and four-wire ohms. For discussion purposes, the A5 Ohms PCA can be divided into several areas: precision thin film resistor networks, relay switch matrix and control circuits, other control circuits, guard circuit, two-wire compensation circuits, monitoring circuits, and diagnostic circuits. See Figure 2-1 for a block diagram of the A5 Ohms PCA. The last part of this section discusses diagnostic capability built into the A5 Ohms PCA. See the A5 Ohms PCA schematics for circuit details.

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Figure 2-2. Block Diagram of the A5 Ohms PCA

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Figure 2-2. Block Diagram of the A5 Ohms PCA

Precision Resistor Networks

Fluke proprietary hermetically sealed thin film resistors (Z1-5) are used in the A5 Ohms PCA. While the values are not exact, the thin film resistors are made to have excellent time and thermal stability with low temperature coefficients. The resistors are made for 4-wire operation but may be used as 2-wire devices with degraded specifications. In four-wire mode, the resistors are connected to OUT_HI, SNS_HI, OUT_LO, and SNS_LO. The resistors have the following nominal ohm values: 0 (short), Z1 (1, 1.9), Z2 (10, 19, 100, 190, 1.0 k, 1.9 k), Z3 (10 k, 100 k, 1 M, 10 M), Z4 (19 k, 190 k, 1.9 M, 19 M). As an example, at 19 M, the maximum peak current is 1.6 ^A. The limit is required because an active circuit used to guard the resistance value from leakage has a maximum range of about +/-33 V. Exceeding that voltage may cause the ZGUARD buffer to cause errors in resistance measurement. Note that the resistors have maximum peak currents that should not be exceeded, and excessive current for extended periods of time may damage the resistors and create a long term offset. Only one resistor is switched onto the out high and out low signal traces at a time. The output and sense resistor traces are directed to the output terminals through a series of relays.

Relay Switch Matrix and Control

The relay switch matrix and resistors are shown on Sheet 1 of the A5 Ohms PCA schematic, and the switch control circuits are shown on Sheet 5 of the A5 Ohms PCA schematic. A block diagram of the high/low and high-sense/low-sense relay routing is shown in Figures 2-2 and 2-3. The relay matrix provides switching between the various resistor values and the A3 Motherboard PCA, and also switches in guard circuits, 2-wire mode compensation circuits, and other compensation circuits. These relays are latching type and can be set or reset.

A3 Motherboard PCA connections are shown on Sheet 1 (8A-8D). K29 and K30 isolate the ohms output and sense lines from the A3 Motherboard PCA. The HIGUARD signal is switched through K25 and can be used to drive the shields of HI cables when the Calibrator output is active. See the Guard Circuits section for more details.

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