Theory of Operation 5
OHMS PROTECTION
The second stage (U404, precision resistor network Z401, and analog switches U402 and U403) is a current amplifier whose gain is controlled by the In-Guard μC. The In-Guard μC sets the output current for each range by controlling U402 and U403. (See switch state table in Figure 5-7.)
5-16. OHMS PROTECTION
The Ohms Protection circuit (Q402, Q403, Q404, Q405, Q406, and Q407) clamps the open circuit voltage of the Ohms Current Source and provides protection for the Ohms Current Source.
The circuit protects the Ohms Current Source from up to +/-300V across the INPUT terminals. The circuit also clamps voltage transients larger than 1560V with four MOVs (RV401, RV402, RV403, and RV404). In addition, a 1 kΩ, 2W fusible wire-wound resistor (R410) in series with the output current path fails open-circuit under extreme overvoltage conditions.
Large positive input voltages are blocked by CR402. Large negative input voltages are dropped equally across three high-voltage transistors (Q402, Q403, and Q404). If -300V is present at the collector of Q404, the voltage drops equally across Z402 so that large negative voltages never reach the current source.
The circuitry associated with Q408 (R406, R407, R408, R409, Q406, Q408, and CR403) clamps the open-circuit voltage of the Ohms Current Source below +6.5V in the lower four ranges and below +13V dc in the higher two ranges. The in-guard uC turns Q408 on or off depending on range. In the lower four ohms ranges, Q408 is on, effectively shorting R409; R406 and R409 then form a voltage divider which clamps the output of the ohms current source below +6.5V. In the higher two ohms ranges, Q408 is off, including R409 in the voltage divider and clamping the output below +13V.
5-17. OHMS FUNCTIONS
5-18. 2-Wire Ohms
In the 2-wire ohms function, the Ohms Current Source is connected to the INPUT HI terminal by ohms relay K401 (Figure 5-8). The Ohms Current Source applies a known current to the resistance under test, and the resulting voltage drop across the resistor is measured ("sensed") as in the VDC function.
