This circuit note describes a technique for auto-ranging a battery operated DVM suitable for panel meter applications. Also, circuit ideas will be presented for conductance and resistance measurement, 9V battery and 5V supply operations, and current measurement.
In the field of DVM design, three areas are being addressed with vigor: size, power dissipation, and novelty. The handheld portable multimeter has gained in popularity since low power dissipation devices enabled battery operation, LSI A/D converters reduced IC count, and novelties such as conductance, automatic range scaling, and calculating were included to entice the user.
Auto Ranging Circuitry:
The control signals necessary for auto-ranging are overrange, under-range, and clock. The over-range and underrange inputs control the direction of a scale shift, becoming active at the completion of an invalid conversion and remaining active until a valid conversion occurs. The clock input controls the timing of a scale shift. This signal should occur only once per conversion cycle, during a time window which will not upset an ongoing conversion and must be disabled after valid conversions.
Auto-range is achieved in two steps:
1. Measure the input and compare it with range that is >2V, >20V and >200V. this is achieved using LM324 operational amplifier.
2. Shifting of range selector switch is done using CD4051 it selects the correct input.
3. Shifting of decimal point according to input voltage is done using 74138 3:8 decoder IC.
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