Thursday, September 25, 2014
Digital Voltmeter and Ammeter Circuit Module
- This V/I display module is eminently suitable for building into an existing DC power supply, where it gives a precise indication of the set voltage or the current consumption of the load.
- In the voltage range, the decimal point lights on LD3, and the resolution is therefore 100 mV Two current ranges are possible: 0-9.99 A (link a) or 0-0.999 (.999) A (link b).
- The 3-digit readout is based on A/D converter Type CA3l62 and BCD-to-7 segment decoder Type CA3l6l, both from RCA.
- The resulting small negative deviation in the volt- age range is compensated by P2.
- These points should be adjusted in the above order. Two presets, P1 and P3, are required to ensure correct nulling of the module. P1 compensates for the quiescent current consumption of the regulator circuit in the supply.
- When voltage measurement is selected, P4-R1 attenuates the input voltage by a factor 100. Also, point D is pulled low so that the decimal point on the LS display, and the
- The current sensing resistor is therefore either 0Rl or lR0. It is important that Rs does not affect the output volt- age of the supply in question.
- When current measurement is selected, the drop across the sensing resistor is applied direct to the HI-LO inputs of DAC IC1.
- The sensing resistor has such a low value as to render the voltage divider ineffective. There are four adjustment points in the module: P1: current range nulling; P2: full-scale current calibration; P3: voltage range nulling; P4: full-scale voltage calibration.
- The V/I display module is conveniently fed from the unregulated voltage available in the supply (max. 35 V) see points E and F in Fig. 2; bridge rectifier B1 may then be omitted.
- It must, therefore, be fitted ahead of the voltage divider that controls the output voltage. DPDT switch S1 selects between l voltage and current readings.
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