Digital Voltmeter and Ammeter Using CA3161E and CA3162E

Figure 1 

The circuits were designed using two A/D converters to create a voltmeter and ampermeter.

• 1N4001 – a 1.0 Amp Silicon rectifier with voltage range of 50 to 1000 Volts and possessing features such as guaranteed high temperature soldering, high current capability, diffused junction, low reverse leakage, utilizes void-free molded plastic technique for low cost construction, and carries Underwriters Laboratory Flammability Classification of 94V-0 by its plastic package
• BC557 – PNP general purpose transistors used for amplification and switching due to its low current and low voltage
• CA3162E – a monolithic analog to digital converter that provides 3 digit multiplexed BCD output with features such as availability of extended temperature range version, over range indication, maintains delay with hold inhibits conversion, high speed (96Hz) and low speed (4Hz) conversion rate, internal timing with no external clock required, differential input, capable of reading 99 mV below ground with single supply, ultra stable internal band gap voltage reference, and dual slope A/D conversion
• CA3161E – a monolithic integrated circuit that performs the BCD to seven segment decoding function and features constant current segment drivers, low standby power dissipation, pin compatible with other industry standard decoders, eliminates the need for output current limiting resistors, constant current segment outputs, and TTL compatible input logic levels
• 7805 – a 3-terminal 1A positive voltage regulator with output transistor safe area compensation, internal short circuit current limiter, output voltage of 5V, 12V, and 15V, internal thermal overload protection, no external components, and current output in excess of 0.5A
• Voltmeter – a device or an instrument used for measuring the electrical potential difference between two points of either alternating current or direct current electric circuit
• Ammeter – also known as amperemeter, is an instrument used for the measurement of electric current flow in terms of the unit called ampere
• 7 Segment LED – is a form of electronic display device for displaying decimal numerals that is an alternative to the more complex dot-matrix displays also known as seven-segment indicator
• Diode Bridge – also known as bridge rectifier which has four diodes arranged in a bridge configuration where the output voltage has the same polarity with either polarity of the input voltage

The circuits are designed with the combination of CA3161E and CA3162E to imposing a complete 3-digit display using a binary coded decimal to 7-segment display/decoder. The combination of the two converters offers a complete digital readout system with a minimum number of external parts. Although the circuit provides precise measurements of the voltage in the terminal and the current that flows within the circuit, they are very basic even for an inexperienced user. The CA3162E corresponds to the IC1 and IC3 in both circuits which functions as the A/D converter for 3-digit display while CA3161E corresponds to the IC2 and IC4 which drives the 7-segment decoder.

From the first figure, which represents the voltmeter, the input comes in series with resistor R1. It is then combined with resistor R3 that creates a voltage divider operation which produces an output voltage that is a fraction of its input voltage. IC1 CA3162E has its pin 10 and 11 receive the input power. The variable resistor RV1is being adjusted just enough to produce a display in the 7-segment which is zero in all the 3 digits. The external voltage in the input has value around 900 mV. The resistor RV2 will operate to adjust and rectify the 900 mV source to produce the necessary display. Doing this will require checking on the voltage for precision and good quality.

In figure 2, which represents the ampere meter, no resistor exists in the input stage thus, creating a drop in the voltage that is proportional to the value of current flowing into the circuit. Since this circuit utilizes the same type of IC, pins 10 and 11 of IC3 would be receiving the input voltage. the variable resistor RV4 will provide the necessary adjustment to provide a zero display on all the 3 digits in the 7-segment display. The positive input is connected to the positive pole of the battery while the negative input has the 10 ohm resistor at 10 W connected in the negative pole. Variable resistor RV3 is adjusted to produce 0.9 A current in the display. To ensure this value, an external ammeter is used for measurement.

In both circuits, the voltage or current converter converts the input voltage applied between pins 10 and 11 to a current that charges the integrating capacitor (C4 and C9) for a predetermined time interval. The V/I converter will be disconnected from the integrating capacitor and a constant current source of opposite polarity is connected. The original value of the number of clock counts will be restored which elapsed before the charge. The comparator senses the restoration which in turn latches the counter. The count is then multiplexed to the BCD outputs.

Voltmeters contain very high resistance and are connected in parallel across the component through which the voltage is being measured. The black or negative lead of the voltmeter is connected normally to the negative terminal of the component like the power or battery supplies while the red or positive lead of the voltmeter to the positive side of the component. Alternatively, alligator clips can be used in case of the black lead since it can be permanently connected to 0V while the red probe measures other points for voltages.

In using the ampere meter, it should be placed in series with the component though which current is being measured. In order to prevent the reduction of that current as it flows through the device, it should be designed with a low internal resistance. Ampere meters for both AC and DC can be made of dynamometer, hot-wire, or moving-iron.

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