Indicating DC Meters

Fundamentally, all meters measure current and in this sense are ammeters. When they have low internal resistance so as to produce no effect on a circuit's performance and are actuated through direct connection in series with the main current path, they are truly ammeters.

When they have high resistance so that they may be placed in shunt or directly across a voltage source they are called voltmeters. Their indication of voltage is based on the product of a known, calibrated internal resistance in series with the meter movement, and the current flowing through that resistance.

Most meters are constructed to use the attractive and repulsive force characteristics of magnetic structures to produce deflection of a pointer. In the most common PM type mutual force is exerted upon a movable, pivoted coil which itself becomes d-c current being measured. When a movable a magnet when it passes magnet is mounted in the field of a fixed permanent magnet and when it is pas sing current of the correct polarity, a force is exerted upon the movable magnet. In the resultant alignment of the fields, definite and proportional movement is produced against the res training force of a spring connected to the pivoted coil. Deflection is arranged to be proportional to the amount of current through the coil, so that when the meter scale over which the pointer moves is calibrated, a true indication of current is obtained.

The basic d-c meter movement uses the pivoted coil and a PM. The electrodynamometer type of instrument, which is less common than the PM type, employs the same basic moving-coil mechanism but uses a fixed electromagnetic field actuated by current which is related to or proportional to that in the moving coil.

When the current through a circuit is sent through the fixed-magnet coil and the voltage across it applied to the movable coil, this instru ment becomes a wattmeter, since deflection is then proportional to the product of voltage and current.

Dynamometers are less flexible than the PM type meters, less sensitive, and generally more expensive. Their basic arrangement, however, allows them to be used on either a-c or d-c when appropriate scale multiplier, and shunt arrangements are included in their internal circuitry.

Indicating DC meters are devices used to measure various electrical parameters such as voltage, current, and power in direct current (DC) circuits. Here are some common types of indicating DC meters:

1. DC Voltmeter: Measures the voltage between two points in a DC circuit. It is connected in parallel with the component whose voltage is to be measured. The scale of a DC voltmeter is typically linear.

2. DC Ammeter: Measures the current flowing through a DC circuit. It is connected in series with the circuit. The scale of a DC ammeter may be linear or non-linear.

3. DC Wattmeter: Measures the power consumed or generated in a DC circuit. It is connected with a combination of series and parallel connections. It typically has two coils: a current coil (connected in series) and a voltage coil (connected in parallel).

4. Galvanometer: A very sensitive instrument used to detect and measure small currents in a circuit. It can be modified to act as a voltmeter or ammeter by adding appropriate resistances.

Key Features of Indicating DC Meters:

• Accuracy: The degree to which the measured value conforms to the true value. High-quality meters have low errors.
• Range: The maximum and minimum values that can be measured by the meter. Ranges can often be selected via a switch.
• Sensitivity: For voltmeters, it is often expressed in ohms per volt and indicates the meter's ability to measure small voltages.
• Resolution: The smallest change in the measured quantity that can be detected by the meter.
• Response Time: The time the meter takes to reach a stable reading.

Types of Indicating Mechanisms:

• Analog Meters: Use a needle and dial to display readings. Commonly use a moving coil mechanism where a coil moves in a magnetic field to deflect the needle.
• Digital Meters: Use digital displays (LED, LCD) to show readings. These are more precise, easier to read, and often include additional features like data logging and auto-ranging.

Applications:

• Laboratories: For experiments and precise measurements.
• Industrial Settings: For monitoring and controlling equipment.
• Automotive: For battery testing and troubleshooting electrical systems.
• Consumer Electronics: For checking and maintaining electronic devices.

Considerations When Using DC Meters:

• Loading Effect: The meter should not significantly alter the circuit it is measuring.
• Polarity: Correct polarity must be observed to avoid damage to the meter.
• Range Selection: Proper range must be selected to prevent meter damage and ensure accurate readings.

Understanding these basics will help in selecting and using the appropriate indicating DC meter for your needs.