Hot-wire instruments are widely used for measuring both AC and DC currents and voltages. These instruments are based on the heating effect of electric current and are especially suitable for high-frequency AC measurements. In this article, we will explore the working principle, construction, advantages, disadvantages, range extension, and applications of hot-wire ammeters and voltmeters.
Working Principle of Hot-Wire Instruments
- Hot-wire instruments operate on the Joule heating effect, where a current-carrying wire heats up and expands due to the power loss (i²R). This expansion causes a pointer to move over a graduated scale.
- Since the heating effect is independent of the direction of current, these instruments can be used for both DC and AC measurements. The deflection of the pointer is proportional to the mean square value of current (I²), which makes them true RMS reading instruments.
Construction of Hot-Wire Ammeter and Voltmeter
The main components of a typical hot-wire instrument are:
- Hot-Wire (AB): A fine platinum-iridium wire stretched between two terminals. This wire carries the operating current and expands when heated.
- Tension Wire (CD): A phosphor-bronze wire connected to the midpoint of the hot-wire to magnify its expansion.
- Silk Thread and Pulley System: A silk thread attached to the tension wire passes over a pulley connected to the spindle. This system translates the expansion into pointer movement.
- Pointer and Scale: The pointer is mounted on the spindle and moves across the calibrated scale.
- Damping System: An aluminum disc moves between the poles of a permanent magnet to produce eddy current damping.
- Spring Control: A spring is used to provide the controlling torque.
Working of Hot-Wire Instruments
When the instrument is connected in a circuit:
- Current flows through the hot-wire.
- The wire heats up and expands.
- The expansion causes slack, which is pulled by the spring and silk thread mechanism.
- This rotates the pulley, moving the pointer across the scale.
- When current stops, the wire cools and contracts, returning the pointer to zero.
As the expansion is proportional to i², the average deflecting torque is also proportional to mean of i² over a cycle.
For DC:
θ ∝ I²
For AC:
θ ∝ I²rms
This results in a square-law scale, which is compressed at the beginning and expanded towards the end.
Magnification of Expansion
The actual expansion of the hot-wire is very small. To enhance the movement:
- The sag in the hot-wire is utilized, which is much greater than the linear expansion.
- Additional magnification is achieved using the phosphor-bronze tension wire, which further increases the deflection.
- The total magnification helps in making the instrument sensitive enough to detect small changes in current.
Advantages of Hot-Wire Instruments
- Can measure both AC and DC.
- Readings are independent of frequency and waveform.
- Immune to stray magnetic fields.
- Ideal for high-frequency AC measurements.
Disadvantages of Hot-Wire Instruments
- Non-uniform scale (square-law).
- Slow response due to thermal inertia.
- Frequent zero adjustment needed.
- Risk of burnout if overloaded.
- Higher power consumption than other types.
Range Extension
To measure higher values:
- For hot-wire voltmeters, a high resistance (multiplier) is connected in series.
- For hot-wire ammeters, a shunt resistor is used.
⚠️ Note: At radio frequencies, special care is taken to minimize inductance in the design and shunt resistors.
Applications of Hot-Wire Instruments
- Widely used for high-frequency AC measurements (e.g., radio and wireless applications).
- Cheaper than dynamometer-type instruments.
- Used in laboratories for accurate AC RMS measurements.
- Suitable for circuits where inductive reactance can affect other types of meters.
Conclusion
Hot-wire ammeters and voltmeters are reliable RMS measuring instruments that are especially useful for AC and high-frequency applications. While they have some limitations like non-linear scale and slow response, their versatility, accuracy in waveform-independent measurement, and immunity to magnetic fields make them an essential tool in many electrical and electronic measurement applications.