Instantaneous Current Surge in Tungsten Infrared Heating Tubes

Users of resistance heating tubes or tungsten wire infrared heating tubes are aware that when these tubes are first started, within the first minute or so, the current in the circuit is very large. This phenomenon is known as instantaneous inrush current, where the initial current is much greater than the rated current. But why does this happen?

First, let’s define instantaneous current. In a circuit, the moment it is powered on, a large current is usually generated. This short-duration large current is called instantaneous current. In circuits with capacitors, this is due to the need for capacitors to charge when first powered on, effectively creating a short circuit. Therefore, the instantaneous current can theoretically be very large. This concept is relatively easy to understand in circuits with capacitors. But why does this happen in infrared heating tube circuits?

Let’s first understand the material of the heating filament in infrared heating tubes—tungsten wire. Tungsten, with its high melting point, high resistivity, and good mechanical strength, is the best material for incandescent lamps among all pure metals. The heating principle of infrared heating tubes is similar to that of incandescent lamps, so tungsten wire is also used for the heating filament.

At 20°C, the resistivity of tungsten wire is 5.3×10−8 Ω⋅m5.3 \times 10^{-8} \, \Omega \cdot m5.3×10−8Ω⋅m. Like other metals, the resistance of tungsten wire increases with temperature. The temperature coefficient of resistance for tungsten wire is 5.5×10−3/100°C5.5 \times 10^{-3}/100°C5.5×10−3/100°C. The surface temperature of the tungsten filament in a 100W incandescent lamp usually ranges between 2300-2800°C. For infrared heating tubes, which typically have a power of around 1000W, the operating temperature is even higher than 2800°C. Thus, the resistance of the infrared heating tube during operation is much greater than its resistance at room temperature (cold resistance).

So, what are the cold resistance and operating resistance of a 220V 500W infrared heating tube? What does the resistance change curve look like at startup? According to test data from a paper in the journal “Lighting Electrical Appliances,” the resistance of a tungsten wire infrared heating tube changes from 7.45Ω to 87.36Ω from startup to stable operation. This means that with a stable voltage of 220V, the instantaneous current at startup is 10 times the stable operating current.

The greatest change in resistance occurs within the first second. So, why does the tungsten wire infrared heating tube have an instantaneous inrush current at startup? It is mainly due to the low resistance of tungsten wire at low temperatures. If the voltage remains constant, the current in the first second of startup will be much larger than the current at rated power.

This principle also applies to traditional resistance heating tubes.

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