How to measure the input power of PTC heating elements

How to measure the input power of PTC heating elements?

PTC is the abbreviation of Positive Temperature Coefficient. It is a commonly used heating element. Due to its low thermal resistance, high heat exchange efficiency, longer service life than traditional heating wires, safer and more reliable, etc., it is widely used in electrical appliances. For example, air conditioners, hot air blowers, dryers, hair straighteners and other electrical appliances all use PTC heating elements.

PTC is mainly composed of resistance with positive temperature coefficient. These resistors are thermistors. When the temperature rises within a certain range, their resistance rises sharply and non-linearly as the temperature rises. Due to this special physical characteristic, the current of the PTC heating element changes with the change of the resistance value during the working process. And its heating power will change with the heating time: when the electricity is first turned on, the initial power will be relatively small, then the heating power will gradually increase to the maximum impact power, and then the power will drop, and the stable power will be reached after the heat; but when the heat is dissipated When conditions change, the power and temperature of the PTC will be readjusted to achieve a new balance. Therefore, the input power and current of the electrical appliance using PTC heating components are not stable during its operation. As the resistance of the PTC heating components changes, the input power and current will also change. Therefore, we need specifications to guide R&D engineers and measurement engineers to correctly measure input power and current.

Schematic diagram of current, voltage and power waveforms of PTC heating components in the working process

Related rules in the IEC60335-1 specification

In Chapter 10 of the input power and current of the IEC60335-1 specification compiled by the International Electrotechnical Commission, related measurement requirements and methods are given for the measurement of the input power and current of PTC heating appliances. *The new version of the specification no longer simply defines the input power and current in terms of the power or the integral average value of the current during the representative period. It defines the 10% rule of the representative period. This rule requires us to arrange the active power or current effective value of the representative period in descending order from large to small, and take the value at the 10% position. And we need to compare the maximum value, the arithmetic average value and the value obtained by the 10% rule, and then determine which value to choose to define the input power or current according to the comparison result.

Whether the input power and current use the maximum value of the representative period, the arithmetic average value, or the value determined by the 10% rule, not only needs to look at the operating conditions of the equipment, but also need to compare these values. Therefore, it is very important to accurately measure these values.

How to accurately measure several values for comparison?

For appliances that use PTC heating elements, the input power and current will change during operation, so we need to record the input power or the effective value of the current during the representative period to determine whether the appliance will have a stable state. In general, as long as we record the data for the entire representative period, we can get the maximum value, the arithmetic mean, and the value determined by the 10% rule based on these data, and then determine which value to use as input according to the flowchart Power or current.

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