EN IEC 60738 Thermistors - Directly Heated Positive Step Function Temperature Coefficient

The standard “EN IEC 60738 Thermistors - Directly heated positive step function temperature coefficient”, which is based on a European standard and later published by the International Electrotechnical Commission (IEC), consists of a series of sub-standards published under the same title. Namely,

• EN 60738-1 Part 1: General requirements” defines test terms and methods for positive step function temperature coefficient thermistors, insulated and non-insulated types, typically made of ferro-electric semiconductor materials. This standard covers standard terms, inspection procedures and test methods for use in detailed specifications for qualification approval and quality assessment systems for electronic components.
• Standard “EN 60738-1-1 Part 1-1: Blank detail specification - Current limiting application - Assessment level EZ”.
• Standard “EN 60738-1-2 Part 1-2: Blank detail specification - Heating element application - Assessment level EZ”.
• Standard “EN 60738-1-3 Part 1-3: Blank detail specification - Surge current application - Assessment level EZ”.
• Standard “EN 60738-1-4 Part 1-4: Blank detail specification - Detection application - Evaluation level EZ”.

In short, this standard is a European and international standard applied to thermistors, which are types of temperature-sensitive resistors. It covers thermistors used in electronic equipment in particular.

The topics covered by this standard are:

• General specifications for positive temperature coefficient (PTC) thermistors
• Performance requirements and test methods
• Application guidelines (especially in electronic circuits)
• Reliability and quality assurance procedures

PTC thermistors are resistors with a positive temperature coefficient, meaning that their resistance increases with increasing temperature. PTC thermistors are divided into two groups according to the materials used, their structure and manufacturing process:

• The first group of PTC thermistors consists of silicon, which uses silicon as a semiconductor material. Due to their linear characteristics, they are used as PTC temperature sensors.
• The second group is the switching type PTC thermistor. The switching type PTC thermistor has a highly non-linear resistance-temperature curve. When a switching type PTC thermistor is heated, the resistance initially begins to decrease until a certain critical temperature is reached. As the temperature increases further above this critical value, the resistance increases significantly. This type of PTC thermistors are widely used in PTC heaters, sensors, and similar equipment. Polymer PTC thermistors, which are made of a special plastic, fall into this group and are often used as resettable fuses.

Key features of PTC thermistors include:

• Silistors have a linear resistance-temperature characteristic with a relatively small slope over most of their operating range. They may exhibit a negative temperature coefficient at temperatures above 150 degrees. The temperature coefficient of resistance of silistors is about 0,7 percent to 0,8 percent/degree.
• Switched PTC thermistors are usually made of polycrystalline ceramic materials that are highly resistive in their original state and are made semiconducting by adding additives. PTC is often used as self-regulating heaters. Most switched PTC thermistors have a switching temperature of 60 degrees to 120 degrees. However, special application devices are manufactured that can switch at temperatures as low as 0 degrees or as high as 200 degrees.

Switching PTC thermistors have a slightly negative temperature coefficient up to the point of minimum resistance. Above this point, they experience a slightly positive coefficient until they reach the transition temperature. This temperature is sometimes called the switching or Curie temperature. The switching temperature is the temperature at which the resistance of switching PTC thermistors begins to rise rapidly. The Curie temperature is often defined as the temperature at which the resistance is twice the value of the minimum resistance.

The minimum resistance of a PTC thermistor is the lowest resistance that can be measured in a switched type PTC thermistor. It is the point on the curve where the temperature coefficient becomes positive.

Nominal PTC resistance is normally defined as the resistance at 25 degrees. It serves to classify thermistors according to their resistance value. It is measured with a low current that does not heat the thermistor enough to affect the measurement.

The distribution constant represents the relationship between the applied power and the temperature rise due to self-heating. Some of the factors that affect the distribution constant are: contact wire materials, the way the thermistor is mounted, ambient temperature, conduction or convection paths between the device and its surroundings, the size of the device, and even its shape. The distribution constant has a large effect on the self-heating characteristics of the thermistor.

The nominal current represents the maximum current that can flow continuously through a PTC thermistor under specified ambient conditions. Its value depends on the dissipation constant and the RT curve. If the thermistor is overloaded to the point where the temperature coefficient begins to drop again, this will result in a runaway power condition and destruction of the thermistor.

Similar to the maximum rated current, the maximum rated voltage represents the highest voltage that can be continuously applied to the thermistor under specified ambient conditions. Its value depends on the distribution constant and the RT curve.

Our organization, which has been trying to support businesses from every sector with its testing, measurement, analysis and evaluation studies carried out in a wide range for years, has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. In this context, testing services are also provided to businesses in accordance with the "EN IEC 60738 Thermistors - Directly heated positive step function temperature coefficient" standard.