Flexible heaters made from silicon and polyimide materials are designed to provide heating features to a wide range of applications. Their flexibility allows the heaters to wrap around odd-shaped surfaces when used inside or outside products. The flexibility of the heater is to provide enough heat for the intended application at the specific area without interfering with its functions.
Heater control systems and controller mechanisms help to monitor the temperatures of the flexible heater to provide enough heat when necessary, without providing too much that could damage the circuitry, heater materials, or the application. The types of mechanisms used to control the heater temperatures will be based on how the heater is customized and the costs.
Types of Heater Controls
The two types of heater controls that will be discussed are thermostats and thermistors. We will review the differences, pros and cons, and controller options.
Most customers have heard and used thermostats in their daily lives. These are temperature sensors that evaluate the heat that is around them and then controls the temperature of the heating. The way the thermostat senses the temperature can be accomplished in different ways, such as ambient sensing and line sensing.
Silicone flexible heater with embedded thermostat.
Ambient temperature sensing is when the thermostat evaluates the surrounding temperatures. The sensor's algorithm will be set at a specific set point where the thermostat will lower or switch off the temperatures coming from the heater. Line temperature sensing involves monitoring the line that extends into the heating system. When that temperature goes over or under the set point, the thermostat will activate to perform the desired function.
The basic function of a thermostat is the use of a bimetallic (or bimetal) strip. This strip connects the heating system with the electrical circuit as electricity passes through it, which places the heater in the "on" position. As the strip becomes hot, one of the pieces of metal bends. Eventually, it bends so much that it interrupts with electrical circuit as the heater shuts down and is in the "off" position. When the temperatures start to cool off, the metal strip starts to move back to its original position until returning to the circuit to switch on the temperature again.
The advantages of a thermostat are that they are simple and low-cost. They can also be used for line sensing in addition to ambient sensing. Some disadvantages are slower reaction times when switching from on/off positions and looser control of its functions. Another disadvantage is that they are not long-lasting as they can wear out due to the on/off cycles of the bimetal strip, lowering its reliability.
The thermostat will typically be encapsulated in the silicon rubber or polyimide materials as it is mounted on a heated area. On some occasions, a thermostat will have a thermistor in its design.
A thermistor (or thermocouple) is also a sensor that monitors temperatures. Considered a thermally resistive resistor, a thermistor has a greater level of resistance than the conducting material, a lower level of resistance than the insulating material, and will change its resistance when there is a change in temperature. They will not amplify or generate a signal. Thermistors are used as ambient sensors that monitor changes in conductivity.
Example of silicone flexible heater with thermistor embedded wiring.
The thermistor will be made from a range of oxides, binders, and stabilizers as it is made into wafers and then cut to the specific shape and size. There are two types of thermistors available: Negative Temperature Control (NTC) thermistors and Positive Temperature Control (TPC) thermistors. A NTC thermistor will have its resistance decrease while the ambient temperature increases. Meanwhile, a PTC thermistor will have its resistance increase while the ambient temperature increases. For temperature measurements such as the ones used in flexible heaters, am NTC thermistor will be used.
The function of the NTC thermistor is that when there is a change in the temperature, the thermistor's resistance will decrease as the temperature increases. This fluctuating resistance works in a non-linear fashion (curve). This resistance is picked up by the controllers used as the flexible heater’s temperatures are adjusted based on the desired set point. The temperature curve will be dictated by the type of materials that will be used to make the thermistor. A higher resistance thermistor will be used for higher temperature flexible heaters. Lower resistance thermistors will be used for low temperature applications.
A big advantage to thermistors is that they can be used in applications where there are electronic noise present. They can also be used in rugged environments that experience extreme conditions. They are long-lasting, durable, and offer more stability.
A downside to thermistors is that although they are very accurate temperature readings, they can be prone to self-heating. This occurs when the temperatures are higher. The reason for this occurrence is due to the resistance being lower on the thermistor's materials. So, the thermistor may absorb the heat around it.
If the flexible heater system is not controlled properly, the heater will continue to rise in temperature, past the set point. In time, heat losses will come to equal heat input that increases by temperature. A controller will ensure that the flexible heater will turn on and immediately reach its set point temperature without going over the programmed input and burning the heater out. Types of controller options that are available.
On/off switching is the most basic type of flexible heater temperature control. It operates by turning on when the temperature is below the set point and staying on until reaching the set point. Once there, the controller will shut off. This type of controller is not stable, as the on and off cycles will oscillate between one and the other as the temperature rises and falls.
For these controllers, they will lower the power to the flexible heater when nearing the set point. Since the heater's wire-wound or etched foil heating elements will still give off some residual heat, the function will allow the flexible heater to reach the desired set point at lower energy output. Many proportional controllers have time-proportional control where the on/off switching oscillates. Sometimes when using these controllers, the temperature will hover slightly over the set point without hitting it or going over it, which is called dropping.
Proportional/Integral/Derivative (PID) controllers have advanced digital algorithms designed to prevent dropping experienced by proportional controllers and the frenetic oscillation instability experienced with on/off switching. PIDs come with tuning parameters for optimal control as the sensors can use other factors besides temperature feedback to make certain decisions.
Selecting the right sensors and controllers will depend on the set point temperature you want to maintain, the materials that will be used for the sensors, and how the sensors will monitor the temperature. Simple heating applications may be able to handle regular thermostats and on/off switching controls, while complex flexible heaters will require a more stable PID controller for more stability.
It is possible to obtain customized controllers beyond the ones mentioned above. Custom controllers will offer the highest level of temperature control. However, keep in mind the higher costs that will play a factor when manufacturing such a controller. Working with an experienced flexible heater manufacturer can help you decide whether to use a pre-designed controller or a customized one.