Temperature Sensing

The typical standard PTC has an extremely high temperature coefficient of resistance at and above the switch temperature. This characteristic makes it ideal for various temperature sensing applications, especially over-temperature detection.

RTI Electronics manufactures units specifically designed for sensing the temperature of various devices including power transistors, heat sinks, motor windings, transformers, etc. Figure 11 illustrates some of the applications.

Self-Regulating Heating

A unique characteristic of PTC's is their ability, when self-heated above their switch temperature, to maintain a nearly constant temperature regardless of large fluctuations in ambient temperature or voltage applied. RTI Electronics produces devices specifically designed for self-regulating heater applications such as temperature control of crystals, oscillators and liquid crystal displays (LCD's).

Automatic Degaussing

Figure 12 shows a PTC in series with a degaussing coil for a CRT in a color television or monitor. When the switch is closed, the low initial resistance of the PTC allows high inrush current to flow. After a short period of time, the PTC switches to its high resistance state thereby reducing the current to a negligible level as illustrated in Figure 13.

The amount of time required for the PTC to switch into its high resistance state is approximated by the following equation:

Time (seconds) = He (Ts - Ta)/Po

Where: He = Heat capacity wan-sec./ºC

Ts = Switch temperature (ºC)
Ta = Ambient temperature(ºC)
Po = Initial power applied (watts)

EQUATION C

Motor Starting

Figure 14 shows a PTC in series with the starting winding in a single phase electric motor. The low initial resistance of the PTC allows sufficient current to flow through the starting winding until the motor starts.

The PTC then switches to its high impedance state reducing current flow through the starting winding to near zero. The switch time can be approximated by equation C.

Time Delay

In Figure 1 5-A the PTC is in series with the relay coil. When the switch is closed, the relay will energize instantaneously and remain energized until the PTC switches to its high resistance state.

In Figure 15-B the PTC is in parallel with the relay coil.. When the switch is closed, the relay will not energize until the PTC switches to its high resistance state.

The time required for the PTC to switch to its high resistance state can be approximated by equation C.