With fixed resistance heaters, the thermal coefficient of resistance is quite low, changing minimally over a few hundred degrees temperature range.

We are used to working with fixed resistance heaters such as MgO tubes or resistance wire. With fixed resistance heaters, the thermal coefficient of resistance (Tcr) is quite low, changing minimally over a few hundred degrees temperature range.

PTC is an abbreviation for positive temperature coefficient. In a PTC or stone-heating element, the material's electrical resistance changes as a function of its temperature -- the higher the temperature, the higher the resistance. This characteristic makes the stone element "self regulating." Peak wattage comes with lower stone temperature, with the wattage decreasing as the stone gets hotter. In many applications, there is no need for an over-temperature protection or limit switch. Check with UL or other agencies to see if this applies to your application.

Another positive feature for a PTC is its change in resistance and wattage. If applied correctly, it may lower operating costs. Fixed resistance heaters are "bang-bang." That is, they are full on until shut off by their thermostat. Your appliance's temperature rises to its upper temperature (sometimes with a bit of overshoot), then falls until the thermostat resets itself. When optimally applied, the stone simply adjusts resistance to maintain a steady-state temperature.

PTCs may also be ordered to operate over a specified voltage range. For example, a small 350 W heater may need to operate at 120 V and 240 V. An assembly can be supplied that works well at either voltage with only about a 10% variation in wattage.

With a fixed-resistance heater, prime concerns are voltage, wattage, geometry and the use environment. These are important to stone heaters also, but there are some additional considerations. The best way to specify a PTC heater is to contact your representative and let him help you with the application.

Terms to Talk About

Here are some terms related to PTCs that will help you make an informed decision when you specify a heater for your oven or other appliance.

The Curie or switch temperature (Ts) is the point at which the element's resistance begins to change in direct relation to its temperature. Graphically, the temperature is represented along the X axis and a log of the resistance is shown on the Y axis. The range of significant positive temperature response lays between Tc and the curve's peak. After the peak, the stone typically shifts from a positive to negative coefficient.

The stones are also characterized by their resistance at a common temperature, usually 77oF (25oC). This specification is usually called R25c. Imax specifies the maximum current and Vmax specifies the maximum voltage. VD specifies the break-down voltage. Top is the operating temperature. These features need to be discussed when developing a heater for specific applications.

Finally, you will need to specify things like physical size, electrical connections, and the method that you will use to transfer heat from the stone to your appliance.

For more information

Contact Eichenauer. Call (603) 863-1454 or visit www.eichenauerusa.com.