The resistance of PTC heating devices differs significantly from traditional resistive heating. The resistance changes inversely proportional to the temperature of the device but in a non-linear fashion.
A traditional resistor
A resistive device in a circuit inhibits current flow and creates heat but at a fixed amount. The resistance of the device does not change. The current and power of the circuit remain constant according to ohm's law.
E = IR, voltage equals current X resistance
For electronic devices, this equation is the fundamental basis for all design.
How to calculate current flow in a circuit
I = E/R, current equals voltage divided by the resistance
Current = A(amps), E = V(voltage), R = Omega(Ohms)
This graph will help those that are not electronic engineers or math majors solve for each variable!
If the voltage is 24 volts and the resistance is 200 ohms, the current flowing through the circuit will be 0.12 amps.
Decreasing the resistance to 6 ohms provides a very different amount of current flowing through the circuit. E/R or 24 volts/6 ohms = 4 amps
If the circuit shorts out and has no resistance, the equation is 24 volts/ 0 ohms = 24 amps. The equation for a home electrical circuit that shorts out is 120 v / 0 ohms = 120 amps, which trip a circuit breaker with an accompanying spark or fire.
Traditional resistive element heating
Resistance in a circuit produces thermal energy, which equates to heat. Nichrome, a non-magnetic alloy of nickel and chromium, is one of the most common resistance wires. The material has a high resistance to current and oxidation at high temperatures.
Other materials and alloys are used for specific applications to create heat with some coated with heat-conducting materials that protect the elements from oxidation at high temperatures. Typical lengths of each type of wire produce the desired heating.
Forced air or natural convection heat exchangers are utilized to transfer heat to the atmosphere, solids, and liquids. This heating device has long been used for electrical heating in homes, transportation, and industrial applications.
The resistance is chosen concerning temperature requirements, voltage, and current. In most applications, temperature sensors and current-limiting components are required to maintain the proper temperature.
The current through such wire resistors is nearly constant and does not change with temperature, nor does the wire's resistance.
PTC Resistors’ Characteristics
Ohm's Law still applies to the voltage, current, and resistance in a circuit containing a PTC heating element but the element itself changes resistance based on its temperature.
The ceramic stones' ambient temperature that makes up a PTC heating element has a positive temperature coefficient(PTC). As the temperature increases, the device's resistance increases.
This characteristic is not linear as in the Ohm's Law equation. It is the logarithmic scale. The resistance raises in a logarithmic fashion, increasing rapidly as the device approaches its set-point temperature.
Note the rapid increase in resistance as temperature increases. PTC devices are designed with a specific maximum temperature. This design makes the circuit self-regulating in that resistance rises as the temperature reaches the maximum temperature and shuts off the current in the circuit.
Ohm law looks more like this, along with the temperature and resistance graph.
At 50°C, the resistance in ohms is 10² or 100 ohms.
Current (I) = 24v/100 or 0.24 amps
At 90°C, the resistance is approximately 104 or 10,000 ohms.
Current (I) = 24v/10,000 = 0.00024 amps
At 90°C, the current flow stops due to the high resistance.
The advantages of the PTC heating element resistance graph
Fixed maximum current
Without additional circuit safety components such as fuses and temperature controllers, a designer knows the max current for a set voltage at a set temperature. These characteristics simplify the design of wiring and power requirements.
Fixed maximum temperature
The PTC ceramic element is constructed for a specific temperature range and max temperature. The device automatically compensates for lower or higher ambient temperatures.
At low ambient temperature, the resistance is lower and the current higher quickly heats a PTC device. There will be more resistance to the current with higher temperatures, limiting the speed of temperature rise.
The device's temperature will never exceed the set-point temperature due to current flow in the circuit, decreasing the chance of over-heating and circuit damage or fire. This built-in safety factor makes the PTC element heaters very reliable.
Want to learn more or have a specific application?
DBK USA has experts standing by to answer your questions. Specialists in PTC heating elements and applications can help you select the right components for your application.
Feel free to call our custom wiring engineers directly at 1-864-607-9047