Selection guide of PTC thermistor for overcurrent and overload protection

1) Maximum operating voltage
PTC Thermistor is connected in series in the circuit , In normal operating state, only a small portion voltage is on PTC thermistor. When PTC thermistor is in high resistance state, it must bear almost all the power voltage. Therefore in PTC thermistor model selection, it must have sufficient high operating voltage, and also power voltage fluctuation must be taken into consideration.
2) Non-trip current and Trip current
To obtain reliable switching function, trip current should exceed at least twice of non-trip current.
As ambient temperature have substantial influence on non-trip current and trip current, Please refer to the following figure, therefore the worst case should be taken into consideration, non-trip current value in maximum operating temperature range should be taken, while the tripping current value in lower ambient temperature should be taken.

3) Maximum current permissible in maximum operating voltage
When PTC Thermistor is required for protective function, it needs to check whether there is the case that the maximum permissible current, which has been listed in data sheet, has been exceeded. Overloading the PTC thermistor by too high a switching current must be avoided, it may lead to PTC thermistor destroyed, or early failure.
4)Selection of Switch temperature (also called Curie temperature)
AMPRON can offer PTC thermistor for overcurrent protection with switch temperature 80 ℃、100 ℃、120 ℃、140 ℃ non-trip current depends on switch temperature and ceramic body diameter. In consideration of cutting down cost, high switch temperature and small dimension PTC Thermistor shall be better, but it may leads to higher PTC surface temperature, and need to check whether it will cause undesired unfavorable effects. Generally, switch temperature should be 20--40 ℃higher than maximum operating ambient temperature.
5) Application environmental effects
If there is any contact with chemicals or use of potting or sealing compounds, all due care should be taken. Special care should be taken for the reduction of the PTC thermistor effect reduction due to titanate ceramic being deoxidized. And the altered thermal condition in the sealant can lead to local overheating of the PTC thermistor and thus to failure.

Selection guide of PTC thermistor part number for overload protection of power transformer.

An transformer has primary voltage 220V, secondary voltage 16V, secondary current1.5A, primary current 330mA in abnormal condition, it shall enter into protective state within 10 minutes. Operating ambient temperature: -10--40 ℃, temperature may rise 15--20℃in normal operating state. PTC Thermistor will be installed near transformer. Please select an appropriate PTC thermistor part for primary protection.
1) Determine maximum operating voltage
Operating voltage 220V, considering power fluctuation, maximum operating voltage should be 220V x (1+20%）=264V
Maximum operating voltage shall be 265V.
2) Determine non-trip current
According to calculation and actual measurement, primary current is 125mA in transformer normal operation. In consideration the ambient temperature in installation position may reach to60 ℃，Non-trip current in60 ℃should be 130—140mA.
3) Determine trip current
As the ambient temperature in PTC thermistor installation position may reach –10℃, non trip current in –10℃should be 320-330mA, tripping time within 5 minutes.
4) Determine rated zero power resistance at 25 centigrade. R25
PTC thermistor is in series in the primary circuit, the voltage decreasing should be possibly small, the heating power of PTC Thermistor itself also maintain possibly small. Generally, the voltage decreasing should be less than total power voltage 1%. We can get R25 through calculation 220V X 1% ÷ 125mA=17.6 Ω
5) Determine maximum current
Trough practical measurement, primary current can reach 500mA in transformer secondary circuit in short circuit state. If considering that larger current may pass through PTC thermistor in the state of primary coil partial short circuit. The maximum current should be more than1A.
6) Determine switch temperature and dimension
Maximum ambient temperature in PTC thermistor installation position may reach60 ℃, switch temperature should be40 ℃higher than that, then the switch temperature can be100 ℃. In considering cutting down cost, also PTC thermistor is not installed in the transformer wire coil, higher surface temperature won’t have unfavorable effect to transformer, therefore switch temperature can also be120 ℃, and then the diameter of PTC Thermistor can be smaller.
7) Determine PTC thermistor part number.
Based on the above technical requirement, in reference of AMPRON technical data, MZ11-10P15RH265 shall be more appropriate.
Maximum operating voltage 265V, rated zero power resistance at25C(R25) 15Ω± 20%，non-trip current 140mA，trip current 350 mA，maximum current1.2A，reference temperature120 ℃，diameter ø11.0mm。

Selection guide of PTC thermistor for lighting time delay starting

Based on switching time can be lengthen by increasing switch temperature, and ceramic body volume, reducing resistance, we determine the following basic principle:
1.As the higher bulb temperature during lighting operation, switch temperature of PTC thermistor can not be too lower, otherwise, it will lead to too short time delay interval, inferior warm-up effect. Switch temperature >100 ℃shall be more suitable.
2.Single capacitor starting circuit has higher demand on PTC thermistor voltage withstanding capacity, general withstanding voltage >800V.
3.PTC thermistor reliability can be increased through higher resistivity, therefore, higher resistivity PTC thermistor shall be chosen first at the premises of satisfying starting characteristics.
4.Low ambient temperature starting should be taken into consideration, in low ambient temperature, heat balance resistance of PTC thermistor is relatively lower, it may cause lamp doesn’t startup.
5.Switching life cycle >100，000 cycles shall be better.
6.Preheating time shall be not less than 0.4 seconds.
7.After starting finished, power consumption of PTC thermistor should comply with relevant stipulation.