Selection Guide for PTC Thermistors for Overcurrent and Overload Protection

Selection Guide for PTC Thermistors for Overcurrent and Overload Protection

1. Maximum Operating Voltage

When a PTC thermistor is connected in series in a circuit, only a small portion of the voltage remains across it during normal operation. When it activates and assumes a high-resistance state, it must withstand nearly the entire power supply voltage. Therefore, when selecting a PTC thermistor, ensure that it has a sufficiently high maximum operating voltage, while also taking into account potential power supply voltage fluctuations.

2. Non-operating Current and Operate Current

To ensure reliable switching, the operate current must be at least twice the non-operating current.

Because ambient temperature significantly affects both the non-operating and operate currents (see the figure below), worst-case scenarios must be considered. For the non-operating current, select the value for the maximum allowable ambient temperature; for the operate current, select the value for the lower ambient temperature.

3. Maximum Allowable Current at Maximum Operating Voltage

When a PTC thermistor is required for protection, check whether there are conditions in the circuit that could generate currents exceeding the maximum allowable value. This generally refers to situations where there is a risk of a short circuit. The data sheet specifies the maximum current value. Exceeding this value may damage or prematurely fail the PTC thermistor.

4. Switching Temperature (Curie Temperature)

Huaju offers overcurrent protection components with Curie temperatures of 75°C, 85°C, 105°C, and 120°C. The non-operating current depends on the Curie temperature and the diameter of the PTC thermistor chip. To reduce costs, a high Curie temperature and a small size component should be selected. Furthermore, consideration should be given to whether such a PTC thermistor's high surface temperature could cause undesirable side effects in the circuit. Generally, the Curie temperature should exceed the maximum ambient operating temperature by 20 to 40°C.

5. Environmental Impact

When exposed to chemicals or using potting compounds or fillers, extreme caution must be exercised. The reduction of the barium titanate ceramic can lead to a decrease in the PTC thermistor's effectiveness. Changes in thermal conductivity caused by potting can also cause localized overheating and damage to the PTC thermistor.

Guide to Selecting Power Transformer Overload Protection

A power transformer has a primary voltage of 220V, a secondary voltage of 16V, and a secondary current of 1.5A. When the secondary is abnormal, the primary current is approximately 330mA, and protection should be activated within 10 minutes. The transformer's operating ambient temperature ranges from -10°C to 40°C, with a temperature rise of 15°C to 20°C during normal operation. The PTC thermistor is installed close to the transformer. Please select a PTC thermistor for primary protection.

1. Determine the Maximum Operating Voltage

Given the transformer's operating voltage is 220V, accounting for power supply fluctuations, the maximum operating voltage should be 220V x (1 + 20%) = 264V. The maximum operating voltage of the PTC thermistor is 265V.

2. Determine the Non-operating Current

Calculations and actual measurements show that the transformer's primary current is 125mA during normal operation. Considering that the ambient temperature at the PTC thermistor's installation location can reach up to 60°C, the non-operating current at 60°C should be 130-140mA. 3. Determine the operating current
Considering that the ambient temperature at the PTC thermistor's installation location can reach as low as -10°C or 25°C, the operating current should be 300-340mA at -10°C or 25°C, with an operating time of approximately 5 minutes.
4. Determine the rated zero-power resistor R25
When a PTC thermistor is connected in series with the primary, the voltage drop generated should be minimized, and the PTC thermistor's own heat generation should also be minimized. Generally, the voltage drop across a PTC thermistor should be less than 1% of the total power supply. R25 is calculated as follows:
220V × 1% ÷ 0.125A = 17.6 Ω
5. Determine the maximum current
According to actual measurements, when the transformer's secondary is short-circuited, the primary current can reach 500mA. Considering the higher current flowing when a partial short occurs in the primary winding, the maximum current of the PTC thermistor should be above 1A. 6. Determine the Curie Temperature and Dimensions
Considering that the ambient temperature at the PTC thermistor's installation location can reach up to 60°C, add 40°C to this when selecting the Curie temperature. For example, consider a temperature of 100°C. However, considering the low cost and the fact that the PTC thermistor is not installed within the transformer winding, its higher surface temperature will not adversely affect the transformer. Therefore, a Curie temperature of 120°C can be selected. This reduces the diameter of the PTC thermistor and reduces costs.
7. Determine the PTC thermistor Model
Based on the above requirements, after consulting the Huaju company's specifications sheet, the MZ12A-120M15RM150 was selected.
Maximum operating voltage = 265V, rated zero-power resistance value 15Ω ± 20%, non-operating current 150 mA, operating current 300 mA, maximum current 1.5A, Curie temperature 120°C, and maximum dimension ø11.5mm.