PTC thermistors for Circuit Overload Over-current Protection

What is PTC thermistors Circuit Protector?

PTC thermistors Circuit Protector, also called PTC thermistor Resettable fuse, are used instead of conventional fuses to protect load, such as motors, transformers, or electronic circuit, against overcurrent for circuit protection. PTC thermistors protector not only respond to inadmissibly high currents but also if a preset temperature limit is exceeded. PTC thermistor over-current protector limits the power dissipation of the whole circuit by increasing their resistance and thus reducing the current to a harmless residual value. In contrast to conventional fuses, PTC thermistor resettable fuses do not have to be replaced after elimination of the fault but resume their protective function immediately after a short cooling-down time.

PTC thermistors for Circuit Over-current Overload Protection Application

Positive Temperature Coefficient PTC thermistor applies in the over-current overload and short circuit protection of transformers, battery charger, switches, switch power supply, adaptor, meter, instrumentation, apparatus, electronic coils, control panels, air conditioner, automotive electron, etc.

How PTC thermistor Resettable Fuse Protect Circuit against Overload Over-current Work? Operating Principle

PTC thermistor Resettable Fuse Circuit Protection operating principle is as indicated in the following.
When a circuit is in the normal status, the current through PTC is lower than the rated current and PTC thermistors protector is in the normal state with small resistance value, which will not affect the normal operation of the protected circuit.
In case of some fault in the circuit and the current is greater than the rated current, the PTC thermistor protector will become hot quickly and present a high resistance state, which sets the circuit relatively "off" to protect the circuit from damage.
After the fault is removed, PTC thermistor protector will automatically restore its low resistance state and the circuit will resume normal operation.

Normally, when PTC thermistor is used as an overcurrent protection component, it is preferred to select maximum operating current, maximum operating voltage and proper specifications. Furthermore such factors should be taken into consideration as the dimensions of the components, rated zero-power resistance, operating temperature range, etc.

Relation between operating temperature, operating current and non-operating current is as shown in Fig.2. Operating current is normally 2~3 times non-operating current, and both of the currents will be reduced as operating temperature increases.

Fig. 3 is Voltage-Current Curve and Load Curve in normal operating state. From point A to point B, the voltage on PTC thermistor protector increases gradually, current through PTC thermistor current limiting device also increases linearly, it indicates that PTC thermistor protector resistance keeps almost unchanged, in low resistance state; From point B to F, the voltage increases gradually, PTC thermistor protector resistance increases sharply due to heating, current through PTC thermistor current limiting device decreases rapidly, it indicates that PTC thermistor current limiting device in protection state. Normal load curve lower than point B, PTC thermistor over current protection device won't enter into protection state.

Fig.2 PTC thermistor Non-trip current versus ambient temperature

3 types over-current and overload protection PTC thermistor circuit protector carries.

1. PTC thermistor for current overload protection(Fig. 4):R_L1 is the loading current in normal operating state, When overloading resistance decreases, such as transformer short circuit, loading curve changes from R_L1to R_L2 , exceeding point B, PTC thermistor protector will enter into protection state.
2. PTC thermistor for voltage overload protection (Fig. 5): Power voltage increases, such as increases abruptly from 220V to 380V, loading curve changes from R_L1to R_L2 , exceeding point B, PTC thermistor protector will enter into protection state.
3. PTC thermistor for temperature protection(Fig. 6): When ambient temperature increases exceeding certain temperature, PTC thermistor voltage-resistance curve changes from A-B-E to A-B₁-F, loading curve R_L , exceeding point B₁, PTC thermistor protector will enter into protection state.

PTC thermistor current temperature protection

PTC thermistor outline and dimension (mm)

Circuit protection using PTC thermistor over-current protection devices examples

PTC thermistor inrush current limit circuit

AMWEI PTC thermistor for Circuit Overload Over-current Protection 265V/250V Series Data

AMWEI PTC thermistor for Circuit Overload Over-current Protection 140V Series

AMWEI PTC thermistor for Circuit Overload Over-current Protection 60V Series

AMWEI PTC thermistor Current Limiting Devices for Transformer Over-current Overload Protection 265V, 100C
(For application of PTC thermistor protector installation inside transformer windings coil)

AMWEI PTC thermistor Current Limiting Devices for Transformer Over-current Overload Protection 265V 120C
(For application of PTC thermistor protector installation outside transformer windings coil)

PTC thermistor for Measuring Instrumentation Meter Overload and Short Circuit Protection

The input circuitry of digital multimeters and other instrumentation requires overcurrent protection against the unintentional fault conditions which can occur. PTC thermistors offer reliable, convenient and cost-effective protection. Solid state construction allows resettable operation, eliminating the inconvenience of fuse replacement.

Modern multimeters, with wide-ranging functions, are particularly at risk. The inadvertent application of a voltage source to an instrument in resistance mode will cause excessive currents within the input circuitry. Multi-channel instruments, such as oscilloscopes, are also susceptible if the ground terminals are raised to high potential. These high currents, if unchecked, would destroy the input circuitry.

The PTC thermistor is connected directly in series with the input terminals. It is chosen so that over the expected range of working conditions (current, voltage, ambient temperature) the device remains in a low resistance state, offering as little series resistance as possible to ensure accurate measurement.

When a fault condition occurs, the abnormally high current causes sufficient I2R heating in the PTC thermistor to switch it from low to high resistance, reducing the circuit current and preventing damage.

After removal of the fault condition, the PTC thermistor reverts to low resistance to reestablish normal operation. Devices can be chosen to achieve automatic reset (fault condition must be removed) or manual reset (power source disconnected).

AMWEI PTC thermistor current limiting devices for measuring instrumentation and meter circuit overload and short circuit protection, Voltage 265V/420V/550V, Reference Temperature 80C, 100C, 120C

AMWEI PTC thermistor Circuit Protector Reliability Data

How to select PTC thermistor circuit protector for overcurrent and overload protection?

1) Maximum operating voltage

2) Rated current ( Non-trip current) and Switching current (Trip current)

3) Maximum current permissible in maximum operating voltage

When PTC thermistor over-current protection device 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 protector by too high a switching current must be avoided, it may lead to PTC thermistor protector destroyed, or early failure.

4)Selection of PTC thermistor Reference Temperature (also called Switch temperature or Curie temperature)

AMWEI offer PTC thermistors for over-current protection with Reference Temperature 80 ℃, 100 ℃, 120 ℃, 140 ℃. The rated current (non-trip current) depends on reference temperature and ceramic body diameter. In consideration of cutting down cost, higher reference temperature and smaller dimension PTC thermistor current limiting devices shall be more economical, but it may leads to higher PTC thermistor surface temperature, and need to check whether it will cause undesired unfavorable effects. Generally, reference temperature should be 20--40 ℃ higher than maximum operating ambient temperature.

5) PTC thermistor Application environmental effects

If there is any contact with chemicals or use of potting or sealing compounds, all due care should be taken. The reduction of the titanate ceramic that can be caused by chemical effects on the surface of the thermistor and the resulting formation of low-resistance conducting paths. And the altered thermal relations in the sealant can lead to local overheating of the PTC thermistor protector and thus to failure.

An example selecting AMWEI PTC thermistor part for power transformer overload over-current protection

A transformer has primary voltage 220V, secondary voltage 16V, secondary current 1.5A , primary current 330mA in abnormal condition, it shall enter into protective state within 10 minutes. Operating ambient temperature: -10C--+40C , temperature may rise 15--20C 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

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 to 60℃ , Non-trip current in 60℃ should be 130--140mA.

3) Determine trip current

As the ambient temperature in PTC thermistor protector installation position may reach -10 ℃ , non trip current in -10 ℃ should be 320-330mA, tripping time within 5 minutes.

4) Determine PTC thermistor 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=17.6Ω

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 than 1A .

6) Determine reference temperature and dimension of PTC thermistor over-current protection device

Maximum ambient temperature in PTC thermistor protector installation position may reach 60 ℃ , reference temperature should be 40 ℃ higher than that, then the reference temperature can be 100 ℃ . In considering cutting down cost, also PTC thermistor is not installed in the transformer windings coil, higher surface temperature won't have unfavorable effect to transformer, therefore reference temperature can also be 120 ℃ , and then the diameter of PTC thermistor can be smaller.

7) Determine PTC thermistor over-current protection device AMWEI part number.

Based on the above technical requirement, in reference of AMWEI technical data, AMWEI part no. AMZ11-10P15RH265 shall be more appropriate.
Maximum operating voltage 265V, rated zero power resistance at 25C (R25) 15 Ω±20% , non-trip current 140mA , trip current 350 mA , maximum current 1.5A , reference temperature 120 ℃ , diameter 11mm .

Read more: ‘PTC Thermistor for High Voltage Current Surge Protection’