PTC Thermistor Current Protect Resettable Fuse

What is PTC Thermistors Circuit Protector?

PTC Thermistor Current Protection Resettable Fuse

PTC Thermistor Current Protection Resettable Fuse

PTC thermistors Circuit Protector, also called PTC Thermistor ResettableFuse, 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 Thermistor Resistance Change with Current

PTC Thermistor Resistance Change with Current

PTC Thermistor Trip Time vs. Current

PTC Thermistor Trip Time vs. Current

PTC Thermistor as Current Protect Resettable Fuse

PTC Thermistor as Current Protect Resettable Fuse

PTC Thermistors Circuit Over-current Overload Protection Application

Simplest circuit for PTC Thermistor protecting a transformer

Simplest circuit for PTC Thermistor protecting a transformer

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 Protective Threshold Current

PTC Thermistor Protective Threshold Current

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.

Different Curie Temperature PTC Non-trip Current vs. Ambient Temperature

Different Curie Temperature PTC Non-trip Current vs. Ambient Temperature

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

When connected in series with the input of an electrical or electronic circuit, such as a small motor or power supply, the PTC thermistor acts as a self-resettable fuse, protecting the circuit against current, voltage and temperature overload conditions.

In normal operating conditions the PTC thermistor resistance is low , and the current is below its hold value (Ihold). However, an overload will quickly heat up the PTC thermistor until, at around switch temperature (Ts), its resistance becomes high, limiting the current to far below its trip value (Itrip), and so protecting the circuit.

Operating of PTC Thermistor as Current Protect Resettable Fuse

Operating of PTC Thermistor as Current Protect Resettable Fuse

PTC Thermistor Replacement Idea for a Resistor and Fuse Solution

PTC Thermistor Replacement idea for a resistor and fuse solution

Benefits

  • Protection against overcurrent situations
  • Automatic reset from protective trip mode
  • Space-saving
  • Various characteristics to meet a suitable resistance value

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

PTC Thermistors for over-current overload protection

PTC Thermistors for over-current overload protection

PTC thermistor for current overload protection(Fig. 4):RL1 is the loading current in normal operating state, When overloading resistance decreases, such as transformer short circuit, loading curve changes from RL1to RL2 , exceeding point B, PTC thermistor protector will enter into protection state.

PTC thermistor for voltage overload protection (Fig. 5): Power voltage increases, such as increases abruptly from 220V to 380V, loading curve changes from RL1to RL2 , exceeding point B, PTC thermistor protector will enter into protection state。

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 RL , exceeding point B1PTC thermistor protector will enter into protection state.

Electrical Characteristics Imax as a Function of Voltage

Electrical Characteristics Imax as a Function of Voltage

Current Deviation as a Function of Ambient Temperature

Current Deviation as a Function of Ambient Temperature

Voltage Derating as a Function of Ambient Temperature

Voltage Derating as a Function of Ambient Temperature

Circuit protection examples using PTC thermistor over-current protection devices

PTC Thermistor Current Protector Application Examples

PTC Thermistor Current Protector Application Examples

PTC Thermistor Current Protector Transformer Primary or secondary protection

PTC Thermistor Current Protector Transformer Primary or secondary protection

PTC Thermistor for inrush current limit circuit

PTC Thermistor for inrush current limit circuit

Current Protection PTC Thermistor Engineering Drawing

Current Protection PTC Thermistor Outlook Engineering Drawing

Current Protection PTC Thermistor Outlook Engineering Drawing

AMWEI PTC Thermistor Current Protect Fuse 265V/250V Series Data

 AMWEI Part NumberR25
(OHM)
Non-trip
current
Int(mA)
Trip current
@25C
It
(mA)
Max.
voltage
Vmax
(V)
Max.
current
Imax
(A)
 Reference
temp
Tref
Dimension
(mm)
 @25C @60C Dmax Tmax Φd
AMZ11-20P2R6H2652.6 ohm 650mA 500mA1300mA265V4.3A120C22.05.00.6
AMZ11-20P3R7H2653.7 ohm530mA430mA1050mA265V4.3A120C22.05.00.6
AMZ11-20P5RH2655.0 ohm480mA370mA970mA265V4.3A120C22.05.00.6
AMZ11-16P5RH2655.0 ohm420mA320mA840mA265V3.1A120C17.55.00.6
AMZ11-16P6RH2656.0 ohm390mA300mA780mA265V3.1A120C17.55.00.6
AMZ11-16P7RH2657.0 ohm350mA280mA700mA265V3.1A120C17.55.00.6
AMZ11-13P10RH26510 ohm300mA200mA600mA265V1.8A120C14.05.00.6
AMZ11-13P12RH26512 ohm270mA180mA540mA265V1.8A120C14.05.00.6
AMZ11-13P15RH26515 ohm240mA160mA500mA265V1.8A120C14.05.00.6
AMZ11-13P50RH26550 ohm130mA100mA260mA265V1.8A120C14.05.00.6
AMZ11-12P7RH2657 ohm320mA250mA640mA265V1.8A120C14.05.50.6
AMZ11-12P8RH2658 ohm300mA230mA600mA265V1.8A120C14.05.50.6
AMZ11-12P9RH2659 ohm290mA220mA580mA265V1.8A120C14.05.50.6
AMZ11-12P10RH26510 ohm250mA200mA500mA265V1.8A120C14.05.50.6
AMZ11-12P12RH26512 ohm240mA180mA480mA265V1.8A120C14.05.50.6
AMZ11-12P15RH26515 ohm200mA160mA400mA265V1.8A120C14.05.50.6
AMZ11-12P27RH27027 ohm120mA90mA350mA270V1.8A120C14.05.50.6
AMZ11-12P101H265100 ohm50mA40mA100mA265V1.8A120C14.05.50.6
AMZ11-10P12RH26512 ohm230mA160mA460mA265V1.2A120C11.55.00.6
AMZ11-10P15RH26515 ohm180mA140mA350mA265V1.2A120C11.55.00.6
AMZ11-10P18RH26518 ohm170mA130mA340mA265V1.2A120C11.55.00.6
AMZ11-10P39RH26539 ohm130mA100mA250mA265V1.2A120C11.55.00.6
AMZ11-08P15RH25015 ohm150mA120mA300mA250V0.8A120C9.55.00.6
AMZ11-08P22RH25022 ohm135mA110mA270mA250V0.8A120C9.55.00.6
AMZ11-08P25RH26525 ohm130mA100mA250mA265V0.8A120C9.55.00.6
AMZ11-08P35RH26535 ohm115mA90mA225mA265V0.8A120C9.55.00.6
AMZ11-08P39RH27039 ohm105mA70mA210mA270V0.8A120C9.55.00.6
AMZ11-08P45RH26545 ohm105mA80mA200mA265V0.8A120C9.55.00.6
AMZ11-08P55RH26555 ohm90mA70mA180mA265V0.8A120C9.55.00.6
AMZ11-08P70RH26570 ohm75mA60mA150mA265V0.8A120C9.55.00.6
AMZ11-07P82RH26582 ohm70mA50mA140mA265V0.6A120C8.55.00.6
AMZ11-07P56RH26556 ohm90mA60mA175mA265V0.6A120C8.55.00.6
AMZ11-06P33RH25033 ohm110mA85mA200mA250V0.4A120C7.54.50.6
AMZ11-05P70RH26570 ohm65mA50mA130mA265V0.3A120C6.55.00.6
AMZ11-05P85RH26585 ohm60mA45mA120mA265V0.3A120C6.55.00.6
AMZ11-05P121H265120 ohm45mA35mA90mA265V0.3A120C6.55.00.6
AMZ11-05P181H265180 ohm40mA30mA80mA265V0.3A120C6.55.00.6
AMZ11-04P70RH25070 ohm50mA40mA100mA250V0.2A120C5.55.00.6
AMZ11-04P121H265120 ohm40mA30mA80mA265V0.2A120C5.55.00.6
AMZ11-03P151H250150 ohm40mA30mA75mA250V0.2A120C4.55.00.5
AMZ11-03P221H265220 ohm30mA24mA60mA265V0.2A120C4.55.00.5
AMZ11-12N12RH26512 ohm200mA140mA400mA265V1.8A100C14.05.00.6
AMZ11-12N10RH26510 ohm220mA180mA440mA265V1.8A100C14.05.00.6
AMZ11-12N8RH2658 ohm250mA200mA500mA265V1.8A100C14.05.00.6
AMZ11-10N12RH25012 ohm180mA130mA350mA250V1.2A100C11.55.00.6
AMZ11-10N12RH26512 ohm210mA160mA420mA265V1.2A100C12.05.00.6
AMZ11-10N15RH26515 ohm175mA120mA320mA265V1.2A100C11.55.00.6
AMZ11-10N18RH26518 ohm145mA110mA320mA265V1.2A100C11.55.00.6
AMZ11-10N22RH26522 ohm140mA100mA260mA265V1.2A100C11.55.00.6
AMZ11-10N27RH26527 ohm120mA90mA240mA265V1.2A100C11.55.00.6
AMZ11-08N25RH26525 ohm110mA80mA230mA265V1.0A100C9.55.00.6
AMZ11-08N35RH26535 ohm95mA70mA190mA265V1.0A100C9.55.00.6
AMZ11-08N45RH26545 ohm85mA55mA170mA265V1.0A100C9.55.00.6
AMZ11-07N22RH25022 ohm120mA80mA210mA250V0.5A100C8.55.00.6
AMZ11-07N35RH26535 ohm95mA70mA190mA265V0.5A100C8.55.00.6
AMZ11-07N50RH26550 ohm85mA60mA170mA265V0.5A100C8.55.00.6
AMZ11-07N60RH26560 ohm80mA50mA160mA265V0.5A100C8.55.00.6
AMZ11-05N82RH26582 ohm55mA40mA105mA265V0.3A100C6.55.00.6
AMZ11-05N101H265100 ohm50mA35mA100mA265V0.3A100C6.55.00.6
AMZ11-05N121H265120 ohm45mA32mA90mA265V0.3A100C6.55.00.6
AMZ11-05N151H265150 ohm38mA30mA80mA265V0.3A100C6.55.00.6
AMZ11-05N201H265200 ohm30mA25mA65mA265V0.3A100C6.55.00.6
AMZ11-05N301H265300 ohm27mA20mA55mA265V0.3A100C6.55.00.6
AMZ11-05N601H265600 ohm20mA12mA40mA265V0.2A100C6.55.00.6
AMZ11-04N151H265150 ohm36mA28mA80mA265V0.3A100C5.55.00.6
AMZ11-03N151H265150 ohm33mA25mA65mA265V0.2A100C4.55.00.5
AMZ11-03N101H250100 ohm40mA30mA80mA250V0.2A100C4.55.00.5
AMZ11-03N70RH25070 ohm45mA35mA90mA250V0.1A100C4.55.00.5
AMZ11-08M25RH26525 ohm90mA50mA180mA265V0.8A80C9.55.00.6
AMZ11-08M35RH26535 ohm80mA45mA160mA265V0.8A80C9.55.00.6
AMZ11-08M40RH26540 ohm75mA40mA150mA265V0.8A80C9.55.00.6
AMZ11-08M50RH26550 ohm60mA35mA120mA265V0.8A80C9.55.00.6
AMZ11-07M101H265100 ohm40mA25mA80mA265V0.6A80C8.55.00.6
AMZ11-05M70RH25070 ohm50mA30mA100mA250V0.3A80C6.55.00.6
AMZ11-05M121H265120 ohm30mA20mA60mA265V0.3A80C6.55.00.6
AMZ11-03M101H250100 ohm25mA18mA55mA250V0.2A80C4.55.00.5
AMZ11-03M151H265150 ohm22mA15mA45mA265V0.2A80C4.55.00.5

AMWEI PTC Thermistor Current Protect Fuse 140V Series Data

Part Number Resistance
@ 25C
R25
(OHM)
Non-trip
current
Int(mA)
Trip
current
@25C
It
(mA)
Max.
operating
voltage
Vmax
(V)
Max.
current
Imax
(A)
 Reference
temp
Tref
Dimension
(mm)
@25C@60C Dmax Tmax Φd
 AMZ12-20P1R5H1401.5 ohm 850mA 680mA1700mA140V4.3A120C22.05.00.6
AMZ12-20P2R6H1402.6 ohm650mA500mA1300mA140V4.3A120C22.05.00.6
AMZ12-16P4R7H1404.7 ohm425mA330mA850mA140V3.1A120C17.55.00.6
AMZ12-16P5R6H1405.6 ohm400mA310mA800mA140V3.1A120C17.55.00.6
AMZ12-13P6R8H1406.8 ohm325mA250mA650mA140V1.8A120C14.05.00.6
AMZ12-12P5R6H1405.6 ohm325mA250mA650mA140V1.8A120C13.55.00.6
AMZ12-12P6R8H1406.8 ohm300mA230mA600mA140V1.8A120C13.55.00.6
AMZ12-10P10RH14010 ohm225mA170mA450mA140V1.2A120C11.55.00.6
AMZ12-10P6R8H1406.8 ohm275mA200mA550mA140V1.2A120C11.55.00.6
AMZ12-08P22RH14022 ohm135mA110mA270mA140V0.8A120C9.55.00.6
AMZ12-06P25RH14025 ohm125mA90mA250mA140V0.5A120C7.05.00.6
AMZ12-16R2RIH1402.1 ohm710mA570mA1420mA140V3.1A140C17.55.00.6
AMZ12-13R3R8H1403.8 ohm500mA400mA1000mA140V1.8A140C14.55.00.6
AMZ12-10R15RH14015 ohm210mA170mA420mA140V1.2A140C11.55.00.6
AMZ12-10R6R7H1406.7 ohm300mA230mA600mA140V1.2A140C11.55.00.6
AMZ12-10R10RH14010 ohm250mA200mA500mA140V1.2A140C11.55.00.6
AMZ12-08R12RH14012 ohm200mA160mA400mA140V0.6A140C9.55.00.6

AMWEI PTC Thermistor Current Protect Fuse 60V Series Data

Part Number Resistance
@ 25C
R25
(Ω)
Non-trip
current
Int(mA)
Trip
current
@25C
It
(mA)
Max.
operating
voltage
Vmax
(V)
Max.
current
Imax
(A)
 Reference
temp
Tref
Dimension
(mm)
@25C@60C Dmax Tmax Φd
AMZ13-16P2R3H602.3 ohm 550mA 450mA 1100mA60V8.0A120C17.54.00.6
AMZ13-12P3R7H603.7 ohm380mA320mA750mA60V5.513.54.00.6
AMZ13-10P5R6H605.6 ohm300mA250mA600mA60V4.3A11.04.00.6
AMZ13-08P9R4H609.4 ohm180mA150mA360mA60V3.0A9.04.00.6
AMZ13-05P25RH6025 ohm100mA85mA200mA60V1.0A6.54.00.6
AMZ13-03P55RH6055 ohm60mA50mA120mA60V0.7A4.54.00.5
AMZ13-08M4R7H604.7 ohm180mA120mA360mA60V3.0A80C9.04.00.6

Note: If not specified, Resistance @25C (R25) tolerance shall be +/-25%.

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)

Part numberResistance
@25C
R25
(OHM)
Non-trip
current
Int (mA)
Trip
current
@25C
It
(mA)
Max.
operating
voltage
Vmax
(V)
Max.
current
Imax
(A)
 Reference
temp
Tref
Dimension
(mm)
@25C@80C Dmax Tmax Φd
AMZ11-05N121H265120 ohm45mA25mA90mA265V0.4A100C6.55.00.6
AMZ11-05N151H265150 ohm38mA20mA80mA265V0.4A6.55.00.6
AMZ11-07N22RH26522 ohm110mA60mA220mA265V0.7A8.04.50.6
AMZ11-07N32RH26532 ohm95mA50mA190mA265V0.7A8.04.50.6
AMZ11-07N55RH26555 ohm70mA40mA140mA265V0.7A8.05.00.6
AMZ11-08N30RH26530 ohm100mA55mA200mA265V1.0A9.05.00.6
AMZ11-08N35RH26535 ohm90mA50mA180mA265V1.0A9.05.00.6
AMZ11-08N40RH26540 ohm85mA45mA170mA265V1.0A9.05.00.6
AMZ11-08N39RH26539 ohm95mA50mA180mA265V1.2A9.55.00.6
AMZ11-08N25RH26525 ohm110mA60mA220mA265V1.2A9.55.00.6
AMZ11-10N22RH26522 ohm125mA70mA250mA265V1.5A11.05.00.6
AMZ11-10N18RH26518 ohm145mA80mA290mA265V1.5A11.05.00.6
AMZ11-10N12RH26512 ohm170mA95mA340mA265V1.5A11.05.00.6
AMZ11-12N18RH26518 ohm180mA100mA360mA265V2.0A13.55.00.6
AMZ11-12N12RH26512 ohm210mA120mA420mA265V2.0A13.55.00.6

Note: If not specified, Resistance @25C (R25) tolerance shall be +/-25%.

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)

Part number Resistance
@25C
R25
(OHM)
Non-trip
current
Int(mA)
Trip
current
@25C
It
(mA)
Max.
operating
voltage
Vmax
(V)
Max.
current
Imax
(A)
 Reference
temp
Tref
Dimension
(mm)
@25C@60C Dmax Tmax Φd
AMZ11-16P6R0H2656.0 ohm390mA300mA780mA265V3.1A120C17.55.00.6
AMZ11-13P10RH26510 ohm260mA200mA520mA265V1.8A14.05.00.6
AMZ11-12P10RH26510 ohm250mA200mA500mA265V1.8A13.55.00.6
AMZ11-10P15RH26515 ohm180mA140mA350mA265V1.2A11.05.00.6
AMZ11-08P25RH26525 ohm130mA100mA250mA265V0.8A9.05.00.6
AMZ11-08P35RH26535 ohm115mA90mA225mA265V0.8A9.05.00.6
AMZ11-08P45RH26545 ohm105mA80mA200mA265V0.8A9.05.00.6
AMZ11-08P55RH26555 ohm90mA70mA180mA265V0.8A9.05.00.6
AMZ11-05P70RH26570 ohm65mA50mA130mA265V0.3A6.55.00.6
AMZ11-03P151H250150 ohm40mA30mA75mA250V0.2A4.55.00.5

Note: If not specified, Resistance @25C (R25) tolerance shall be +/-25%.

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

Part number Resistance
@25C
R25
(OHM)
Non-trip
current
Int(mA)
 Trip current
@25C It
(mA)
 Max. operating
voltage
Vmax
(V)
 Max. current
Imax
(A)
 Reference
temp
Tref
Dimension
(mm)
@25C@60C Dmax Tmax Φd
AMZ11-03M151N265150 ohm20mA15mA50mA265V0.2A80C4.55.00.5
AMZ11-03M102N2651000 ohm10mA7mA20mA265V0.1A4.55.00.5
AMZ11-03M152N2651500 ohm7mA5mA15mA265V0.1A4.55.00.5
AMZ11-04M45RN26545 ohm40mA30mA80mA265V0.3A5.55.00.6
AMZ11-05M601N420600 ohm20mA16mA40mA420V0.2A6.55.00.6
AMZ11-05M232N4202300 ohm8mA6mA17mA420V0.1A6.55.00.6
AMZ11-05M312N4203100 ohm5mA4mA13mA420V0.1A6.55.00.6
AMZ11-07M251N265250 ohm25mA20mA50mA265V0.5A8.05.00.6
AMZ11-07M112M5501100 ohm15mA10mA30mA550V0.5A8.07.00.6
AMZ11-08M12RN26512 ohm120mA70mA220mA265V0.8A9.05.00.6
AMZ11-03N151N265150 ohm30mA25mA60mA265V0.2A100C4.55.00.5
AMZ11-03N401N265400 ohm25mA20mA45mA265V0.2A4.55.00.5
AMZ11-05N751N420750 ohm25mA20mA45mA420V0.2A6.55.00.6
AMZ11-05P39RN26539 ohm85mA70mA170mA265V0.4A120C6.55.00.6
AMZ11-05P65RN26565 ohm70mA60mA140mA265V0.4A6.55.00.6
AMZ11-05P601N420600 ohm20mA16mA39mA420V0.2A6.55.00.6
AMZ11-05P122N5501200 ohm15mA10mA30mA550V0.1A6.55.00.6
AMZ11-05P152N5501500 ohm12mA9mA24mA550V0.1A6.55.00.6
AMZ11A-08P70RN42070 ohm64mA50mA127mA420V1.4A8.57.00.6
AMZ11A-08P121N420120 ohm49mA40mA97mA420V1.4A8.57.00.6
AMZ11A-08P151N420150 ohm43mA35mA86mA420V1.4A8.57.00.6
AMZ11A-08P501N550500 ohm24mA19mA48mA550V1.0A8.57.00.6

Note: If not specified, Resistance @25C (R25) tolerance shall be +/-30%.

AMWEI PTC Thermistor Circuit Protector Reliability Data

TestStandardTest conditions|ΔR25/R25|
Switching test at room temperatureIEC 60738-1Imax, Vmax, 50 cycles.<25%
Rapid change of temperature in airIEC600628-2-14,
Test Na
T=TLCT, T=TUCT 5 cycles,
30minutes.
<10%
Endurance at max. operating temperature
and max. operating voltage
IEC 60738-1Ambient temperature: +60℃,
Maximum Operating Voltage,
Current limited to Imax.
Time: 24 hours
<25%
Storage in damp heatIEC 600628-2-3Temperature of air: 40℃,
Relative humidity of air: 93%,
Duration: 56 days
<10%
Lead wire tensileGB2423.29Test Ua: pull strength 10N, 10 seconds,
Test Ub: bending 90°, pull strength 5N,
successively twice.
Test Uc: revolving 180°,
successively twice.
≤20%

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

In selecting PTC thermistor circuit protector for overcurrent and overload protection, following 5 factors should be taken into consideration.

1) Maximum operating voltage

PTC thermistor over-current protection device is connected in series in the circuit, in normal operating state, only a small portion voltage is on PTC thermistor protector. When PTC thermistorcurrent limiting devices is in high resistance state, it must bear almost all the power voltage. Therefore in PTC thermistor protector model selection, it must have sufficient high operating voltage, and also power voltage fluctuation must be taken into consideration.

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

PTC thermistor over-current protection device should have sufficient high rated current (that current at which the PTC thermistor protector will under no circumstances turn off) within the suitable voltage class. Consider whether the overall layout of the circuit can handle the increased power for the short time until the PTC thermistor protector reduces it. Here a worst case estimate is necessary. Rated current ( Non-trip current) and Switching current (Trip current) depend on the ambient temperature. So, as the worst case for the rated current, the maximum permissible temperature for the application should be taken, and for the switching current the lowest possible ambient temperature. In order to get reliable switching function, tripping current should be at least twice of non-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 for transformer overload 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. 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 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 .

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