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How to Calculate PPTC Holding Current

How to Calculate PPTC Holding Current is an important question when selecting a PPTC resettable fuse for overcurrent protection. Holding current, commonly identified asIhold, determines how much current a PPTC can carry continuously under specified conditions without transitioning into its high-resistance state.

However, calculating the required PPTC holding current is not simply a matter of matching Ihold to the nominal load current.

The actual selection should consider the maximum normal operating current, ambient temperature, PCB thermal conditions, current tolerance, startup behavior, and the manufacturer's thermal derating curve.

This guide explains how to calculate the required PPTC holding current and how to use a PPTC datasheet for practical component selection.

What Is PPTC Holding Current?

PPTC holding current (Ihold) is the maximum current that a PPTC resettable fuse is specified to carry without tripping under defined test conditions.

The exact test conditions vary by manufacturer and product series. Therefore, the Ihold value shown in a datasheet should always be interpreted together with its specified temperature, mounting and test conditions.

A PPTC is a thermally activated protection component. When current flows through the device, power is dissipated according to its resistance. If the generated heat exceeds the heat transferred to the surrounding environment, the device temperature rises and its resistance increases substantially.

This means that PPTC holding current is temperature-dependent.

As ambient temperature increases, the available holding-current capability generally decreases. Manufacturers therefore provide thermal derating curves or temperature-specific current ratings to help engineers determine the appropriate PPTC for the actual operating environment.

Why Is PPTC Holding Current Important?

The purpose of selecting the correct Ihold is to achieve two objectives:

The PPTC should remain in its low-resistance state during all legitimate operating conditions.

The PPTC should provide appropriate protection when an abnormal overcurrent condition occurs.

If Ihold is too low, the device may trip during normal operation.

If Ihold is selected excessively high, the PPTC may provide less effective protection against moderate overcurrent conditions.

Therefore, PPTC selection is a balance between normal operating current and fault protection requirements.

How to Calculate PPTC Holding Current

A practical calculation can be performed in several steps.

Step 1: Determine the Maximum Normal Operating Current

Start with the maximum current that the protected circuit can legitimately draw.

Do not use only the typical operating current.

Consider:

Maximum continuous load current

Power supply tolerance

Load variation

Startup current

Charging current

Motor or actuator startup

Temperature-related current changes

Other legitimate transient conditions

Define this value as:

Iload(max) = maximum normal operating current

For example:

Typical load current = 0.60 A

Maximum normal load current = 0.75 A

The PPTC should be selected based on 0.75 A, not 0.60 A.

Step 2: Determine the Maximum Ambient Temperature

Next, determine the highest temperature that the PPTC will experience during normal operation.

This is not always the same as the equipment's specified ambient temperature.

For example:

Equipment ambient temperature: 50°C

Temperature near the PPTC: 60°C

If the PPTC is mounted close to a DC/DC converter, MOSFET, power resistor or other heat-generating component, its local temperature can be higher than the surrounding air temperature.

Therefore, use the worst-case temperature at or around the PPTC location when possible.

Temperature is particularly important because PPTC holding current decreases as temperature increases.

Step 3: Find the PPTC Thermal Derating Factor

After determining the maximum operating temperature, check the manufacturer's thermal derating curve.

The datasheet may provide either:

A derating percentage

A current-versus-temperature table

A thermal derating graph

Temperature-specific Ihold values

For example, suppose a PPTC has a nominal Ihold of 1.0 A under the reference condition, and the manufacturer's thermal derating factor at the application's maximum temperature is 70%.

The approximate available holding current would be:

Ihold(actual) = 1.0 A × 70% = 0.70 A

This means the device should not be treated as a 1.0 A holding-current device under that higher-temperature condition.

The exact derating factor must always come from the specific PPTC manufacturer's datasheet. Different PPTC constructions and product series can have substantially different temperature characteristics.

Step 4: Calculate the Required Nominal Ihold

Once the maximum normal operating current and thermal derating factor are known, a useful engineering relationship is:

Required nominal Ihold ≥ Maximum normal operating current ÷ Thermal derating factor

For example:

Maximum normal current = 0.75 A

Thermal derating factor = 70% = 0.70

Then:

Required nominal Ihold ≥ 0.75 A ÷ 0.70

Required nominal Ihold ≥ 1.07 A

In this example, a PPTC with a nominal Ihold of approximately 1.1 A or higher may be a candidate for further evaluation.

However, this calculation is only the beginning of the selection process. The engineer should still verify the actual datasheet, time-to-trip characteristics, voltage rating, maximum fault current, resistance and application conditions.

Littelfuse's published PPTC selection methodology similarly recommends determining normal operating current, checking ambient temperature, applying thermal derating, and then verifying the remaining electrical parameters.

PPTC Holding Current Calculation Formula

For a first-pass engineering calculation:

Ihold(required) ≥ Iload(max) / Kt

Where:

Ihold(required)= required nominal PPTC holding current

Iload(max)= maximum normal operating current

Kt= thermal derating factor at the actual operating temperature

For example:

Calculation:

1.20 A ÷ 0.75 = 1.60 A

The next available PPTC rating should then be evaluated against the complete datasheet.

Important:The 75% value in this example is illustrative only. The actual derating factor must be obtained from the selected PPTC series datasheet.

Example: Calculating PPTC Holding Current at 25°C

Suppose an electronic circuit has:

Normal operating current: 0.50 A

Maximum normal operating current: 0.60 A

Maximum ambient temperature: 25°C

If the selected PPTC's reference Ihold is specified at 25°C and the datasheet does not require additional derating at that operating condition, the first-pass requirement is:

Ihold ≥ 0.60 A

A device with an Ihold of 0.75 A may therefore be considered as a candidate.

But the engineer should still check:

Itrip

Vmax

Imax

Initial resistance

Time-to-trip

Package

PCB thermal conditions

Example: Calculating PPTC Holding Current at High Temperature

Now consider the same circuit operating at a higher temperature.

Requirements:

Maximum normal current = 0.60 A

Maximum operating temperature = 70°C

Datasheet thermal derating factor at 70°C = 60%

Calculation:

Required nominal Ihold = 0.60 A ÷ 0.60

Required nominal Ihold = 1.00 A

Therefore, a PPTC with a nominal Ihold of only 0.60 A should not automatically be treated as suitable for this application.

A PPTC with a nominal Ihold of approximately 1.0 A or higher may be considered, subject to complete datasheet and application verification.

This example illustrates why 25°C Ihold cannot simply be copied into a high-temperature design.

Why Temperature Changes PPTC Holding Current

The relationship between current, resistance and temperature is fundamental to PPTC operation.

When current passes through the PPTC, the device generates heat.

A simplified relationship for electrical power is:

P = I²R

where:

P= power dissipated

I= current

R= device resistance

The PPTC also transfers heat to its surroundings.

The actual device temperature therefore depends on:

Current

Resistance

Ambient temperature

PCB copper area

Airflow

Component placement

Package construction

Nearby heat sources

As the ambient temperature increases, less additional heating may be required for the PPTC to reach its transition region.

Consequently, its holding-current capability decreases. Littelfuse's technical explanation describes this thermal relationship and notes that design choices affecting heat transfer can influence holding current.

Do Not Use a Universal PPTC Derating Percentage

One common mistake is to assume:

"PPTC Ihold should always be 20% or 30% higher than the load current."

There is no single percentage that is correct for every PPTC design.

The required margin depends on:

Temperature

PPTC series

Package

PCB layout

Load profile

Manufacturing tolerances

Thermal environment

Startup current

Required reliability

For example, one PPTC series may retain a relatively high percentage of its nominal holding current at 60°C, while another series may have a different derating curve.

Published manufacturer data demonstrates that thermal derating can vary substantially with temperature and device construction.

Therefore:

Use the manufacturer's derating curve instead of applying a generic percentage.

PPTC Holding Current vs Trip Current

Ihold and Itrip are often confused.

They describe different aspects of PPTC behavior.

The region between Ihold and Itrip should not be treated as a precise electronic switching threshold.

A PPTC is a thermal device, so the actual behavior depends on temperature, current duration, mounting conditions and heat dissipation.

PPTC Holding Current and Startup Current

Startup current deserves special attention.

Some circuits draw substantially more current during startup than during steady-state operation.

Examples include:

DC motors

Solenoids

Capacitive loads

Battery charging circuits

Power converters

LED drivers

Communication equipment

If startup current is a legitimate part of normal operation, the PPTC must tolerate it without unwanted tripping.

However, simply increasing Ihold to accommodate every startup event may reduce the effectiveness of overcurrent protection.

The better approach is to examine the time-current behavior of the PPTC and compare it with the duration and magnitude of the startup current.

How to Select PPTC Ihold From a Datasheet

A practical datasheet workflow is:

1. Find the maximum normal load current

Determine the highest legitimate steady-state current.

2. Determine the actual operating temperature

Use the worst-case temperature at the PPTC location.

3. Read the thermal derating curve

Find the available Ihold or derating factor at that temperature.

4. Calculate the required nominal Ihold

Use:

Ihold(required) ≥ Iload(max) / Kt

5. Select the next suitable PPTC

Choose a commercially available device whose specifications satisfy the requirement.

6. Check Itrip

Make sure the device can enter the protection state under the expected fault condition.

7. Check Vmax and Imax

Verify that the maximum circuit voltage and prospective fault current are within the PPTC ratings.

8. Check resistance

Verify that normal-state resistance does not cause unacceptable voltage drop or power dissipation.

9. Check time-to-trip

Confirm that the PPTC can provide adequate protection within the required fault duration.

10. Validate the actual application

Final PPTC selection should be tested under representative electrical, thermal and mechanical conditions.

Littelfuse's selection guide specifically recommends independent testing and evaluation of the PTC in the actual application.

What Happens If PPTC Ihold Is Too Low?

If the selected Ihold is too low, the PPTC may enter its high-resistance state during legitimate operation.

Possible symptoms include:

Unexpected power interruption

Voltage drop

Equipment restart

Intermittent operation

Production test failures

Nuisance tripping at high temperature

For example, a PPTC may operate normally during a 25°C laboratory test but begin tripping during a 70°C burn-in test.

This does not necessarily mean that the PPTC is defective.

The actual operating temperature and thermal environment may simply be outside the conditions assumed during the original selection.

What Happens If PPTC Ihold Is Too High?

Selecting a PPTC with an excessively high Ihold can also create problems.

The device may tolerate larger overcurrents for longer periods before entering its high-resistance state.

This can reduce the level of protection provided to sensitive downstream components.

Therefore, the goal is not:

"Choose the largest Ihold possible."

The goal is:

Choose an Ihold high enough for the maximum legitimate operating current, while maintaining the required fault-protection behavior.

PPTC Holding Current and PCB Layout

The PPTC datasheet is normally generated under specified test conditions.

The actual PCB can behave differently.

PCB factors that can affect PPTC thermal performance include:

Copper area

Copper thickness

Trace width

Thermal vias

Ground planes

Nearby components

Airflow

Enclosure design

A large copper area may improve heat spreading and change the thermal behavior of the PPTC.

Conversely, placing the PPTC near a hot power component can reduce its effective thermal margin.

Therefore, for critical designs, engineers should evaluate the PPTC under the actual PCB configuration rather than relying exclusively on room-temperature calculations.

A Practical PPTC Holding Current Calculation Example

Consider a 24 V industrial control circuit.

Circuit requirements

Nominal voltage: 24 V

Maximum normal load current: 1.5 A

Maximum ambient temperature: 60°C

SMD mounting

Short-circuit protection required

Suppose the candidate PPTC's datasheet indicates a thermal derating factor of 75% at the required operating temperature.

Calculation

Required nominal Ihold = 1.5 A ÷ 0.75

Required nominal Ihold = 2.0 A

A PPTC with a nominal Ihold around 2.0 A or higher may therefore be considered for the next stage of evaluation.

But the selection is not finished.

The engineer must still verify:

Vmax ≥ maximum circuit voltage

Imax ≥ prospective fault current

and confirm:

Itrip

Time-to-trip

Initial resistance

Post-trip resistance

Package

Operating temperature

PCB thermal conditions

This is the correct engineering approach to How to Calculate PPTC Holding Current.

PPTC Holding Current Calculation Checklist

Before selecting a PPTC, verify the following:

Load

What is the maximum continuous operating current?

What is the maximum legitimate startup current?

Are there load variations?

Temperature

What is the maximum ambient temperature?

What is the temperature near the PPTC?

Is the PPTC close to a heat source?

PPTC Datasheet

What is the rated Ihold?

What is the Itrip?

What is the thermal derating curve?

What is Vmax?

What is Imax?

What is the initial resistance?

What is the time-to-trip?

PCB

What is the copper area?

What is the expected airflow?

Are thermal vias used?

Are there nearby heat-generating components?

Validation

Has the design been tested at maximum temperature?

Has the maximum normal load been tested?

Has the expected fault condition been tested?

Has startup behavior been evaluated?

FAQs

What is PPTC Ihold?

PPTC Ihold is the specified holding current of a PPTC resettable fuse. It represents the maximum current the device can carry under defined conditions without transitioning into its high-resistance state.

How do you calculate the required PPTC holding current?

A useful first-pass calculation is:

Required nominal Ihold ≥ Maximum normal operating current ÷ Thermal derating factor

The thermal derating factor must come from the manufacturer's datasheet for the selected PPTC series and operating temperature.

Should PPTC Ihold be higher than the load current?

Yes. The PPTC should have sufficient temperature-adjusted holding-current capability above the maximum legitimate operating current.

However, the required margin should be determined from the actual application rather than a universal percentage.

Does temperature affect PPTC holding current?

Yes. Higher temperature generally reduces the available holding-current capability of a PPTC. This is why manufacturers provide thermal derating curves and temperature-specific ratings.

Can I use the 25°C Ihold value directly?

Only if the actual operating conditions are consistent with the datasheet reference conditions and the manufacturer's specifications permit it.

For higher-temperature applications, the Ihold value should be evaluated using the appropriate thermal derating information.

What is the difference between Ihold and Itrip?

Ihold describes the current the PPTC is specified to carry without transitioning to the high-resistance state. Itrip describes the current associated with triggering that transition under specified conditions.

They should not be treated as a simple ON/OFF threshold.

Why does my PPTC trip below its rated Ihold?

Possible causes include higher-than-expected local temperature, PCB thermal conditions, nearby heat sources, startup current, mounting differences or differences between the actual application and the manufacturer's test conditions.

Is there a fixed safety margin for PPTC Ihold?

No universal margin applies to every PPTC design. The required margin depends on temperature, load characteristics, PPTC construction, PCB thermal environment and reliability requirements.

Should I calculate Ihold or simply choose the next higher PPTC rating?

Both calculation and datasheet selection are useful. First calculate the required nominal Ihold based on actual operating conditions, then select a standard PPTC and verify its complete electrical and thermal specifications.

Can Ruilin Semiconductor help calculate PPTC holding current?

Yes. Ruilin Semiconductor can support engineers and purchasing teams in evaluating PPTC Ihold requirements based on load current, operating temperature, package, PCB conditions and application requirements.

About Ruilin Semiconductor

Ruilin Semiconductoris a semiconductor manufacturer focused on circuit protection and related electronic components.

As a PPTC manufacturer, Ruilin Semiconductor provides PPTC resettable fuse solutions for OEM, ODM, EMS and industrial electronic applications.

Our PPTC products are evaluated across key electrical and thermal parameters including:

Holding Current (Ihold)

Trip Current (Itrip)

Maximum Voltage (Vmax)

Maximum Fault Current (Imax)

Initial Resistance

Post-Trip Resistance

Time-to-Trip

Operating Temperature

Package and dimensions

Ruilin Semiconductor supports customers in selecting PPTC components according to actual application requirements rather than relying only on nominal current ratings.

For projects requiring a specific PPTC holding current, operating temperature, voltage rating, package or resistance range, our technical team can assist with product selection and application evaluation.

Ruilin Semiconductor — PPTC manufacturing and technical support for reliable resettable overcurrent protection.

Conclusion

Correctly determining PPTC holding current is essential for reliable resettable overcurrent protection.

The basic calculation is:

Required nominal Ihold ≥ Maximum normal operating current ÷ Thermal derating factor

But this equation should be treated as a first-stage engineering calculation, not the complete selection method.

A final PPTC selection should also verify:

Ihold + Itrip + Vmax + Imax + Resistance + Time-to-Trip + Temperature + PCB Thermal Conditions

The most reliable approach is to combine the electrical requirements of the circuit with the manufacturer's PPTC datasheet, thermal derating curve and time-to-trip characteristics, followed by validation under actual application conditions.

For OEM and ODM projects, Ruilin Semiconductor provides PPTC manufacturing, component selection and technical support for a wide range of electronic protection applications.

About Ruilin Semiconductor

Ruilin Semiconductor (Shenzhen) Co., Ltd.is a manufacturer specializing in PPTC resettable fuses and polymer positive temperature coefficient (PPTC) circuit protection components.

Our PPTC products are designed to provide resettable overcurrent protection for electronic and electrical equipment exposed to overload and short-circuit conditions. The product portfolio covers different current ratings, voltage ratings, resistance levels, package sizes, and application requirements.

Ruilin Semiconductor supports customers from product selection and engineering evaluation to samples, datasheets and volume production, helping OEMs, ODMs, EMS companies, electronics manufacturers, and distributors identify suitable PPTC solutions for their applications.