Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Positive Temperature Coefficient Overcurrent Protection Devices: SAE J2685:2014 Guide
Standard J2685:2014 from SAE International defines test conditions, procedures, and performance requirements for Positive Temperature Coefficient (PTC) overcurrent protection devices. Intended for ground-vehicle electrical systems, it covers both polymeric (PPTC) and ceramic (CPTC) types in radial-leaded, surface-mount, and other form factors.
Scope and Key Requirements
The standard addresses devices that are normally low resistance and that trip (switch to high resistance) under overcurrent or overtemperature conditions. PTCs are self-resetting: after the fault clears and power is removed, the device cools and returns to low resistance. Maximum operating voltage must match the vehicle system. Required ratings are 16 V for 12 V batteries, 32 V for 24 V batteries, and 58 V for 36 V batteries / 42 V powernets (all DC).
| Battery Voltage | System (Powernet) Voltage | Maximum PTC Operating Voltage |
|---|---|---|
| 12 V | 14 V | 16 V |
| 24 V | 28 V | 32 V |
| 36 V | 42 V | 58 V |
Devices may be polymer-based (PPTC) or ceramic-based (CPTC). The standard applies to all form factors provided they meet the performance definitions.
Testing, Performance, and Design Insights
SAE J2685 defines a comprehensive test sequence covering physical, electrical, and environmental performance. Power supplies must be accurate to within ±2 % of set point, and load transients must recover within 100 ms. Test categories include:
| Test Category | Examples |
|---|---|
| Physical | Dimensions, solderability, lead strength |
| Electrical | Resistance, trip time, power dissipation, hold/trip current |
| Environmental | Temperature cycling, humidity, thermal shock |
🛠️ Design Insight: Because PTCs self-reset, they eliminate the need for fuse replacement after a fault. However, the circuit must experience a momentary loss of power (removal of voltage) to allow the device to cool and reset. System design should account for this behavior.
Hold current (Ihold) is the maximum current the PTC can carry without tripping under specified conditions; trip current (Itrip) is the current that triggers the high-resistance state. These values differ significantly and must both be considered in circuit design.
⚠️ Arc Hazard Warning: The standard recommends avoiding live connection or disconnection of PTCs. Electrical arcing can damage equipment and harm personnel. Always de‑energize the circuit before making or breaking connections.
Application guidelines in Appendix A advise on mounting, thermal management, and derating for ambient temperature. Higher temperatures reduce trip current; lower temperatures increase it. For 42 V powernets, verify manufacturer data for higher-voltage performance.
Frequently Asked Questions
- What is the difference between hold current and trip current?
- Hold current (Ihold) is the maximum steady-state current the device can pass without switching to high resistance. Trip current (Itrip) is the current at which the device begins to trip. Itrip is always higher than Ihold; the ratio depends on the PTC design.
- Can a PTC rated for 16 V be used in a 24 V system?
- No. The maximum operating voltage of the PTC must equal or exceed the maximum system voltage (32 V for a 24 V battery). Using an underrated device risks breakdown and arcing.
- How does ambient temperature affect a PTC’s trip point?
- Higher ambient temperature reduces both hold and trip current. Lower ambient temperature increases them. Always apply manufacturer derating curves for the expected operating temperature range.
- When should I choose a PPTC over a CPTC?
- The choice depends on application requirements such as voltage rating, trip speed, current capacity, and physical size. PPTC devices are typically used in lower‑current automotive circuits; CPTC devices can handle higher currents and voltages. Consult manufacturer data.
