Engineers concerned about protecting against high current or temperatures often use circuit breakers and fuses. These have drawbacks. Fuses need to be replaced by someone. Circuit breakers are often large and may not be compatible with surface-mount processes. Tyco Electronics is now offering its resettable fuses based on polymeric positive temperature coefficient technology (PPTC.) These give automotive engineers some new options, since the devices do not to be replaced when tripped.
The now automotive-qualified PolySwitch SMD devices include more than thirty products in various voltage ranges, according to the company. They are lead- and halogen-free as well as RoHS-compliant. The company points out they can be used to protect electronic control units, I/O ports and traces, telematics and infotainment systems, as well as motor and motor control circuits.
“The small form-factor and resettable functionality means you can place these in tight spots where users never have to access them,” Tyco's Tony Cilluffo, Automotive Strategic Accounts Manager, said at the July 19-20 SAE Convergence 2010 conference in Detroit.
“These devices were first developed to support computer applications in the consumer market,” Cilluffo explained. “USB ports, PCMCIA cards, and keyboard ports on a computer—anything where you have a user accessible port—there is a need for an overcurrent limiting device protecting that port. If there is something shorted in that peripheral, a mouse cord or USB device, your computer is protected from an overcurrent, often using PPTC.”
A PPTC device such as the PolySwitch actually works as a nonlinear thermistor. When current exceeds the rated threshold, the device warms up and greatly increases resistance, reducing current. When it cools after the current is reduced, it becomes conductive again.
“The PolySwitch is self-sufficient in limiting current. It shuts off the device without relying on a microcontroller or other higher intelligence,” Cilluffo said.
The Tyco family of PolySwitchs that are now AEC-Q200 qualified include its nanoASMD, microASMD, miniASMD, ASMD, and AHS surface-mount devices. The devices provide current ratings from 0.05 to 3.0 A and voltage ratings from 16 to 60 V dc. Most devices are rated from -40° to +85°C (-40 to +185°F), but for higher-temperature underhood applications the PolySwitch AHS device family is rated up to 125°C (257°F).