Circuit Protection

by Levi Bennett, Clemson Electrical Engineering Student

Overvoltage Protection

With the use of electronics and the microcontrollers that control them, protection against unwanted conditions is necessary to maintain the level of performance needed in automotive applications. With most electronics inside the car running on a 5 volt reference source, it is important that the voltage not exceed this value. Since all power in a car comes from the 12-volt battery it is necessary to scale down this voltage somehow, then have a device that will not exceed a certain threshold. Most times overvoltage protection consists of a combination of resistors and zener diodes. The resistors in the protection scheme are usually current limiting resistors, which means they only allow a certain amount of current for a given application. The current to the load can be varied by variable resistors. The Zener diode in the protection scheme is the most important element and it serves to maintain the voltage across the load at a constant level.

Overcurrent Protection

To protect both the ECU and the communication bus from overcurrent and voltage spikes a resettable fuse is used between the voltage source and the two buses. Resettable fuses are kind of like thermal resistors in that the resistance changes as the temperature of the wires or device being monitored changes. When an overcurrent or voltage spike situation occurs the temperature of the wires increases, which increases the resistance of the resettable fuse to an open circuit. The unique thing about these fuses is that once the fault situation is removed, the fuse is reset to its original resistance, allowing for normal operation. The purpose of a resettable fuse is to be able to protect from fault situations without having to replace a fuse every time a fault occurs.

Manufacturers

Littlefuse, TE Connectivity

For More Information

[1] Circuit Protection Considerations for Automotive Information Buses, Tyco Electronics Application Note.

[2] Resettable Fuse, Wikipedia.

[3] Resettable Overcurrent and Overvoltage Protection Devices for Automotives, Electronics Bus website.

[4] Overvoltage Protection in Automotive Systems, Maxim website.

[5] Over Voltage Protection Circuit for Automotive Load Dump, Texas Instruments Application Note.