Resistance measurement circuit for external deployment path of sir system

摘要

An inflatable restraint system includes a diagnostic circuit which measures the resistance of an external deployment path containing a squib. The circuit supplies a test current through the path and measures the voltage across the path which is monitored by a microprocessor. A vehicle battery supplies the power to the system and is connected to a constant current source in series with the external path to provide the test current. A current limiting device between the external circuit and ground comprises a current sink allowing a maximum safe current in the external circuit in the event of inadvertent application of battery voltage to the external circuit and, during a normal resistance test, imposes only a small voltage drop on the test current circuit.