2203997-1

Automotive Relays Application Notes Automotive Applications Typical Automotive Applications Application examples Typical current curve Resistive Loads - Heatings (rear window heating, seat heating glow plug, air/water preheating) current Load Capacitive Loads - Lamps (front and rear beam, fog lights, flasher) - Filter capacitors in electronic modules (engine management module, ABS module) current time Inductive Loads - Solenoids (vales, clutches, relay coils) - Motors and pumps (power window, central lock, cooling fan) current time Motor Solenoid time Introduction The range of applications can be classified into resistive loads, capacitive loads and inductive loads. The current curve of resistive loads is specified by the load voltage and load resistance. Capacitive loads have a high inrush current and a low steady current. Therefore lamps are counted to the capacitive loads, because the cold filament has a significantly lower resistance, than the hot filament. Inductive loads are characterized by an exponential current increase and a remarkable switch off arc, induced by the demagnetization of the magnetic circuit of the load. Power supply relays (clamp relays) can switch or feed a mixture of different loads. The circuit design of resistive and capacitive loads is usually a simple switch on and switch off. Motor load circuits are often more complex. The most typical circuits are described hereafter. Short-Circuit Brake The short-circuit brake is used, wherever an electric motor must be braked (e.g. wiper). The short-circuit brake transforms the rotational energy of the motor into electrical energy. The shortcircuit brake can be critical at higher load voltages. If the switchoff arc does not extinguish during the transition time of the movable contact, the arc creates a direct shortage of the power source. Particularly in 24VDC systems, the resulting extremely high arc current could cause almost instantly severe damage to the contacts and could destroy the entire relay. Ubatt Relay M Fig. 1 Short-circuit brake Motor-Reverse Circuit (H-Bridge) The H-bridge is used to operate a motor in two directions (e.g. door lock, steering lock, power window, seat adjustment, etc.). The operation time is typically very short compared to the thermal time constant of the relay (e.g. door lock