Automotive Relays Application Notes Automotive Applications Typical Automotive Applications Load Application examples Typical current curve Resistive Loads - Heatings (rear window heating, seat heating glow plug, air/water preheating) time Capacitive Loads - Lamps (front and rear beam, fog lights, flasher) - Filter capacitors in electronic modules (engine management module, ABS module) time Inductive Loads - Solenoids (vales, clutches, relay coils) Motor - Motors and pumps Solenoid (power window, central lock, cooling fan) time Introduction Ubatt Relay The range of applications can be classified into resistive loads, capacitive loads and inductive loads. The current curve of resis- tive 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, M 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 Fig. 1 Short-circuit brake 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 Motor-Reverse Circuit (H-Bridge) more complex. The most typical circuits are described hereafter. The H-bridge is used to operate a motor in two directions (e.g. door lock, steering lock, power window, Short-Circuit Brake seat adjustment, etc.). The operation time is typically very short compared to the thermal time constant of The short-circuit brake is used, wherever an electric motor must the relay (e.g. door lock <1s, power window <10s). be braked (e.g. wiper). The short-circuit brake transforms the This means, H-bridge relays must be designed for rotational energy of the motor into electrical energy. The short- high current-switching-capability, but not for high circuit brake can be critical at higher load voltages. If the switch- current-carrying-capability. Higher load voltages can off arc does not extinguish during the transition time of the be critical, due to possible short-circuit-arcs (see also movable contact, the arc creates a direct shortage of the power short-circuit brake). source. Particularly in 24VDC systems, the resulting extremely high arc current could cause almost instantly severe damage to Ubatt the contacts and could destroy the entire relay. Relay Relay turn right turn left M Fig. 2 H-bridge 1 03-2015, Rev. 0315 Catalog and product specification according Catalog and product data is subject to the Catalog, product data, Definitions section, www.te.com to IEC 61810-1 and to be used only together terms of the disclaimer and all chapters of application notes and all specifications are 2015 Tyco Electronics Corporation, with the Definitions section. the Definitions section, available at subject to change. a TE Connectivity Ltd. company. Automotive Relays Application Notes Automotive Applications Wiper Circuits Cooling Fan Circuits To stop the wiper in the correct position, a short-circuit brake Depending on the size of the engine, either one or two fans are is used. This can be done either by an internal slipring (usually used to cool the engine. There are different possibilities to control used for rear wiper) or with the normally closed contact of the the speed of the fans. Single fan systems are usually controlled by relay (usually used for front wiper). This requires a high switching one or more serial resistors. The disadvantage is the electrical power capability of the N/C-contact, especially in combination with loss of the resistors. Double fan systems are normally controlled by intermittent wipe function or rain sensor control. Dual speed switching them either in series (low speed) or in parallel (high speed). wipers have two windings, which are commuted by a second If more speed steps are needed, additional serial resistors are used. relay. There are also wiper systems without mechanical gear, A more sophisticated method is a PWM-controlled brushless motor. which are electrically reversed with a H-bridge circuit. In commercial vehicles the cooling fan is mostly driven directly by the motor shaft. Ubatt Ubatt Relay Relay Relay highspeed lowspeed on/off R M M slipring Fig. 5 Single fan circuit Fig. 3 Wiper with slipring Ubatt Ubatt Relay highspeed Relay on/off M Relay highspeed Relay lowspeed Relay high/low M M Fig. 6 Double fan circuit Fig. 4 Dual speed wiper-circuit 2 03-2015, Rev. 0315 Catalog and product specification according Catalog and product data is subject to the Catalog product data, Definitions section, www.te.com to IEC 61810-1 and to be used only together terms of the disclaimer and all chapters of application notes and all specifications are 2015 Tyco Electronics Corporation, with the Definitions section. the Definitions section, available at subject to change. a TE Connectivity Ltd. company.