Automotive Relays
Application Notes
Automotive Applications
Typical Automotive Applications
Load
Resistive Loads
Application examples
current
Typical current curve
- Heatings
(rear window heating, seat heating
glow plug, air/water preheating)
time
current
Capacitive Loads
- Lamps
(front and rear beam, fog lights, flasher)
- Filter capacitors in electronic modules
(engine management module, ABS module)
- Solenoids
(vales, clutches, relay coils)
- Motors and pumps
(power window, central lock, cooling fan)
time
Inductive Loads
current
Motor
Solenoid
time
Introduction
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,
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.
Relay
Ubatt
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 <1s, power window <10s).
This means, H-bridge relays must be designed for
high current-switching-capability, but not for high
current-carrying-capability. Higher load voltages can
be critical, due to possible short-circuit-arcs (see also
short-circuit brake).
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 short-
circuit brake can be critical at higher load voltages. If the switch-
off 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
turn right
Relay
turn left
M
Fig. 2 H-bridge
03-2015, Rev. 0315
www.te.com
© 2015 Tyco Electronics Corporation,
a TE Connectivity Ltd. company.
Catalog and product specification according
to IEC 61810-1 and to be used only together
with the ‘Definitions’ section.
Catalog and product data is subject to the
terms of the disclaimer and all chapters of
the ‘Definitions’ section, available at
http://relays.te.com/definitions
Catalog, product data, ‘Definitions’ section,
application notes and all specifications are
subject to change.
1
Automotive Relays
Application Notes
Automotive Applications
Wiper Circuits
To stop the wiper in the correct position, a short-circuit brake
is used. This can be done either by an internal slipring (usually
used for rear wiper) or with the normally closed contact of the
relay (usually used for front wiper). This requires a high switching
capability of the N/C-contact, especially in combination with
intermittent wipe function or rain sensor control. Dual speed
wipers have two windings, which are commuted by a second
relay. There are also wiper systems without mechanical gear,
which are electrically reversed with a H-bridge circuit.
Cooling Fan Circuits
Depending on the size of the engine, either one or two fans are
used to cool the engine. There are different possibilities to control
the speed of the fans. Single fan systems are usually controlled by
one or more serial resistors. The disadvantage is the electrical power
loss of the resistors. Double fan systems are normally controlled by
switching them either in series (low speed) or in parallel (high speed).
If more speed steps are needed, additional serial resistors are used.
A more sophisticated method is a PWM-controlled brushless motor.
In commercial vehicles the cooling fan is mostly driven directly by the
motor shaft.
Ubatt
Relay
on/off
Ubatt
Relay
highspeed
Relay
lowspeed
M
slipring
M
R
Fig. 5 Single fan circuit
Fig. 3 Wiper with slipring
Ubatt
Relay
Relay
Ubatt
highspeed
on/off
M
Relay
lowspeed
highspeed
Relay
Relay
high/low
M
M
Fig. 6 Double fan circuit
Fig. 4 Dual speed wiper-circuit
2
03-2015, Rev. 0315
www.te.com
© 2015 Tyco Electronics Corporation,
a TE Connectivity Ltd. company.
Catalog and product specification according
to IEC 61810-1 and to be used only together
with the ‘Definitions’ section.
Catalog and product data is subject to the
terms of the disclaimer and all chapters of
the ‘Definitions’ section, available at
http://relays.te.com/definitions
Catalog product data, ‘Definitions’ section,
application notes and all specifications are
subject to change.
Automotive Relays
Application Notes
Automotive Applications
Coil Suppression Circuits
The disconnection of the relay coil generates a transient
voltage peak, which is only limited by the parasitic inductivity
and capacity of the electrical system. In most of the cases it is
necessary to suppress the transient voltage peak to typically
60VDC – 100VDC for the protection of the relay driver or the
vehicle electrical system. Any voltage suppression of the relay
coil influences the dynamics of the electromechanical system
and can reduce the lifetime.
The best protection method for the driver is a diode in parallel to the
relay coil. But this method has the worst influence on lifetime of the
relay. Typical suppression methods are a resistor in parallel to the coil
(preferably as internal component in plug-in relays) or a Zener diode
in parallel to the relay driver (preferably for PCB relays).
Suppression in Parallel to Relay Coil
(External or Internal)
Suppression
in Parallel to Driver
UbattUbatt
R
R
D
D
UbattUbatt
UbattUbatt
Z-Diode
Z-Diode
Fig. 7 Coil suppression circuits
ISO Micro Plug-in relay
Relative lifetime [%]
100
80
60
40
20
0
Lampload
220W
Motorload
20A, 1mH
(Rpar = 6 x Rcoil)
Without
Z-Diode
Resistor
Suppression method
Diode
Fig. 8 Influence of coil suppression on the lifetime
03-2015, Rev. 0315
www.te.com
© 2015 Tyco Electronics Corporation,
a TE Connectivity Ltd. company.
Catalog and product specification according
to IEC 61810-1 and to be used only together
with the ‘Definitions’ section.
Catalog and product data is subject to the
terms of the disclaimer and all chapters of
the ‘Definitions’ section, available at
http://relays.te.com/definitions
Catalog, product data, ‘Definitions’ section,
application notes and all specifications are
subject to change.
3
Automotive Relays
Application Notes
Diagnostics of Relays
Introduction
With the increase of electronic circuits and safety requirements in
cars, more and more switching contacts are monitored by diagnostic
routines. Similar routines are used by the system suppliers before,
during and after the assembly processes (In-Circuit and End-Of-Line
tests) as well.
Automotive relays are originally designed to switch real automotive
applications (e.g. motors, lights, heating, etc.) with a load voltage of
12VDC (or 24VDC or 42VDC) and load currents above 1A. Lower
diagnostic current and voltage levels can result in wrong interpreta-
tions of the state of the relay contact.
This paper explains the technical background of the relay contact
system, coil system and dynamic behavior and gives recommenda-
tions for applicable diagnostic routines.
The breakdown voltage depends on the thickness and specific resis-
tance of the layer and can reach theoretically some hundred volts. In
the practice of automotive relays the breakdown voltage can be up
to 3VDC. A current of min. 10mA is needed to start the A-fritting.
After the electrical breakdown a small current is forced through very
thin channels in the layer. The resulting local high current density
heats the conducting channels up quickly, destroying the layers, until
finally (within a few ms) a metal to metal bridge is established. This
process is called B-fritting. The B-fritting voltage depends again on
the thickness and specific resistance of the layer and can reach in
the practice of automotive relays up to 300mV.
300
U
contact
250
mV
200
150
100
50
A-fritting
Contact System
The most common contact materials for automotive relays are fine
grain silver (AgNi0.15) and silver tin oxide (AgSnO
2
). Both materials
are affected by sulfidation and oxidation. These layers of oxides, sul-
fides and other compounds will be formed on the surface of metal-
contacts by absorption of gas molecules from the ambient atmos-
phere within a very short time. The layers will increase the contact
resistance. The resistance of such layers depends on the thickness
of the layer, effective contact area and the specific resistance of the
contact material/layer. To get a reliable electric contact these layers
have to be destroyed. This can be done by mechanical, electrical or
thermal destruction. A mechanical destruction requires high contact
forces. With the very high degree of miniaturization of automotive
relays, those contact forces cannot be safely achieved, especially for
normally-closed contacts. An electrical destruction requires a specific
breakdown voltage and current. This destruction process is called
A-fritting.
B-fritting
1.5
50m
19m
I
contact
A
Fig. 2 Fritting of a relay contact
A thermal destruction requires high temperatures, which will only be
generated by high contact currents or electric arcs (disconnecting
inductive loads). The thermal destruction can only take place, after
an electrical destruction.
+
Contact
Layer
Contact
Coil System and Dynamic Behavior
Automotive relay coils are designed for DC-voltages. The switching
times (i.e. operate time or switch-on time and release time or switch-
off time) are usually in the low millisecond-range. The operate time
depends on the applied coil voltage. Fig. 3 shows the relation of the
operate time and coil voltage, related to the actual pull-in voltage.
The nominal voltage in the displayed case is 170% of the actual pull-
in voltage at an ambient temperature of 23°C (e.g. 7VDC pull-in vol-
tage at 12VDC nominal voltage). Higher coil temperatures increase
the pull-in voltage and subsequently increase the operate time at the
same applied coil voltage. Including possible contact bouncing, the
operate time can be three times as high as the typical values shown
in the datasheets.
Electric breakdown
of layer (A-fritting)
Fig. 1 Contact layers
4
03-2015, Rev. 0315
www.te.com
© 2015 Tyco Electronics Corporation,
a TE Connectivity Ltd. company.
Catalog and product specification according
to IEC 61810-1 and to be used only together
with the ‘Definitions’ section.
Catalog and product data is subject to the
terms of the disclaimer and all chapters of
the ‘Definitions’ section, available at
http://relays.te.com/definitions
Catalog product data, ‘Definitions’ section,
application notes and all specifications are
subject to change.
Automotive Relays
Application Notes
Diagnostics of Relays
Recommendations for Diagnostic Routines
A contact diagnostic routine must:
Take into account, that the contact resistance may be non-linear.
Due to the explained fritting phenomena, the contact resistance can
be non-linear. This means, that the contact resistance, measured
at low voltage and current levels (e.g. standard multimeter) can be
significantly higher than the contact resistance under real condi-
tions (e.g. supplying a 100W load). We recommend to perform the
diagnostic routine with the real application and real board net voltage
connected.
Supply a sufficient voltage and current to force the A-fritting. If the
diagnostic routine cannot be performed with real application and
voltage, the measurement voltage level must secure an electrical
breakdown of possible layers. We recommend a voltage level of min.
5VDC and current of min. 100mA for min. 1ms.
Accept, that the voltage drop can be 300mV. The B-fritting is a
physical phenomenon, which can occur on all silver based contacts.
For signal-applications, special signal relays are available. We re-
commend to set the diagnostic threshold voltage to 500mV per relay
contact (important for H-bridges or serial contact arrangements).
Regard the maximum possible switching times. If the status of the
contact has to be changed for the diagnostic routine (energize or
de-energize relay), the routine must wait until the intended contact
status is reached. Depending on ambient conditions (temperature,
voltage levels, coil circuits) the times can be significantly longer than
the specified typical times. We recommend a delay time of min. 10
times of the typical switching times or min. 20ms after first contact
status change at min. 110% pull-in voltage.
A coil diagnostic routine must:
Secure, that the status of the contact does not change during the
diagnostic. If the coil driver is monitored by a watchdog routine, the
energizing/de-energizing time of the coil must not result in an unin-
tended closing or opening of the contacts. We recommend times of
max. 0.5ms.
Fig. 3 Operate time
The release time depends mainly only on the used coil suppression.
A low ohmic device (e.g. a diode) in parallel to the relay coil can in-
crease the release time by a factor of up to 4 compared to the typical
values shown in the datasheets.
Fig. 4 Release time
Coil Resistance
The inductivity of automotive relay coils can exceed (in the unsatura-
ted range) 1H. This results in time constants tau between 1 to 50ms
for the exponential inductive current increase (jump response). If the
ohmic coil resistance is measured with a 4-pole measurement, the
resistance value will be wrong, if it is measured during the inductive
current increase after switch on.
Remarks
1. During the lifetime of the car, the relay parameters can alter due to ageing processes like contact erosion, fretting corrosion and relaxa-
tion. Moreover the environmental requirements (temperature, vibration etc.) in the car are higher than in IC- or EOL-tests. Therefore we
recommend for diagnostic routines in the car a higher threshold voltage (approx. 3VDC) and higher delay times (min. 15 times of the
typical switching times).
2. Most of the applications, switched by relays, are not safety related. If a potential relay failure is detected, we recommend to not block any
further activation of the relay but to write the occurrence into the error memory.
03-2015, Rev. 0315
www.te.com
© 2015 Tyco Electronics Corporation,
a TE Connectivity Ltd. company.
Catalog and product specification according
to IEC 61810-1 and to be used only together
with the ‘Definitions’ section.
Catalog and product data is subject to the
terms of the disclaimer and all chapters of
the ‘Definitions’ section, available at
http://relays.te.com/definitions
Catalog, product data, ‘Definitions’ section,
application notes and all specifications are
subject to change.
5
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