Application Note
A u t o m o t i v e
The Design Challenge:
Two considerations in automotive engineering areweight and reliability. As cars become equipped withmore options and safety requirements, the weight ofthe car increases. As the weight increases, performance
characteristics decrease and fuel efficiency decreases.
Standard fusing requires wiring through the fuse panel
to the circuit being protected. This adds to a vehicle’sweight, while the added wiring increases the opportunity for malfunctions. A standard blown fuse must be
manually changed, and the circuit is inoperative while
the fuse is in the non-functional state. The fuse box
must be accessible in order to service fuses, limiting
the locations the fuse box can be placed.
Automotive door lock circuits require overcurrent
protection. If a door lock is jammed or forced into a
position so the lock cannot move in the ordered direction, the IC chips controlling the system can bedamaged. A multitude of hazardous situations canD o o r

L o c k s

occur, such as physical damage to the door or shortcircuiting due to water leakage into the door panel orlow temperatures causing freezing of mechanicalmechanisms. Current designs include wiring from thecontrol module in the cockpit of the car to the door
locks via the fuse box under the dash. Using standard
fusing, once the fuse is blown, the door locks remain
in a non-operative state until the fuse is replaced.
Agency certifications such as those issued by Underwriters Laboratory (UL), are concerned strictly withsafety standards. Automotive standards far exceed
the testing required by agencies such as UL. The
Automotive Electrical Council (AEC) sets forth thestandards for passive electrical components in the
QEC 200 test standards. These tests are concernedwith reliability as well as safety. Companies such as
Visteon, Delphi, and Daimler/Chrysler are currently
testing to QEC 200 requirements. (For a copy ofAEC’s QEC 200, please contact your local Bourns
sales representative.)
The Application:

DOOR
AJ
AR
LO
CKSE/UNS
NL
OR
DO
OC
ORK
BU
LORETTMO CKE
ONR
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MO
COTI
NTONROSEL
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RG
1018100
0TIn a typical door lock design, the lock motor is energized through a relay by a control processor via aninverter IC (See Figure 1). A pulse is sent to theprocessor from the contact lock/unlock switch, keylock, motion sensor, door ajar circuit or remote control. The coils of the relay have a positive voltage
from V(Batt) at pin 5 to prevent relay chatter fromnoise or nuisance tripping of the relays (See Figure 1).
V(Batt) is typically 12 volts, but will soon be upgraded
for the 42 volt automotive platform. The signal fromthe logic controller is sent to an inverter to bias thecoil for current flow through pins 6 or 7 depending on
the required workload of unlocking or locking.

RG
300

CPU

WINDOW UP

I
N
V
E
R
T
E
R
S

VBATT 112 VDC
L11756L1243

WINDOW DOWN

RG
700

WINDOW MOTORRELAYS

21PTC
GRD

Figure 1

DOOR
LOCK
MOTOR

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OR
AJ
AR
LO
CK
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NS
NL
OR
DO
OC
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BU
L
O
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MO CKE
ON
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NT
ON
RO
SE
L
NS
OR
WINDOW UP

I
N
V
E
R
T
E
R
S

VBATT 112 VDC
L11756L1243

WINDOW DOWNCPUWINDOW MOTORRELAYS

21PTC
GRD

Figure 2

DOORLOCKMOTORDOORAJARLOCKSE/UNSNLORDOOCORKBULO
RETT
MO CKEONRTEMOCOTINTONROSELNSOR

The unlocking portion is similar. The coil from pin 5to pin 6 of the relay will be energized by the inverterand controller circuit. The armature at pin 1 willchange state to pin 3. When this happens, theV(Batt) will let current flow through the contacts atpins 3 and 2 (See Figure 3) through the motor(changing lock position) back through pin 4 to pin 1to the PPTC and ground, thus changing the state ofthe lock position.

WINDOW UPCPUWINDOW MOTORRELAYS

INVERTERSVBATT 1
12 VDC
L11756L1243

WINDOW DOWN

If the coil from pin 5 to pin 7 is energized, the armature from pin 4 will change state to pin 3 conducting
current through the door lock motor in a direction tolock the door (See Figure 2). The path of the current
will continue from the door lock motor to pin 2 of therelay. At pin 2 the second armature will not beengaged and will continue contacting pin 1, giving aground through the PPTC. If a short or fault condition were to occur, the excess current flow fromV(Batt) through the door lock motor and relay wouldcause I2R heating in the PPTC. As the PPTC heats
up, the resistance of the PPTC goes exponentiallyhigh, isolating the ground and negating circuit flow.
Once the overcurrent fault condition is removed, thepolymer will return to its semi-crystalline structurewith the carbon chains reconnected, and current will
once again flow through the PPTC.

2DOOR
LOCKMOTOR

1PTC
GRD

Figure 3

Consult you local Bourns, Inc., representative for
additional automotive applications such as:• Window lifts• Modules• Electric seats• Underhood applications• Wiring harnesses• Accessory outlets• Lighting• Moon/sun roofs