Test circuit for detection of malfunctions in an electric triggering device

A test circuit for detection of malfunctions in an electric triggering device (10, 11) for protection of vehicle occupants. The triggering devices (10, 11) are arranged with a first connector (12) connected to earth (13) and a second connector (14) connected to a control unit for supply of a triggering current to the triggering devices when detecting a triggering condition. The test circuit comprises a voltage regulator (17), a current generator (18,19) connected in series between the voltage regulator and the connection of the triggering devices (10,11) to the control unit for generating a test current to the triggering devices, and two voltage comparators (24,25) connected between the earth connection (13), the connection (14) to the control unit and the voltage regulator for detecting the voltage across the triggering devices (10,11).

TECHNICAL FIELD 
The present invention relates to a test circuit for detection of 
malfunctions in an electric triggering device for protection of vehicle 
occupants, in which the triggering devices are arranged with a first 
connector connected to earth and a second connector connected to a control 
unit for supply of a triggering current to the triggering devices when 
detecting a triggering condition. 
BACKGROUND OF THE INVENTION 
Triggering devices according to the above are used i.e. in motor vehicles 
for activating so-called air bags, i.e inflatable air cusions which 
protect the driver and passanger in the event of an accident. It is 
important that the condition of these triggering devices is continuously 
monitored so that they actually function if an accident should occur. One 
problem is that the resistance of various common triggering devices can 
vary between 1.80 and 2.50 .OMEGA.. For example, a normal tolerance zone 
in an application with two triggering devices connected in parallel can 
lie between 0.9 and 1.25 .OMEGA., outside of which range an error must be 
indicated. 
Several test circuits for continuous monitoring of triggering devices are 
known. Because of the problem of the large tolerance zone of the 
resistance of the triggering device, use has been made of circuits in 
which voltage division takes place across the resistance and with 
detection of changes in resistance. For example, U.S. Pat. No. 4,016,426 
describes test circuits which include a plurality of diodes, transistors 
and other electrical components. The complicated construction is due 
partly to the fact that the current must be limited so that the triggering 
devices will not be activated by the test current. This implies that the 
arrangement is relatively expensive to manufacture. 
OBJECT OF THE INVENTION 
It is therefore an object of the present invention to provide a simple, 
cheap and reliable test circuit. 
SOLUTION 
This object is achieved according to the present invention in that the test 
circuit comprises a voltage regulator, a current generator connected in 
series between the voltage regulator and the connection of the triggering 
devices to the control unit for generating a test current to the 
triggering devices, and two voltage comparators connected between the 
earth connection, the connection to the control unit and the voltage 
regulator for detecting the voltage across the triggering devices. 
Preferred embodiments of the invention are detailed in the dependent claims 
.

BEST MODE OF CARRYING OUT THE INVENTION 
A triggering device for a so called air bag is provided with a hot-wire 
whose function is to ignite a powder charge. Error in the triggering 
device is indicated if its resistance deviates from a resistance range. 
Usually two triggering devices connected in parallel in a circuit are 
used. The triggering of the two triggering devices occurs in parallel via 
a common detector. In the following, a test circuit is described in an 
application with two parallel connected triggering devices. 
In the drawing, reference numerals 10 and 11 denote the two 
parallel-connected triggering devices which are arranged with a first 
connection 12 connected to earth 13 and a second connection 14 connected 
via a lead 15 to a control unit for detecting a triggering condition. The 
control unit is connected to the positive terminal of the vehicle's 
battery (not shown). 
A second lead 16 connects a voltage regulator 17 via the ignition lock to 
the previously mentioned battery's +ve terminal and provides voltage to a 
current generator which comprises an operational amplifier 18 and a 
current amplifier 19 (transistor). 
The current generator receives a suitable reference potential in a 
potential divider formed by two resistors 20, 21 which in turn is supplied 
from the voltage regulator 17. The operational amplifier 18 with 
additional current amplification from the amplifier 19 gives a correct 
test current by means of the choice of resistance of the resistor 22. The 
diode 23 serves to prevent reverse current flow through the circuit during 
activation of the triggering devices via the lead 15. 
The voltage comparators 24 and 25 are connected such that an error is 
indicated if the resistance between the points 12 and 14 is too great or 
too small. The resistors 26, 27 and 28 give the two reference potentials 
and the resistors 30, 31 and 32 are included in the positive feed-back 
coupling for distinct detection. The resistor 29 is a "pull-up" resistor 
when the voltage comparators 24, 25 present "open-collector" output. 
The voltage comparator 24 indicates an error in the case of too high a 
voltage whilst the voltage comparator indicates an error when the voltage 
is too low. The error signal is attained at 33 as an output voltage. 
In a preferred embodiment of the invention, the maximum test current 
through the triggering devices is 40 mA. The parallel connection of the 
two triggering devices means that the resistance between the points 12 and 
14 ought to lie between 0.90 and 1.25 .OMEGA.. At this low resistance a 
voltage of approximately 40 mV is attained. This low voltage is sufficient 
for the monitoring of the triggering devices, though not so high that they 
are activated. 
The invention is not restricted to the above-described embodiment, but may 
be varied within the scope of the appended claims.