Eurolights and autocheck system

[DeWitt Harrison <de@aztek-eng.com>]

There was a recent question about how to deal with the autocheck
system when installing euro spec headlights. Another q-lister
recommended the instructions available at http://www.geocities.
com/MotorCity/Speedway/3229/light.htm (Sorry, I forget the
site's author at the moment.)  Good info.

The author discusses the lamp current sensing module and notes
that the sensing chip is a proprietary one marked ULX245M. This
sent me to the data books and I found an Automotive Lamp Monitor
chip having the P/N UL2454M made by Allegro MicroSystems, Inc.,
in Worcester, MA, phone 508-853-5000.    Well, hummmm.

It is a 8 pin plastic DIP, dual lamp circuit, rated for operation between
-40C and +85C.  A quad circuit part is also available (ULN2455A).
Absolute max supply voltage is 30V.

The outputs are open collector NPNs so they can just be wired together.
Output A is pin 1, output B is pin 5. There is an ORed output in
addition, pin 8, which is also OC. Pin 4 is ground. There is no
dedicated supply voltage connection pin, the chip draws power via
the sense pin connections to lamp power.  Each detector will trip
when the differential voltage between sense pins (pair A is pins 2
and 3, pair B is pins 6 and 7) exceeds about 13 mV (7 mV min,
20 mV max).

In the application example circuit, switched lamp voltage is
connected to pin 2 (6)  and one end of the current sense resistor.
The lamp's positive terminal is connected to the other side of
the sense resistor and pin 3 (7). In other words, lamp current
develops a small voltage across the resistor. This voltage is
sensed by pins 2 and 3 (or 6 and 7). The resistor should
be at least 0.020V divided by the lamp current. For example,
in a 60W circuit, lamp current would be roughly 60/14 = 4.3A
so that R should be greater than 4.7 milliohms. Depending on
where the sense pins are connected, the headlight wiring may
or may not be included in the effective sense resistance value.

An alternate connection, were pin 2 (6) was connected to the
left lamp positive terminal and pin 3 (7) was connected to the
right lamp positive terminal, with each lamp feed through
its own sense resistance, would detect a difference in current
between the two lamps. With this arrangement, two pairs of
lamps could be monitored with a single part. The info at the
referenced web site suggests that this may be the modus
operandus of interest. I would be interested to know whether
there are two or four sense wires in the 443 919 469B
lamp monitor module.

I haven't familiarized myself with the headlight wiring details yet
so I'm not sure yet what use we may make of this info. Cheers,

DeWitt Harrison    de@aztek-eng.com
Boulder, CO
88 5kcstq