Ross said:
Everybody seems to forget that the lamp test must correctly detect an
open circuit LED lamp. A dummy load which allows the computer to
record a successful test must actually include the LED lamp in a fail
safe manner. In other words, assume the dummy load is in place and the
LED lamp was not connected in parallel. The computer would still carry
out a successful test I would imagine. Now how meaningful would this
result be?
Would this make you happy, or would it still violate
some design standard?
----+--------------[Rload]-------------+
| |
[R] |
| /
+-----[comparator]---[oneshot]---|
| | \
[LED] Vref |
| Gnd
Gnd
Comparator triggers one-shot when it sees Vfled.
I'm clueless as to how the testing is done by the computer -
I don't know if it is a one time test at turn on, or if it
is continuous. If it is continuous, remove the oneshot.
Ed
Ed,
Your arrangement would appear to trick the vehicle computer into
providing a fail safe test result, however the cost of including this
custom modification into each LED lamp assembly would imo make it
uneconomical compared to simply using incandescent lamps.
I would imagine that the additional components would also need to be
integral to the LED lamp itself so that it formed a complete unit in
order to meet the relevant vehicle design standards (for Sth. Africa
in this case). I would also ssurmise that the design standards for the
trailer would specify the type of lighting fixtures, wiring,
connectors etc, but additional circuitry to make LED lighting
agreeable to an existing vehicle computer would not be covered. Since
the modified lamp assembly as proposed by your suggested circuit is
not a standard product then it would need to be approved under the
design standards before being allowed to be installed.
On the subject of LED automotive lights I did some further research;
When attempting to use LED lighting in high reliability, long life and
efficient operation, it appears that some type of regulated driver
circuitry is required which preferably incorporates diagnostics.
Including LED lamps to work with existing vehicle computers is bound
to create some difficulties particularly where the computer only
caters for incandescent lamps which are standard on the vehicle
itself. Trying to mix incandescants and LED's on the same vehicle
appears to be a no-go situation.
A search for LED drivers for automotive applications revealed that
using LED's is not as simple as just wiring a series of LEDs into an
array and connecting them to the vehicle power supply. Some sort of
control module may be needed.
Melexis make an IC specific to this application. However, to include a
diagnostic function seems to be relatively complex.
http://www.melexis.com/relinfofiles/applicationnotemlx1080103.pdf
ST Microelectronics have an IC which does includes a diagnostic
function
http://www.st.com/stonline/products/literature/an/8414.pdf
Analog Devices have the AD8240
http://www.analog.com/en/prod/0,2877,AD8240,00.html
This recent EDN article sheds a little light (pardon the pun) on the
subject of automotive LED lighting and some considerations
http://www.edn.com/article/CA490413.html