Come up with a brilliant solution .......


I work in the trailer manufacturing business.
We use LED tail-light clusters.
The modern truck/tractors utilise a computer system to perform diagnostics
on various systems within the rig. One of these is the lighting system.
A pulse is sent to each tail light, the system monitoring the current. If
there is current flow, it is assumed the incandescent globes are OK.
Unfortunately, the LED lamps draw such a small amount of current that the
computer does not see a "filament" and flags a major fault on the truck
dashboard. In addition to the error message, the system continues to send
curent pulss in the forlorn hope that things at the rear of the rig have
improved. This causes the entire suite of LED lamps to flash like a
low-class disco!
There are ways around it. Some manufacturers have placed incandescent globes
in parallel with the LED lamps, others have used high wattage resistors.
Neither is acceptable for obvious reasons.
There must be a way to "tell" the computer that the LED lamps are fine by
emulating the current drawn by an incandescent globe, without using the
solutions noted above.
Do any of you outstandingly brilliant contributors have any novel and
innovative ideas?????
Reply to
Geocacher
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The only ways I can see to "emulate" current draw is to either:
(a) actually draw it (resistor, incandescent bulb or similar, as you mention);
(b) modify the sense circuitry for greater sensitivity; or
(c) interpose a "box" that senses LED array current and fudges the pooter input.
If the current sense circuitry simply senses volt drop across a sense resistor, you'll need to increase it. If it uses a transformer approach with an open toroid, wind more turns on it. This assumes that the circuitry can be modified (legality, access, information availability).
Nope, only basics.
Reply to
budgie
Does the manufacturer offer an updated ROM for your model?
Cheers, Nicholas Sherlock
Reply to
Nicholas Sherlock
No. Manufacturers of the truck/tractors offer no assistance. They will not permit any tinkering with their computer systems - will void the warranty - and have not come up with any viable solution themselves!
Reply to
Geocacher
On Thu, 22 Sep 2005 14:02:39 +0200, " Geocacher" put finger to keyboard and composed:
You need some way of drawing the required amount of current for the duration of the pulse, and zero amps thereafter. What about a simple circuit based around a parallel PTC resistor, ie something like what happens inside a TV set during automatic degaussing at switch-on?
-- Franc Zabkar
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar
Franc Zabkar furiously typed the following on 23/09/2005 8:22 AM:
What about a feedback amp?
Reply to
Richard Waters
That would work for the pulse but would it work when the light was switched on (as in tail lights - the PTC would go high resistance and the computer would no longer detect enough current)? I think just a parallel resistor is going to be required. Certainly the least complexity solution. Or maybe, dare I say it, the light bulb.....
Ken
Reply to
Ken Taylor
how about, you make a circuit that AC-couples (use a capacitor) the brake line to a transistor that switches in a low impedance load (resistor) for a short time (e.g. 50ms). each time the brake line pulses high, the pulse turns on the transistor for a moment.
the constant "on" signal of the brakes won't trigger the circuit due to the AC coupling.
such a short on-time won't effect brakelight performance any, but will be long enough to allow the computer to 'sense' the higher load.
globes
Reply to
Craig Hart
Surely the obvious solution is an electrolytic capacitor with large valued parallel discharge resister and smaller series resister. When the current pulse is sent, the series resister determines the initial current draw. The parallel resister enables the cap to discharge for another cycle.
How big a capacitor you need is determined by the required current draw and the amount of time after switch-on that the current is sensed. The idea will only be feasible of the time is very short. Normal V=I*C*T rule applies.
Clifford Heath.
Reply to
Clifford Heath
globes
Is the current only checked during the pulse?
Ross
Reply to
Ross Marchant
Or use the charge time of the cap to turn off a PNP power transistor which would allow the use of very much smaller values.
Reply to
Ross Marchant
As far as we've been able to check this mess, yes, the current pulse is sent when the ignition is switched on, and will keep on pulsing until the load sense is satisfied. Vehicle diagnostics are performed (obviously, the lights are only one of the tests) and any errors are indicated on the dashboard. However - it would appear that the system has a continuous checking routine to determine whether a lamp has failed during the run. Sorry I sound so hazy over this, but trying to get a truck to play with is not easy. Suppliers are not too interested, and the operators are not keen on taking a truck out of service for us to investigate further. Sometimes, I hate my job!!!!!
Reply to
Geocacher
How long is the pulse? Is is a consistant duration? I would go for a somple circuit that switches in a load for a breaif period, perhaps half a second, then switches off.
Reply to
The Real Andy
This was a problem with LED traffic lights and the traffic controllers which sense failed lights.
Have you any idea how much current is required for the sensing circuit to be happy with the load? What kind of power rating is the normal truck incandescent globe?
Found any decent caches recently?
Reply to
swanny
The Brake and Indicator lamps are 24V 21W, the Park lights are 5W
Yes I have found some good caches. Placed some too!!! Are you also a geocacher?
Reply to
Geocacher
Maybe I am missing something here - it seems simple. Power transistor thru resister from active side of tail lamp supply to earth - resistor set so required current is drawn when transistor is biased on - transister base fed thru zener so conducts when pulse arrives. Pulse sent, transistor conducts, computer thinks happy thoughts.
David - or is this just too easy (and cheap - $5 worth of bits at the most)
Nicholas Sherlock wrote:
Reply to
quietguy
I don't think there is any legitimate way around this, for the following reason.
The whole idea of the computer check for blown lamps is to alert the operator to a safety issue.
If you use ANY system which "fudges" the computer into thinking that the led tail-lights are in fact incandescants (such as dummy loading, transistor current loads etc) then what happens if somebody actually disconnects the led lamps (try it out using a parallel incandescant bulb)? With the dummy load in place the computer will think the led tail lights are connected when in fact they are not. Such work-arounds are potentially hazardous and should not be contemplated. Either the manufacturer of the truck computer modifies his program to cater for led lamps or you should stick to using incandescant bulbs.
Ross
Reply to
Ross Herbert
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Well I'm glad that someone thought of this at last! :-) The simple resistor is probably the only method that would permit 'proper' operation of the sensor if you selected it to 'trick' the processor only when the LED lights were working. It'd be a trial-by-error job of selection, though.
Ken
Reply to
Ken Taylor
Hello
Is this a common problem or a one-off? You'd think the truck manufacturers would have a solution in software or an add-on module.
It would be possible to make a simple circuit using a mosfet and a few other small parts to apply a brief controlled load when 12/24V was first applied. A sort of simulation of a large capacitor. It would be convenient and more reliable if the circuit was potted into a small module that connected in parallel with the LED lamp; preferably at the LED lamp itself so that the wiring from the front of the truck was still protected. Modules could be preset (before potting) to correct for 5W, 21W etc. Field configurable modules would probably not be long-term reliable.
To get it right you need to know the test pulse duration, the check delay, the pass/fail window and wether the computer checks at other times. It may just read the current a short time after the circuit is energised.
I recall years ago we had to fake fuel injectors so that when a dual fuel engine was running on CNG it didn't bring up the check engine light on the dash. It was more difficult than expected because the engine computer monitored the inductance of the injectors.
Regards Paul
--
Regards
Paul
Reply to
PB
On Fri, 23 Sep 2005 10:04:18 +1000, Clifford Heath put finger to keyboard and composed:
The LED array may itself provide an appropriate discharge time constant.
That should be dV = I * dT / C
or C = I * dT /
dV
-- Franc Zabkar
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

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