E38 High level brake light - replaced with LED now minor problem.....

Hi Folks
BMW E38 V8 740i RHD
The High level brake lamp as fitted to later models - 1996> used a regular 21W incandecent bulb.
However the earlier models did not have this fitted and mine came with an
aftermarket version with 6 x 3W bulbs. When this decided to die on me I bought exactly the same unit but the designe had been changed to use LED instead of incandescent bulbs.
Problem - the OBC unit checks all bulbs every 5 seconds by sending a low voltage pulse through the system. The incandescent bulbs do not react to this and the resistance is too high but the OBC knows that all filaments are in circuit.
THe replacement LED unit is wired exactly the same as the old unit but it flashes every 5 seconds due to the OBC check.
Any ideas how to eliminate the flashing and keep the LED unit (Looks nice)
Hugh
Sir Hugh of Bognor
The difference between men and boys is the price of their toys. Intelligence is not knowing the answer but knowing where and how to find it!
Hugh Gundersen snipped-for-privacy@h-gee.co.uk Bognor Regis, W.Sussex, England, UK
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snipped-for-privacy@h-gee.co.uk wrote:

I think you could wire an R-C circuit with a time constant of 100 ms or so in parallel with the light but the faulty bulb warning light (if your car is so equipped) would probably be permanently illuminated. That condition would be better than spooking (and then desensitizing) people behind you, though.
Note that you would be sacrificing the desirable property of LEDs that they light up more or less instantly, giving the people behind you more notice of an emergency stop.
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I think the RC circuit is the right solution to this problem.
What you need to do is to add an electrolytic capacitor across the terminals of the LED. This will cause a slight delay when the circuit is energized and will eliminate the flashing if the capacitor is large enough. If you know someone with a scope you could find out what the actual pulse width is but failing that I think that a reasonable delay to try would be 10ms and normally this would required a capacitor value of C=T/R or .010 seconds divided by the ballast resistor value which might get you a capacitor spec of 500 microfarad. Unfortunately, the LED will light up when the capacitor voltage gets up to .6 volts so we need to increase the capacitor spec by a factor of 15 to get the 10ms delay. This would require a 7500 mfd capacitor at 25 volts working voltage. These are pretty bulky and hard to come by so I think I would try a 4700mfd at 25 volts as a first attempt. These are about 3/4in in diameter and 1 1/2 in long so there's some hope that six of them might fit in your housing. I think you should be able to get them at Radio Shack or failing that at Digi-Key for sure. Don't skimp on the working voltage. Be very sure that the positive terminal of the electrolytic capacitor is connected to the positive side of the LED. If you hook it up backward, the capacitor will short out. Because we don't know the actual value of the ballast resister and the required capacitor value is uncertain, I would just try it on one LED at first to see if it works. If the test LED still flashes put another capacitor in parallel with the first one. If you know the actual value of the ballast resister let us know and we can be more specific with the fix.
Good Luck
wrote:

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Have you thought of adding a resistor in line so the LED looks like a light bulb to the computer?
HINT An LED has virtually no resistance and the light bulb assembly has something like 240 ohms of total resistance.
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The LED may be low resistance, but couple it across a 12 volt supply and it will die instantly. So you need a current limiting resistor wired in series. For car use and most single LEDs this might be around 330 ohms.
A bulb with a resistance of 240 ohms would be approx 0.6 watt at 12 volts.
The centre brake light on my E39 is 21 watts, so the resistance when on will be approx 7 ohms - less when cold.
I've got Hella aftermarket LED taillights on my E39, and there are two rather large units which fit under the spare wheel to compensate - for the warning device. I dunno how they work, but suspect there's a bit more to them than just a resistor.
--
*Monday is an awful way to spend 1/7th of your life *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

What's the math on that Dave?
I thought that volts X watts = resistance. If there is a bulb that measures 21w or 6 bulbs that measure 3w each or 18w in total, then we have approx. 20w X 12v = 240 ohms.
I understand what you are saying, but I used the wrong math. We are both saying the same thing, but one of us is saying it wrong. I suspect it is me ...
You know, there is going to be considerable heat generated on said resistor, it will need to be one of those ceramic type I'd guess.

I suspect you're right on that, but I see no reason one can't add a resistor to the center light that lives in the back window. I suspect the tail light assemblies that go into the rear corners of your car get more stuff to support the LEDs than the center light demands, which explains why you have a box in the spare tire well.
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wrote:

A hot bulb has a resistance given by Ohm's law. The resistance of a cold bulb is actually very low.
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wrote:

I understand that, but Ohm's Law says that resistance can be calculated if one knows the voltage and power. This calculatiion should be the same without regard to heat. Since heat will destroy the load resistor that we are talking about, then one must select a rexsistor that can handle the heat that will be generated in the circuit, but the resistance of the circuit will remain the same.
The issue we are trying to solve is, what is the resistance one might have to add to a circuit that is now running LEDs as a load where a lightbulb (or series of light bulbs) is the designed load for the circuit? We know that the OP has either a 21w bulb or six 3w bulbs, that have been replaced by an LED assembly. The circuit monitor is looking for a load that no longer exists, and hense is detecting the no-load condition as an error. The challenge is to add a load resistor that looks to the monitor like the original bulbs looked.
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Jeff
Can I stop you there.
Nothing wrong with the circuit monitor. No fault is shown and everything is tickety boo. It's ONLY the 6 x LED lamps flash momentarily every 5 seconds as the OBC unit check the "bulbs".
I had thought of using a 741 Op-Amp to output a negative voltage to the LED unit (0v) and biasing the input to see 9V on the negative input so when the brakes are applied in reality the 12V feed will override the 0V and turn it high triggering a darlington pair to feed the LED unit.
Seems reasonable but where do I pick up a live feed and would I need to run the + side from the brake light feed in all cases so the board is only powered when the brake circuit is in operation.
there must be a simpler device. Another way might be a rather large capacitor - say about 2000uF @ 24V ?
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wrote:

Okay, let's back up a step or two.
Everything with the monitor is good, the trouble is that the LED Center Brake Light flashes when the monitor makes its check?
I thought the problem was a false report of brake light failure.
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Volts = Amps x Resistance
Watts / Volts = Amps
Amps = volts / resistance
resistance = volts / amps
watts = volts x amps
Hugh

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V No - it's R = ----- W

The problem is space - there's only just enough room for the bulb, so it would have to be external.
--
*White with a hint of M42*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:
Hi Dave and all
Problem fixed but first thanks for the help with ideas but we must remember that incandescent light bulbs do not obey Ohm's Law as they get hot.
Cure involved a minor adaptation of a simple Darlington pair I use with my year 7 (11 years old) pupils as a moisture sensor.
Instead of the base resistor being used as part of the input I have used a potential divider feeding the base of a BC238. This is made up of a 56K 1/4W resistor and a 10uF capacitor. The Darlington pair is made up from a BC328 and a BFY51 transistors dropping the LEDs to 0V or chassis earth. The feed is taken from the brake light circuit which also feeds the LEDs.
The time is delay is about 10ms but just enough to stop the lamps flashing every 5 seconds. The full on braking display delay is not noticeable.
Circuit available if anyone wants it.
Hugh

Bognor Regis, W.Sussex, England, UK
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wrote:

Hi Dave
I thought it wouldn't be that easy ----------------- unless it's a f***ing big resistor!
Must be some way.................
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Been there done that - I teach electronics to 11 - 16 year olds and have thought of most things. 21W bulb has a current draw of about 1.7A and about 7 ohms resistance. 6 x 0.02A = 0.12A current so the resistance is around 100 ohms in theory.
Taking theory out of the equation the LEDs actually short circuit the system and light up because of the 100 ohm resistance or thereabouts and the incandescent bulbs just let the current pass.
I am not shutting you down just putting the theory to the test. I have stuffed a 10 ohm resistor in series and even tried it in parallel but still it flashes
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I was trying to defeat the error message on the OBC, b ut your trouble is different. The bulbs do not respond to the relative low signal that comes from the monitor, but the LEDs do and the result is constantly flashing center brake light. I would think this would take some sort of RC device that would soak the monitor signal before the LEDs, then discharge almost immediately to ground so the LEDs never light until the brake pedal is depressed and then you need a diode that blocks the relative high voltage that is normal for the brake light circuit.
As much as I understand your facination with the LED center light, I can't think of a way to defeat the false signal but allow the desired signal to pass. And, since you teach this stuff and can't figure it out either, me thinks you need to get light bulbs again.
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Bognor Regis, W.Sussex, England, UK
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On Sun, 15 Oct 2006 15:12:44 +0100, snipped-for-privacy@h-gee.co.uk wrote:

Hi Folks
Fixed it.
Method - 56K resistor feeding a 10uF capacitor. Centre take off feeding a 2K2 resistor to bias a BC238 transistor and a BFY 51 transistor arranged as a darlington driver.
Power supply is from brake light circuit and also feeds LEDs + . LEDs - (neg) is connected to the collector of the Darlington pair and the 0V line or negative is fixed to chassis ground.
If anyone else is interested I can let you have the circuit diagram and list of parts.
Alternatively I can let you have a kit of parts or ready made unit for a small payment.
Sir Hugh of Bognor
The difference between men and boys is the price of their toys. Intelligence is not knowing the answer but knowing where and how to find it!
Hugh Gundersen snipped-for-privacy@h-gee.co.uk Bognor Regis, W.Sussex, England, UK
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