LED "driver"box for car LED lights

I own a Fiat Punto 12+. This car can not be fooled with canbus LED lights, other than using a resistor. But then the car will never detect a defect bulb anymore, untill the resistor will be defect to. So I am planning to build a box with some components to fool the canbus, but also with a faulty LED "detector" in it. So the plan is as following:

Form the light switch of the car, the cable will run in to the box. It will run thru a relay (the switchable side), then a resistor is placed and then it is grounded to the chassis.
That way, when the relay is in closed position, the car thinks there is a working light, so no canbus light warning. Before the relay, the wire will be tapped and connected to the LED light and the LED light is also connected to the ground. Then the hard part: I want to have someting to have the relay switch to the open position, as soon as the LED light is faulty.

So what I first need to know is: can I measure a faulty LED bulb, in my case it will be a BAU15s 5w LED bulb and if yes, what difference in voltage/amps/resistance will I meausure when a LED is faulty and it what ways can a LED be faulty?
Visual experience of my own have seen LED bulbs that just won't lightup/went out and LED bulbs that are flickering.

My father in law had the following idea:


BT1 will be the cars light switch.
Will this work, is there a better or other way to "open" the relay when the LED bulb is faulty or maybe a solution that will detect more faults?

I hope you can help me with this.

With kind regards,

Tinjo

I know it's common to call these CANbus lights, and "fooling the CANbus", but the communications bus on the car has nothing to do with the current sensing. The car might use the CANbus to carry the information about whether the lights are working, but loads of other information is on the CANbus as well. Historically, CANbus appears on cars at a similar point in time to bulb failure sensing, and they became thought-of as linked, which the're not.

Anyhow, rant over, and your scheme would most likely work. I have a few points to make.

I'm assuming that the LED in the circuit is actually the BAU15s 5w LED lamp, so it is rated to 12 V.

Most LED lights will take less current if they fail. However, many LED lights use many LEDs so the failure of one or some will still leave the lamp taking significant current.

If you are using a relay, you should put a diode or a resistor in parallel with the coil, to avoid the inductance of the coil generating huge voltage spikes as the transistor turns off. A diode is common, but that can slow the turning off of the relay, causing excessive contact arc at turn off. A resistor will result in more power consumption, and a larger voltage that the transistor has to withstand, but the relay will turn off faster.

You don't need the relay at all. Just put the big resistor in place of it's coil, and make sure the transistor is suitably rated. You will need a big transistor anyhow, as most of the LED lamp current will go through the base-emitter junction.

The resistor rating is marginal. It will dissipate around 21 W at 12 V, but around 29 W at 14 V, which is about what voltage you get with a car engine running. I guess that these will be direction indicators, so the average power will only be around 15 W, which isn't too bad, but the resistor will get really hot at that power. I suggest a 50W resistor. I have a similar concern about the 1.8 Ohm resistor.

You might want to put a capacitor in parallel with the 1.8 Ohm resistor. Some LED lights contain buck converters so the current taken has high frequency ripple. 10 uF or so here will help to keep the transistor from turning on and off rapidly.

You should check if the BAU15s 5w LED lamp has any resistors in parallel. Those are often called CANbus resistors. They should be removed, or if the LEDs themselves fail, the detection circuit would still be operated, and the blown bulb detection wouldn't work. I've seen some automotive LED lights where the resistors are in parallel with the whole circuit, and some where they are just in parallel with the LEDs. If there are resistors in parallel with the LEDs themselves, the current limiting circuit in the lamp be set for a suitable LED current, plus what the parallel resistor takes. If you remove the parallel resistor, more current will flow in the LEDs, which may damage them or make the light too bright, so you may need to adjust the current limiting circuit.

The resistors in parallel often result in the lamps running really hot, making the LEDs unreliable, which is another reason to remove them.

Some cars try to detect lights that are turned off. There are two methods of doing this, and both cause problems with LED lamps. One method is to put a few mA of current into the lights at all times, and to detect the voltage. With LED lamps, a few mA will often cause the LEDs to glow, and there will be a significant voltage across the LEDs, so the car thinks that the bulb is blown, while the lights glow all, or some of, the time. This is what the resistors in parallel with the LEDs are generally there to deal with.

The second method for detecting lights that are off is for cars to just turn the lights on to full power for around 1 ms, so too short a time for the filaments to get hot. This, of course, causes LED lights to flash. It's quite difficult to make a circuit to stop that happening.

Some cars will not turn lights on if the driver circuit hasn't already detected the bulb. If that is the case in your car, then your circuit won't work.

Thank you for your detailed reply. Yes I know that the CANbus is a lot more then for what I use the term in this thread, but as it is so common for people to call it CANbus lights and CANbus fooling, everyone will understand what I say haha.

For what I read and experience about the light warning system of my car, is that it only detects a broken bulb when the bulb is activated. If I start the car and leave all lights out, no warnings. If I indicate, then the warning comes up. After the warning, the warning light stays on, but the message will not return, until I use the car again, after it has been shutdown. The way it detects a broken bulb seems to be of the drawn current. However I did experienced, with the brake lights, that when both bulbs are detected as faulty, the lights will not turn on at all, but only after activating the brake lights, not initially.

What I meant with flashing LED's was that they are flickering fast and uncontrolled. I had that in another car. The LED bulbs worked fine for about a month and after a month they started flickering after a fews seconds/minutes, after they where turned on. After replacing the LED for a new one, it was fine again, so it had to be a broken bulb, but what causes those symptoms? Driver circuit faulty? LED's are to weak for the running voltage or something else?

I don't think it is possible to remove any resistors or other components within the LED bulb itself easy, without the risk of molesting the bulb. In almost all cases, the bulbs are glued/sealed. So I would like to use a circuit that can be used without modifiyng a bulb, because it should be easy to replace a faulty bulb.

You say that using a 50W resistor will be better, but for what I guess a 50W resistor will get a lot hotter than a 21W or is that wrong?

htevents said:

You say that using a 50W resistor will be better, but for what I guess a 50W resistor will get a lot hotter than a 21W or is that wrong?

Click to expand...

What I am saying is that running a 25 W resistor at 21 W will make the resistor very hot. Running a 50 W resistor at 21 W will make the resistor run at a lower temperature than the 25 W resistor at 21 W.

I was not suggesting changing the resistance of the resistor. It is the voltage and the resistance the affect the heat power that the resistor has to deal with. The power rating of the resistor is the maximum power that the resistor can handle without overheating.

If you run a resistor near its maximum power rating it will be really hot.

https://www.farnell.com/datasheets/1498243.pdf shows that with a heatsink, the 25 W resistor runs at about 80 °C above ambient at 21 W. The 50 W resistor isn't actually much better and it runs at about 60 °C above ambient at 21 W. You might want to go to a 75 W resistor so that it's not too hot to touch. The 50 W resistor is only rated at 20 W if it is used without a heatsink.

htevents said:

What I meant with flashing LED's was that they are flickering fast and uncontrolled. I had that in another car. The LED bulbs worked fine for about a month and after a month they started flickering after a fews seconds/minutes, after they where turned on. After replacing the LED for a new one, it was fine again, so it had to be a broken bulb, but what causes those symptoms? Driver circuit faulty? LED's are to weak for the running voltage or something else?

I don't think it is possible to remove any resistors or other components within the LED bulb itself easy, without the risk of molesting the bulb. In almost all cases, the bulbs are glued/sealed. So I would like to use a circuit that can be used without modifiyng a bulb, because it should be easy to replace a faulty bulb.

Click to expand...

I suspect that the resistors fitted to the bulbs are what is causing the failure. Most electronics, including LEDs, will have a shorter lifetime if run hot. The LED lamp manufacturers put resistors in parallel to fool the car detection circuits, but the heat generated by the lamp is then too much and the LEDs degrade quickly.

Many indicator circuits are designed to flash twice as fast if one bulb is blown. On method is to measure the overall current that the indicators on one side of the car takes, and the circuits expect the current taken by 2 x 21W incandescent bulbs. Therefore the LED lamps are going to have to take around 1.5 A so that two of them will take enough current for the car to think that the lights are OK.

The problem is that the LED lamp will be far too hot if 1.5 A is consumed and the indicator is on for more than a few flashes.

I have removed the parallel resistors from LED lamps to reduce the operating temperature. I agree that it can be difficult, and there are many different constructions of LED lamp.

It has been a rule for a long time that cars will tell the driver if an indicator has stopped working. That has resulted in the current detection, which has resulted in LED lamps being run far too hot, which has resulted in unreliable lamps. So the rule has resulted in exactly the opposite of what was intended, after the technology has moved from incandescent bulbs to LEDs.

Diver300 said:

What I am saying is that running a 25 W resistor at 21 W will make the resistor very hot. Running a 50 W resistor at 21 W will make the resistor run at a lower temperature than the 25 W resistor at 21 W.

I was not suggesting changing the resistance of the resistor. It is the voltage and the resistance the affect the heat power that the resistor has to deal with. The power rating of the resistor is the maximum power that the resistor can handle without overheating.

If you run a resistor near its maximum power rating it will be really hot.

https://www.farnell.com/datasheets/1498243.pdf shows that with a heatsink, the 25 W resistor runs at about 80 °C above ambient at 21 W. The 50 W resistor isn't actually much better and it runs at about 60 °C above ambient at 21 W. You might want to go to a 75 W resistor so that it's not too hot to touch. The 50 W resistor is only rated at 20 W if it is used without a heatsink.

Click to expand...

Sorry, had misunderstood the power rating change. You are right.

Diver300 said:

I suspect that the resistors fitted to the bulbs are what is causing the failure. Most electronics, including LEDs, will have a shorter lifetime if run hot. The LED lamp manufacturers put resistors in parallel to fool the car detection circuits, but the heat generated by the lamp is then too much and the LEDs degrade quickly.

Many indicator circuits are designed to flash twice as fast if one bulb is blown. On method is to measure the overall current that the indicators on one side of the car takes, and the circuits expect the current taken by 2 x 21W incandescent bulbs. Therefore the LED lamps are going to have to take around 1.5 A so that two of them will take enough current for the car to think that the lights are OK.

The problem is that the LED lamp will be far too hot if 1.5 A is consumed and the indicator is on for more than a few flashes.

I have removed the parallel resistors from LED lamps to reduce the operating temperature. I agree that it can be difficult, and there are many different constructions of LED lamp.

It has been a rule for a long time that cars will tell the driver if an indicator has stopped working. That has resulted in the current detection, which has resulted in LED lamps being run far too hot, which has resulted in unreliable lamps. So the rule has resulted in exactly the opposite of what was intended, after the technology has moved from incandescent bulbs to LEDs.

Click to expand...

My dash indicator lights are flashing twice as fast indeed, but the exterior indicator light are still flashing at the normal tempo, but I only had the rear indicators changed for LED, maybe if I also change the fronts to LED, all the indicators will flash faster.

When you removed the parallel resistors, how did you managed to get control the current flow into the LED's?

Diver300 said:

I'm assuming that the LED in the circuit is actually the BAU15s 5w LED lamp, so it is rated to 12 V.

You should check if the BAU15s 5w LED lamp has any resistors in parallel. Those are often called CANbus resistors. They should be removed, or if the LEDs themselves fail, the detection circuit would still be operated, and the blown bulb detection wouldn't work.

The resistors in parallel often result in the lamps running really hot, making the LEDs unreliable, which is another reason to remove them.

Some cars will not turn lights on if the driver circuit hasn't already detected the bulb. If that is the case in your car, then your circuit won't work.

Click to expand...

I would rather not have to modificate the LED bulb itself, because it should be a "foolproof" operation, like: "Faulty bulb? Just replce it for a new one", but I understand you say that the "fail detection" will not work properly?
And even if there is a resistor in parallel over the LED's or whole circuit, wouldn't there still be a measurable difference in current flow, voltage or resistance when the LED's themself fail, so a "fail detector" can still detect a failing bulb or would that make the "fail detector" too complex?

I still do not know what fails are the most common with most car LED bulbs and what the measurable difference could be between a working and a faulty one. Or is this also too complex to explain or detect with a relatively simple system and will need an IC or Arduino type circuit?

About cars not turning faulty lights on at all, I think I experienced that with the brake lights. When I exchanged the bulbs for LED bulbs, set the car on ignition, no warning, than when hitting the brake, I get the warning and the LED bulbs stay off, only the third brake light did work, but that one is not monitored. I dropped this question at a Fiat forum, so maybe something else happened. I do have to test it with only one bulb exchanged for LED and the other one just the standard bulb, if then both lights will light, I can be for shure that the car won't turn on the lights if "all" the lights of that section are faulty.

If this is the case, is there any other way to build a circuit that will work?

P.S.: I see that a lot of (chinese) Car LED bulbs do have IC drivers, but I don't see any resistor.

l

htevents said:

My dash indicator lights are flashing twice as fast indeed, but the exterior indicator light are still flashing at the normal tempo, but I only had the rear indicators changed for LED, maybe if I also change the fronts to LED, all the indicators will flash faster.

Click to expand...

The car will respond to one indicator not working, because the rules says it has to. That doesn't have a lot to do with how it detects that one indicator isn't working. It might detect each light, or it might detect the total current.

You could try a pair of 10 W incandescent bulbs. If the car sees them as OK, then it is probable that the car is detecting each end separately. If it doesn't, it's probably looking at the overall current.

When cars had flasher units, the usual method was measuring the overall current and increasing the flash rate if the current was too low. There was no possibility of having a different flash rate on the dash light.

Clear. Will test some more with incandescent bulbs with lower wattage and see if and what warning come up.

I hope someone can still help me with an answer on the measurable difference of a faulty LED bulb. If a bulb fails, wouldn't there be a measurable difference in the drawn current, used voltage or in resistance, so we can build a "detection" system for that?

htevents said:

When you removed the parallel resistors, how did you managed to get control the current flow into the LED's?

Click to expand...

LED lights will have some current control device, which may be a resistor, and IC, or a combination of both. An LED light that is designed to fool current sensing devices will have one or more additional resistors, and it is those resistors that I have removed. The current control device is unchanged.

In some cases, the circuit is arranged so that removing the additional resistors causes extra current to flow in the LEDs, so I have adjusted the current control device to cut down the overall current.

htevents said:

I hope someone can still help me with an answer on the measurable difference of a faulty LED bulb. If a bulb fails, wouldn't there be a measurable difference in the drawn current, used voltage or in resistance, so we can build a "detection" system for that?

Click to expand...

There are so many varieties of LED lights, and so many components in each one, that it's not possible to be specific, but usually the current will drop if the light fails.

The LED lights that have parallel resistors to fool the current sense devices will often still take a lot of current if the LEDs fail.

Again, thank you for your fast reply. I see the complexity here.
If I sum it up simplified: Take a LED bulb without a "canbus" resistor. If it fails with open circuit, it can be dected, if it fails with short circuit, it can be detecten and is the LED's themselfs fail, but the driver stays up, we can detect a current drop. Am I right?

htevents said:

Again, thank you for your fast reply. I see the complexity here.
If I sum it up simplified: Take a LED bulb without a "canbus" resistor. If it fails with open circuit, it can be dected, if it fails with short circuit, it can be detecten and is the LED's themselfs fail, but the driver stays up, we can detect a current drop. Am I right?

Click to expand...

I think that's about right, but there could be rare partial failures where the current doesn't change much.

Diver300 said:

I think that's about right, but there could be rare partial failures where the current doesn't change much.

Click to expand...

Well, there is always someting as visual inspection, but this project is pretty interesting, so I tought I would try this. You where great help
The circuit is now updated to this:
I think we are getting closer.

I don't know how much current the LED lamps will take, but the 1.8 Ohm resistor might need to be changed for a larger value, as with 1.8 Ohm it will take about 350 mA before the transistor turns on, and many LED lights will take less than that.

The 6 Ohm resistor will give well over 2 A at 14 V. As the total current will include the 350 mA, you will get quite a bit more current than a 21 W incandescent light. I think that the 6.8 Ohm or 8.2 Ohm would be fine.

The power rating of the used LED bulbs are 5W. They work between 9-18V DC.

Diver300 said:

I don't know how much current the LED lamps will take, but the 1.8 Ohm resistor might need to be changed for a larger value, as with 1.8 Ohm it will take about 350 mA before the transistor turns on, and many LED lights will take less than that.

Click to expand...

Possibly this is the answer on detecting a faulty LED bulb by a current drop. If R2 is adjustable, maybe with a selector switch, it can switch off Q1 when the current is below the threshold. I can select what power of bulb I am using if I decide to use another LED bulb. The current LED bulbs are 5w each, if I decide to use 10W bulbs, I can switch to another R2 resistance, so the threshold will change.
Maybe I have to power the LED bulbs by a separate power line, with a 12V regulator in front, so we can have more consistant values?

It would probably be a good idea to measure the current that one LED lamp takes, with and without the engine running.

Some LED lamps take more current at lower voltages.

Will do that tomorrow and will report back. Also I think I have to keep in mind that R2 will take a bit of voltage I guess, so the LED bulbs can be a bit dimmer with this setup. Am I correct?

Maybe. It depends on the current limiting circuitry inside the LED lamp. Some will be slightly dimmer with the extra 0.7V drop, but some won't.

All tested and have found the right value for R2 so when a 5W LED lamp takes less then 2.5 Watt, Q1 will switch off the power to R1 and the car will detect a "broken bulb". With this I have 3 detection points. Shots circuit, open circuit and a dimmed LED caused by failed/burned LED's.

Thank you for all the help. I really appreciate it.