LED bulbs - would this work?

Birchall

Birchall

Dick Chown Award 2016
Hi,

I have always wanted to fit LED bulbs for the stop and tail on the A2.

The problem is that the lack of current drain results in a "bulb out" warning on the tail lights and the instrument cluster warnings would light up like a Christmas tree when the car doesn't sense the current drain from the stop lights.

One way of resolving this is to fit the special resistors that "mimic" the current drain from the bulb.
The problem with that solution is that you have to fit four of them (one each for the stop and tail and the same the other side).
Breaking into the wiring on the rear cluster to achieve this is all too much messing for me and adds some additional failure points (especially since my standards of wiring would give Tom (Timmus) sleepless nights if he ever saw it!)

So could I do the following do you think?

Instead of having a separate resistor on the live side of each of the contacts of the stop and tail, could I half this by just having one resistor in the earth side of the circuit. I would obviously have to break into the earth feed, but that is simpler than breaking into the two live feeds (stop and tail)?

I am sure that the power of the two circuits (Stop and Tail) when both are illuminated (braking at night) would be ok for the resistor because the drain from the LEDs is so low?

My electrical skills only go as far as pure electrics and resistors, diodes and capacitors etc. are all "Electrickery" to me.

I am still hoping that someone can build me a "timed" flasher / hazard relay, so I can fit my LED bulbs into the indicators (again without resistors)

I am sure that anyone with basic electronics could do this, but it is way beyond me.

Steve B
 
It would be good if we could find the 'off' setting for the bulb warning in vcds, but it seems the A2 is not labelled up in vcds so its not that easy.

I wonder if the vag com guys will ever update it so that everything is labelled properly so that we can tick a box for settings.

Have you thought about buying some newer Audi rear LED lights off eBay and use the internals as a project?

Thats what im thinking of doing.
 
manhattan$ said:
It would be good if we could find the 'off' setting for the bulb warning in vcds, but it seems the A2 is not labelled up in vcds so its not that easy.

I wonder if the vag com guys will ever update it so that everything is labelled properly so that we can tick a box for settings.

Have you thought about buying some newer Audi rear LED lights off eBay and use the internals as a project?

Thats what im thinking of doing.
Click to expand...

HI,

It can be bypassed, if you retrofit DIS you get the bulb out warning constantly unless you earth out the right pin (or so I believe) so it can be bypassed, but that wouldn't help with the stop light. The brake switch sensor would throw up faults because it would sense the car slowing down without sensing that the brakes have been applied.
This is where you get the EMS light, the Brake warning and the pre-heater lights all lit on the instrument cluster. So even if you "switch off" the bulb out warnings, you still have the stop light problem.

Steve B
 
Hi,

the load resistor fitted to increase the current drain of an LED light would be normally fitted in parallel, that is across the light itself. That way the LED unit is provided by the full supply voltage, which it expects, with the resistor providing an additional drain to increasing the overall current passing though the circuit. That would mean that each LED in its simplest form would require a separate resistor as each light has a separate switched positive feed:

LED with parallel resistor.png

If a single resistor was fitted in series to the LED light, this would cause the voltage and current on the LED itself to be reduced. The LED may be dim or not light at all depending on the values chosen and how the LED light is driven electronically:

LED with series resistor.png

As most of the lights (indicators, side lights) have separate switched feeds anyway and thus would require separate resistors, the simplest way to fit them is probably is at the point of use, in the light itself.

Note that the circuits above are a simplification and the LED light unit itself shown as an single LED symbol would require additional components to limit the current flowing through the LED such as either a simple series fitted resistor or more complex power control circuilt.


What would be better would be to re-engineer the failure detection to reduce the threshold current such that the lower current draw of the LEDs is accepted as a valid working rather than fault state. Thus the light failure feature could be preserved but operate correctly with LEDs.

regards

Andrew
 
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Birchall said:
I am still hoping that someone can build me a "timed" flasher / hazard relay, so I can fit my LED bulbs into the indicators (again without resistors)
Click to expand...

Hi Steve,

Do we know if the A2 uses a Bi-Metallic Strip or resistance and capacitance circuit to time the indicator flash? If the latter, I may be able to help. :)

Regards,

Matt.
 
HI,

I am not sure and I wouldn't want to guess, has anyone got an old broken relay that they would like to donate for investigations?

Tom has repaired these so he would know.

Steve B
 
Hi Andrew,

Thank you for the detailed and very helpful reply.

My lack of understanding about "Electrickery" meant that I thought that wiring it in parallel would have the same effect as wiring it in series.


As for reducing the threshold current, the stop light check is different to the "bulb out" check, the tail light "bulb out" just gives a warning, the stop light check tricks the system into thinking there is a major brake fault.

Steve B
 
Hello again,

As Andrew has said, the best way to go about this, in my opinion, is to fit resistors at the light bulbs themselves. Yes, they have the disadvantage of the same current drain as filament bulbs, but they will work exactly as they should; hopefully without problems. Andrew is absolutely correct in that you will need the resistors 'across' the load, such that the current is split and not the voltage.

To determine the resistor values needed, you will need to calculate the following.

So, total resistance in parallel is calculated by (1/Rt) = (1/R1) + (1/R2). In short, the more resistance in parallel, the lower the overall resistance.

I assume standard indicator bulbs are 21W? If so, 21W at 12V = 1.75A. 1.75A at 12V gives a resistance of 6.857 Ohms. This resistance is the total resistance of each 21W fitting.

I looked at some cheap LED's on eBay, designed for indicator use, with a rating of 2.7W. I will base my calcs on this. 2.7W gives a current draw of 0.225A. This means an effective resistance of 53.333 Ohms.

Using the parallel resistance calculation above, this means a resistor of 7.869 Ohms is needed, in parallel with the bulb. They don't make a resistor of that exact value, but you can use smaller ones in series to achieve a value close to that, which will do the same.

Theres another added complication, being that you'll need resistors able to drop the effective wattage between the 2.7W bulb and 21W bulb. Most are quarter watt, and hence, not suitable.

7.869 Ohms means a current of 1.525A. This in turn, means a power of 18.3 Watts. A resistor above this power rating will be suitable.

Hope this is of some use. :)

Regards,

Matt.
 
wilco184 said:
Hope this is of some use. :)

Regards,

Matt.
Click to expand...

Hi Matt, it certainly is helpful (as you always are).

I don't like using resistors at all, it just seems like another thing to fail, especially when you have to wire them at the bulb, too fiddly for me.
Also it kinda removes the main advantage of the LED bulb (the low drain).

So I think I will pass on fitting resistors.

But an interesting subject!!

Steve B
 
And a 20W resistor of even a few ohms will be a monster wirewound type.

I was musing about this recently. And was thinking you could get a raspberry Pi and program it to do what you want it it to do. I'm pretty sure you could write some nifty program to fool the DIS and drive your LED bulbs at the same time. Bit of an expensive experiment though.

Chris
 
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KekseKaempfer said:
And a 20W resistor of even a few ohms will be a monster ceramic type.

I was musing about this recently. And was thinking you could get a raspberry Pi and program it to do what you want it it to do. I'm pretty sure you could write some nifty program to fool the DIS and drive your LED bulbs at the same time. Bit of an expensive experiment though.

Chris
Click to expand...

A raspberry Pi?

My wife made me a Rhubarb Pie once (it was 14 inches long and an inch wide!!!)

Couldn't "resist" that one !!!!

Steve B
 
wilco184 said:
Hi Steve,

Do we know if the A2 uses a Bi-Metallic Strip or resistance and capacitance circuit to time the indicator flash? If the latter, I may be able to help. :)

Regards,

Matt.
Click to expand...

No modern day relay uses a bi-metallic strip. You used to get those in 2 pin flasher relays that WOULD rely on load to heat up the strip and cause the disconnection. If it transpires that the A2 relay uses a bi-metallic strip i'll do a Paddy Ashdown and eat my hat. Modern day relays have a load sensing circuit in them that just needs to be fooled into thinking the load of the bulbs is OK.

They can be easily hacked to be compatible with LED bulbs.

Chris
 
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KekseKaempfer said:
No modern day relay uses a bi-metallic strip. You used to get those in 2 pin flasher relays that WOULD rely on load to heat up the strip and cause the disconnection. If it transpires that the A2 relay uses a bi-metallic strip i'll do a Paddy Ashdown and eat my hat. Modern day relays have a load sensing circuit in them that just needs to be fooled into thinking the load of the bulbs is OK.

They can be easily hacked to be compatible with LED bulbs.

Chris
Click to expand...

Hi Chris,

wouldn't a simple "no" have been suitable, Matt was merely asking, he has not had the benefit of years of car ownership, so some things that are obvious to others are not always obvious to everyone. I have heard of people stripping the A2 relay box and commenting on cleaning "points" and so even after my 45 years of car tinkering I could not be sure that there was no bimetal strip involved inside the "black box"?

Steve B
 
Birchall said:
Hi Chris,

wouldn't a simple "no" have been suitable, Matt was merely asking, he has not had the benefit of years of car ownership, so some things that are obvious to others are not always obvious to everyone. I have heard of people stripping the A2 relay box and commenting on cleaning "points" and so even after my 45 years of car tinkering I could not be sure that there was no bimetal strip involved inside the "black box"?

Steve B
Click to expand...

If it came across in any way offensive or abrupt then apologize wholeheartedly. It wasn't meant to be. I'd love to get my hands on a relay just to try and put this to bed once and for all. I have been looking for a cheap one on and off.
 
KekseKaempfer said:
If it came across in any way offensive or abrupt then apologize wholeheartedly. It wasn't meant to be. I'd love to get my hands on a relay just to try and put this to bed once and for all. I have been looking for a cheap one on and off.
Click to expand...

Hi Chris, I didn't see it as anything other than just the way it was phrased, I know you are not the type to offend in any way whatsoever.

In fact it was probably me that read something into it that wasn't there. I am "dying" with a "cold from hell" and so not myself tonight, so please accept my apologies too.

Thanks for your positive reaction.

Steve B
 
Hi,

No worries, Chris; no offence was taken. As Steve says, my experience of car electronics convention is somewhat limited, although my job involves electronics, and I was simply using two methods which are commonly used with what I do.

It would be great to see this one solved. I like the idea of using LED's, especially for the indicators because of the instant switch on.

Regards,

Matt.
 
Yes
a replacement relay with a timed (electronically) flash instead of one based on current drain woukd be great.
Mir would even be ok to use with standard bulbs I woukd imagine.
Mot may therefore be an ideal replacement when anyone's relay packs up. No moving parts would mean long term durability too.

I mi people were to send any old failed relays they could be converted to electrickery instead.

So so I believe that there is some value in pursuing the feasibility of this.
The added advantage being the ability to fit LED bulbs and so reduce the current consumed (handy if you breakdown late at night. (How many times have you seen a stranded car with the hazards on completely drain the battery in next to no time)
the lower current may also extend the life of the control box.
The instant illumination of the LED bulbs is cosmetic only but still nice.

Steve B
 
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