LED Advice Please

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Mad Professor

Senior Member
Good Day To You All.

I am looking for advice with regards to LED’s.

But 1st I better let you know what it is I am trying to do.

More and more new cars on the market use LED based rear lights, for side lights, brake lights, ect.
My car is 20years old now, so still uses incandescent light bulbs.
I would like to remove the old incandescent light bulbs and replace them with LED’s.

The problem I have is that LED brightness is given in lux, mcd, lumens, not in watts

From my understanding lumens is the right way of finding out the brightness of a LED or Bulb, ect.

Most of the web sites I have looked at so far list the LED brightness in mcd and not in lumens.

All the bulbs in the rear lights of my car are clear, as the lens (cover) of the lights is frosted with the colour needed, So I will only need wight LED's.

I know a lot of the users here don’t like giving out any advice when it comes to Main’s Power related or Automotive Related questions.

I am not going to be trying to change the front main lights to LEDs, but they are being up graded to HID lights.

I am just trying to feel like I am bringing my poor old car into the new age.

So my main question here is: Are there any charts or conversions so that I can find out what brightness led would be compaible of replacing say a 5watt & 21watt incandescent light bulbs?

Thanks for your time.

Best Regards.
 
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moxhamj

New Member
I did this on one brake light about 4 years ago. I used a cluster of about 20 leds and it was most useful to just do one side, and then stand behind the car and compare the brightness with the old globes. The leds were/are dimmer, which is a negative, not so dim as to upset the police, which is a positive, and the bulb on the other side has blown twice since then but the led side is still functioning, which is a positive. One could certainly argue that a dimmer brake light is better than no brake light at all.

I think leds have got better in the meantime. Maybe just go and buy some and see how they compare.
 
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Dippy

Moderator
Mad P,

Let me know how you get on as i'd like to do something similar.

May I just make a couple of observations and suggestions.

1. Many (all?) car manufs PWM the LEDs for side-lights and put them full-on for brakes. If you go this route please PWM them fast as some people (including me) get upset by flicker.

2. Side lights can be on for many hours. PWMing them means they can have a low duty and gives them a greater chance to dissipate heat. Heat is ultimately the cause of premature LED failure.

3. If you PWM them make sure no PWM gets upstream of the light fitting. Many/all new cars use the loom for coded signals for other things so interfernece is a no-no.

4. If you live in the UK one of the favourite MOT failures is 'being able to see white light' from the rear light cluster - it allows MOT garages to flog you another bulb. So use coloured LEDs.

5. Brightness. Like Drac says: buy some and try them out. It really doesn't matter what a Data Sheet says, it's what they look like in your application as your rear light lens will diffuse the light. And it will be diffused differently depending on the angle spec of the LED.

I'll leave others to go into doom&gloom mode about car electrics and regulations etc.....

mcds/lumens... pain in neck. Hi power white LEDs are usually given in lumens e.g. those Golden Dragon things or whatever they're called.
Go for it.
 
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Mad Professor

Senior Member
Dippy: Thanks for your reply.

I was going to use two set's of LEDs in the light cluster for 5w and 21w, using PWM on the brighter LEDs seems like a much better way about going down this road.

I know heat is the big killer of LED's I have always used higher current limting resistors in my projects in the past to get the max life out of LED's.

I guess by the PWM interfernece, you are on about newer cars that use a CAN-BUS system.
My car is a Peugeot 205 built in 1988 so very limited, PWM interfernece will not be a problem.

I do indeed live in the UK well Isle Of Wight, all my bulbs in my light cluster are clear / white, so white LEDs should not be a problem there.

As for brightness I guess I will have to order a few LEDs from rswww.com and try them.
 
hippy

hippy

Technical Support
Staff member
Not that you shouldn't do what you are suggesting but the easy option seems to be to buy ready made LED clusters designed for the job. Nowhere near as expensive as they were a few years ago and if you buy the right ones they' will have whatever marking on them which proves to you, MoT testers and police that they are perfctly okay for use.
 
hippy

hippy

Technical Support
Staff member
One thing to bare in mind is that while LED lights which are too dim are a hazard, lights which are too bright won't do you any favours and can be equally dangerous. I'm not sure what any laws say about brightness ranges.

OTOH, it is nice to have a super-bright rear window brake light which can blind the %$@&* who's been tail-gating when you stop in a traffic queue. Deep breath, pull-over, let them pass probably gives a longer stress-free life though.
 
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Dippy

Moderator
hippy: "and if you buy the right ones they' will have whatever marking on them which proves to you, MoT testers and police that they are perfctly okay for use"

- I have never seen the "right ones" anywhere. The last time I looked in places like Halfords and the Maplin catalogue they said "Not legal for use on public Highways" - things may have changed.

But, let's be honest; if they look OK then no-one is going to open up the thingy and have a look are they. If you feel like panicking you could buy a light meter. Blimey, this isn't Rocket Science it it??

But, frankly, manufs get away with all sorts. Check out the VW Golfs circular rear lights; side/brake outside and inidcator in the middle. Dark rainy night, brakes on and indicating, you can hardly see the indicator at distance. Poor design, but VW wrote a big cheque out to the correct authorities and got they're E authorisation, job done.

The point I was making about coloured LEDs was: if your lens colour fades (and you have white LEDs) then white light will be visible and your car will fail the MOT. If you use coloured LEDs then that is impossible. What's the big deal with wanting to use white anyway?? Is there a reason? Have you got a job lot of cheapies from Ebay or something?
 
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kevrus

New Member
I would use the same colour LED as the lens that it sits behind, my thinking is that for example, a red lens would allow most of the light through from a red LED, but block more light from a white LED as it is emitting all the spectrum colours...just my thoughts.
The biggest give-away that LEDs are fitted is the near-instantaneous reaction times in comparison with filament lamps. This could obviously be addressed but opens up another debate on reaction times and sfety etc...
 
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Mad Professor

Senior Member
Dippy: I don't have any problem fitting coloured leds, I just prefer white ones, don't ask me why.

Heres what my rear light lens look like.


 
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Dippy

Moderator
"I just prefer white ones, don't ask me why"

- I won't ask. A man's fettish is his own business ;)
Do you have leather seats?

Just seems odd and kevrus makes a good point re colours/lens colours. But,
instant on/off is no big prob is it? Now that thousands/millions of LED-shod cars are kicking about.

Next years cars will have ramped PWM so they look like quaint old bulbs. Ah, ain't that sweet.

I'd be more worried about cracked/faded lenses .... MOTs.... and a greasy monkey opening it up when white light is seen.

Anyway, carry on. Suck it and see. I'm off.
 
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Michael 2727

Senior Member
Jaycar here in OZ have a range of drop in LED replacements for several auto lamps.
But if you read the fineprint they say "Only suitable for Show or Off Road use"
LEDs are dimmer in many cases but they are getting better day by day.
Jaycar may only be covering their own butt legally with the note, but I suspect it's more
likely with the advances in technology that the RTA here have not made any ruling as
to what is legal or what is not as far LEDs go.( they can be a little behind the times )

They have been using LED clusters in Heavy Vehicles on OZ for many years but you
will notice that there can be up to 80 to 100 LED LAMPS in the cluster, what you may
not have noticed that each LAMP has up to 8 or 10 individual LEDs inside it. Lotta LEDs.
(5 to 10 yr old technology above)

The only real way to check would be the side by side comparison test for visibility
even then you may have to check with your local Road or Traffic athourity and or
your Auto Insurance company to see if you are still covered.
( you know what they can be like, ANY excuse to void a policy works for them )

I'm sure in the near future auto manufacturers will start selling approved LED
replacement Lamps for vehicles ( I'd hate to be doing the paperwork worldwide for that )

And there will probably come a day when some LEDs will becme illegal to use on
vehicles because they are too bright.
Nothing like a 3W Red Luxeon tail light smacking you in the retina to mess up your day :)
 
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Eclectica

Member
Just wonder if it might be worth mentioning about wavelength and brightness...:rolleyes:

5mm red LEDs always used to be 660nm, but the human eye perceives 625-635nm as over 3 times as bright! [hence 660nm red LED development has fallen by the wayside and 635nm red is now 'normal' and adopted as red by the 'power LEDs'.]

Power LEDs are great, but as has been said here they need a heck of a lot of cooling/heatsinking if they are going to last.

I am currently building a 36W red/white/blue LED lamp which is fan cooled (based on a 100mm x 100mm PC processor cooler TR2-R1) each LED is 1W rather than 3W so as to spread the point sources of heat as much as possible and keep junction temperatures low (also 1W are still more efficient on a lumen per £ basis).

The choice of LED driver and its ability to handle more than one LED junction is a serious consideration. We built a unit with 10 LED junctions and using a certain nameless constant current LED driver within its specification but the switched output was only delivering 60% of its rated output when measured on the scope. When we reduced it to 9 LED junctions this went up to 70% and visibly brighter. With 1 LED it was 99%! It worked better doubling the current and having the LEDs in // as 2 strings of 5.

Also it would be worth putting it on a separate fused circuit (may be part of MOT or other regulation these days?). I'd be tempted to put a load of 15V transorbs around the LED driver supply to limit the effect of spikes (which are a feature of most car electrics) as these transients often find their way through the LED drivers and give the LEDs a pasting!

Lastly Rapid electronics sell a wide range of power LEDs and competitive prices - compared to RS that is.;)

All the best with your venture.

Regards
M.
 
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westaust55

Moderator
Also . . . . don't forget losses.
The basic old incandecent lamp is not very efficient. 95% of the power goes into heat and just 5% becomes light.

So in a 21 Watt bulb (std Brake light here in Oz) only 1 Watt is effectively converted to light. They also use clear globes - no coloured glass.

As such a 1 Watt Super LED being more efficient, should be a lot closer to the same light output as the 21 Watt bulb than the Watt rating along may suggest.
 
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papaof2

Senior Member
But also remember that the incandescent bulb has a wide illumination angle, which can only be duplicated in an LED by having multiple LEDs pointing at various angles. The single 1W LED typically has a narrow angle of illumination.

John
 
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Mad Professor

Senior Member
I have ordered a few samples of white and coloured LED's from rswww.com

If I get my order today, and if I get some free time over the weekend, I will be trying out the LED's.

I will let you know how I get on.
 
G

gengis

New Member
I did convert my motorcycle stop light to LEDs. My first attempt was to use 20 small high brightness leds - in one side of the two lamp brake light - eventually a cop stopped me to tell me I had a light go out - it was working but not well.

The problem I had was the lens had a crosshatch retro reflector built into the part of the lens that the lamp sits directly behind. The reflector is supposed to carry the light around it and the LEDs with their 6 degree beam spread were wasted effort.

The solution for me was to spring for 4 - one watt leds and mount them to an aluminum plate (heatsink) I cut to fit the housing - with the LEDs spaced behind the clear (non retro reflector) parts of the lens. Now the lamp is brighter than the original incandescents and uses lots less power. I use 60 ma for the tail lamp and 600 for the stop light - original 1157 two filament incandescent bulbs. The leds set me back $8 each back in 2004, but were worth it, since the charging system was maxed out on that bike.

The one watt LEDs have a 120 degree beam spread.

Talking about luminous intensity becomes meaningless. The whole assembly has to put the light where you want it with the proper beam angle.
 
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westaust55

Moderator
Dippy said:
hippy: "and if you buy the right ones they' will have whatever marking on them which proves to you, MoT testers and police that they are perfctly okay for use"

- I have never seen the "right ones" anywhere. The last time I looked in places like Halfords and the Maplin catalogue they said "Not legal for use on public Highways" - things may have changed.
Click to expand...

And besides brightness another possible reason many LED based lmaps are "Not for Highway Use" is due the the EMI limitations for the automotive industry and the use of switching mode supplies within some LED lamps.
Where efficiency is not an issue 9and when is it not these days), then linear regulators can be used.
 
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westaust55

Moderator
Mad Professor said:
I am looking for advice with regards to LED’s.

The problem I have is that LED brightness is given in lux, mcd, lumens, not in watts

From my understanding lumens is the right way of finding out the brightness of a LED or Bulb, ect.
Click to expand...

In simple terms, a lamp with an output of 1 candela distributing light equally in all directions creates a light level of 1 lux (1 lumen/m2) over an area of 1 square metre at a distance of 1 metre.
 
M

Michael 2727

Senior Member
Quote: Originally Posted by Mad Professor

I am looking for advice with regards to LED’s.

The problem I have is that LED brightness is given in lux, mcd, lumens, not in watts

From my understanding lumens is the right way of finding out the brightness of a LED or Bulb, ect.


Quote: Originally Posted by westaus55 -
In simple terms, a lamp with an output of 1 candela distributing light equally in all directions creates a light level of 1 lux (1 lumen/m2) over an area of 1 square metre at a distance of 1 metre.
Click to expand...
This is where the problem lies - An incandescent lamp usually radiates light in all directions.
LEDs can have a narrow beam (typ) 7º Deg to 30º Deg and up to 120º Deg for Luxeon style
LEDs
 
D

Dippy

Moderator
All important stuff of course.
Bottom line:
1. Blow the theory. Just start off with the brightest LEDs you can buy.
2. Use some common sense re beam spread. You will have to experiment anyway based on LED-cluster lens distance.
3. Buy some and try them.
4. Make sure your circuit doesn't send noise upstream. Though if it upsets adar uns that will be a bonus.
5. If you are worried, get a cheap light meter and compare.
You will have to experiment anyway, so you're going to waste some time and money. Goes with territory.

I really doubt if switched mode are used in rear lights. Maybe a linear reg + PWM control.
Personally, I'd just use transient suppression + diode + linear reg + PIC + FET drivers.
Or maybe forget the regulator & PIC and get some kind of 555 type thing or other astable which can easily stand 15V.

For LED arrays you can easily parallel up some seriesed LEDs so that any R component is minimised to reduce wastage. This also provides some backup should a single LED fail.

Does this really need such Nesbitian levels of thought? I'll be back in a few weeks, I hope its still being discussed as it is so complex :)
 
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Eclectica

Member
Maxims suggestions of LED drivers for automotive use include a mixture of linear and switchmode drivers: http://www.maxim-ic.com/solutions/hb_led_drivers/parts.mvp/scpk/1996/pl_pk/0

The usual trouble is getting hold of them in small numbers :rolleyes:.

With switchmode drivers there are all the normal design issues plus additional ones such as circuit layout, stability, and distance of LED from driver to contend with.

This does make linear regulator based drivers a more attractive proposition if you are not worried about a bit of extra heat.

Lighting design with LEDs is becoming a big industry which is just emerging from its infancy into childhood - there is still a long way to go ;).

There is no better way to trying things out, but a bit of considered design (especially on thermal management and careful driving of the LED side) will help big time.

Also it will keep the red and blue flashing LED lights away from your rear!:eek:

All the best.
M
 
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