Driving 3w leds in a vehicle - help please!

[gledy2]

Hi, needing a little help with this, new on here and new to high power leds, played with smaller ones before with resistors, that is all. Only some very basic knowledge of electronics so needing a little help please!

I am looking to illuminate behind the wheels on my car, when i open the doors wired to the interior light. I have wired it up with normal leds but it isnt bright enough. I have done this with 4 leds in series, one at each wheel. I have bought some 3w leds, not realising they are so easy to power! Here are the specs:

DC Forward Voltage: Min: 3.6V Max: 4.2V
DC Forward Current: 700mA
1 led per wheel arch so 4 in total


The vehicle voltage will vary from 12-14V. After reading up I see it is not good to run these with resistors with much more power! I found out about buckplugs for each led, but not keen as they aren't cheap, looking for a cheap solution.

Please excuse my most likely stupid questions, happy to build the things and solder them up, just not sure what. Got the idea now of linear and switching regulators, but not exactly understanding FET and NPN transistors yet!

My questions are:
1)If i went with a linear supply, would it be better to wire 2 sets of 2 in series for a larger voltage drop meaning less heat? How hot would they get if they are only on for as long as the door is open, (not long). No idea on how quick they would heat up?

2) would this circuit let the leds fade out like the smaller powered ones with the interior lights?

3)off topic - would a 28mmx28mmx10mm heatsink be enough for the leds for my needs as they wont run for very long, the ones i have seen suggested for 3w are way bigger at 45x45x10mm but less easy to hide!

4)While learning on google I saw these
https://en.wikipedia.org/wiki/78xx -
I watched afromans great voltage regulator video which helped me understand! If i got one of the 8v ones of this and wired it in to two leds in series, would this not have the same effect? i realise this is a voltage not current regulator but would this work and be an easier fix for me? I now understand voltage and current i think, just not sure how it applies to driving the led. Surely a 12v for all 4 in series wouldn't work due to voltage dropout? It would mean less heat though, as according to afromans video, if i had a 14v source and 8v power from leds, both at 300ma, this would mean 3.6w of heat?

Thanks in advance for anyones time and sorry if my knowledge is shocking, just trying to learn, thanks, allan

 

[ronv]

Constant Current Driver

Here is a circuit that is fairly easy and often used.
With the values shown it will drive 2 LEDs in series with about 260 ma. with a battery voltage of 12 to 14 volts.
This should work for your smaller heat sink, but we should look at a spec sheet to make sure.
If you are shopping radio shack the resistor could be 1/2 watt but it will be hot. The 317T should have one of your small heat sinks as well.

 

[gledy2]

Thanks so much ronv, exactly what i was needing. I have had a read up about lm317's and constant current circuits with them in now i know what to look for, thanks. I am guessing the 5 beside the resistor stands for a 5 ohm resistor? New to the world of circuit diagrams!

I dont have the spec sheet on the leds, they were an ebay purchase, the only data i got was this:
**broken link removed**
Item Conditions Min. Typ. Max.. Unit
Forward Voltage VF If=700mA 3.5 4.0 4.2 V
Dominant Wavelength λp IF=700mA 510 / 515 nm
Luminous Intensity IV IF=700mA / 80 100 lm
50% Power Angle 2θ1/2 IF=700mA / 140 / degree

Note:

Absolute maximum ratings Ta=25℃

Tolerance of measurement of forward voltage±0.1V

Tolerance of measurement of peak Wavelength ±100K

Tolernce of measurement of luminous intensity±15%

Absolute Maximum Ratings (Ta = 25oC)
Item Symbol Absolute Max. Rating Unit
Power Dissipation PD 3000 mW
Forward Current (DC) IF 700 mA
Peak Forward Current IFP 1000 mA
Reverse Voltage VR 10 V

Would 260ma be enough to drive them or would i need a different resistor to the one in your circuit? Or am I missing something? This was my thoughts:

I read that the resistor would be calculated by ohms law R = Vref / I. I take it from reading Reference voltage on a LM317 is 1.25V
So for 2x leds in series 700ma forward voltage R=1.25/0.7 which is 1.79ohms. So should i be using a 1.8 ohm resistor? If a 5 ohm resistor is used this would mean a current of 250ma, which is close to your 260ma figure?

Also i calculated wasted power (maybe if i am using the right values) P = (Vin – Vref – Vf) * I (13.5-1.25-8)x0.7=2.98W. Would my heatsinks be ok for this?

Thanks if you can help me again! Allan

 

[ronv]

I am guessing the 5 beside the resistor stands for a 5 ohm resistor? New to the world of circuit diagrams!

Right you are.

Would 260ma be enough to drive them or would i need a different resistor to the one in your circuit? Or am I missing something? This was my thoughts:

I read that the resistor would be calculated by ohms law R = Vref / I. I take it from reading Reference voltage on a LM317 is 1.25V
So for 2x leds in series 700ma forward voltage R=1.25/0.7 which is 1.79ohms. So should i be using a 1.8 ohm resistor? If a 5 ohm resistor is used this would mean a current of 250ma, which is close to your 260ma figure?

Right again. I derated the current for the LEDs because they are only good for 700 ma at 25C. So that is with a huge heat sink. Since the LEDs will dissipate up to 1 watt (.25X4.2) they will be quite hot with your heat sink. I think they will still be pretty bright.

Also i calculated wasted power (maybe if i am using the right values) P = (Vin – Vref – Vf) * I (13.5-1.25-8)x0.7=2.98W. Would my heatsinks be ok for this?

The total power will be 3.3 watts (13.5X.25) with 1 watt in the LEDs .3 in the resistor and the rest (2watts) in the regulator. It is hard to tell if 250 ma. is to much or if you could use more because there is no spec on the temperature rise of the LED vs. power, but my guess is they will be hot with 250 ma. and the small heat sink - maybe 50 or 75C. You can also see why the regulator will need a heat sink. I would try it with the lower current first.

Good luck with the project. Let us know how it works.

 

[gledy2]

Thanks ronv, for your time and explanations, now i understand it thanks to your circuit diagram and explanations. I have ordered up some lm317's and resistors, and will use the heatsinks i have for the leds and for the lm317's. will let you know how i get on once i get the parts and get it installed. THanks again, allan

 

[alec_t]

....but don't try driving them from the car's interior light circuit. You will almost certainly overload it and cause wiring damage or destroy the PWM circuitry which dims the light.

 

[gledy2]

Ah ok thanks alec, yeah i never thought about that side of things with the higher power leds drawing more current. I had a circuit wired up directly but the leds weren't bright enough. However, i did the same thing on a previous car and it wasnt a problem- i had 3 smd leds in each wheel well, unsure of the ratings as they came in a waterproof block ready to accept 12V, and another wire running off the interior lights to 2x ccfl inverters in the headlights running ccfl rings. Now I think about it and have a little more understanding about electronics, the wiring must have been under a lot higher load. Still the dimming worked ok! Here is a pic of it before...