Driving 3w leds in a vehicle - help please!

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Thread Starter


Joined Jun 11, 2013
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.

I saw this similar thread on here...

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 linear, 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 these 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
http://en.wikipedia.org/wiki/78xx - 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 the led .

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

Thread Starter


Joined Jun 11, 2013
Oh ok sorry i read through the faq's before posting and didnt see it, whaty if it was for a house with a 14v dc source voltage?! lol


Joined Apr 26, 2005
whaty if it was for a house with a 14v dc source voltage?! lol
Some will always suspect your asking for auto modifications when the voltages are within the range of automotive products. There are other forums that will answer that question.


Joined Mar 24, 2008
The owners of All About Circuits has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

This thread is against the AAC forum rules, Chapter 6, as seen here:

6. Restricted topics. The following topics are regularly raised however are considered “off-topic” at all times and will results in Your thread being closed without question:

  • Any kind of over-unity devices and systems
  • Automotive modifications
  • Devices designed to electrocute or shock another person
  • LEDs to mains
  • Phone jammers
  • Rail guns and high-energy projectile devices
  • Transformer-less power supplies
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Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

Please try to understand the reasons behind this action, and feel free to browse and use the forums.

You might find answers to your questions in one of these forums:


Another good forum that shares many of the same membership is http://www.electro-tech-online.com/ . Good luck.
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