Im thinking of upgrading some drls on my car.

LED SPEC: 4V max 750mA max. Daylight white. http://www.ebay.co.uk/itm/141679345010?_trksid=p2060353.m1438.l2649&var=440967222198&ssPageName=STRK%3AMEBIDX%3AIT

There is going to be 4 leds per headlight.

Im wondering which is the best way to drive these, since the car will give out anything between 12-14.4v.

Ive been thinking about just putting some resistors, but these leds output goes down substantially when voltage is dropped.

Ive also thought about building a C.C circuit to the leds, is it possible to use LM317t for these leds? Would these leds have the same brightness when the car is on / off?


Maybe the best thing is to buy a finished driver from ebay?

I'm sorry for the noob question.


Thank you for answering

There are leds in it right now, however, they arent that bright, they consume around 3w per headlight, they look to be driven with resistors.

Im planning on having the electronics inside the headlight, so, only limited space avalible.

I do have 5 dc to dc step up laying around, maybe i should use those.

i dont have the leds at home yet.

The leds are placed in the bottom, one pointing each direction.


  • \$\begingroup\$ Lights must be defined by the optical specs first, then the electrical requirements, then choose the correct parts with a good mechanical, thermal and electronic design. Can you understand this and re-phrase your question ? rather than ...I have 4 LEDs how do i make it work? \$\endgroup\$ – Sunnyskyguy EE75 Feb 12 '17 at 0:22
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    \$\begingroup\$ Which standard do you intend to be compliant with? ECE or FMVSS 108 (USA only) or Canada Motor Vehicle Safety Standards (almost similar to USA) to be street legal. Whatever you do must be street legal and ought to be SMPS regulated for effective , efficient output. \$\endgroup\$ – Sunnyskyguy EE75 Mar 17 '17 at 14:58
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    \$\begingroup\$ It will be nearly impossible to get these street legal. (You'll endanger yourself and the cars around you and likely will be fined.) \$\endgroup\$ – JWRM22 Jun 22 '17 at 6:35
  • \$\begingroup\$ Just search for "3W led driver", or "700ma constant current driver". The c.c. devices often support more than one LED in series, so you might connect 2x2 LEDs to two drivers. \$\endgroup\$ – JimmyB Jul 28 '17 at 10:38
  • \$\begingroup\$ There are also these small modules. Note though that none of those are specified for automotive applications, hence may fail after some time at elevated temperatures, vibrations, voltage transients, moisture, ... \$\endgroup\$ – JimmyB Jul 28 '17 at 10:39

You are best to use some sort of Switchmode Current source .If you use parallel linear regs you will get an efficiency no better than the filament lamps that you replaced.One option is to boost up to 16V with all the leds in series .This keeps current down making the DCDC convertor cheap and easy to build .The LED currents are equal despite spreads in the individual terminal voltages .You can protect your series string from LED open circuit failure by using an amplified Zener arrangement .If you buy a driver from ebay you will learn nothing .

  • \$\begingroup\$ The only difficulty with this is that the input voltage range and the LED operating range are likely to overlap. It may need to be a buck/boost converter. \$\endgroup\$ – Brian Drummond Oct 8 '16 at 9:31
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    \$\begingroup\$ @ brian drummond .. Yes I had used a sepic on another job .I guess that a cuk or a zeta or any boost convertor that has some buck properties would work . \$\endgroup\$ – Autistic Oct 8 '16 at 9:42

Using resistors to drive LED for lighting applications is something I wouldn't advise for. This video can provide you more perspective.

LM317t in itself doesn't control current, depending upon load(LED in your case) the current will vary, but on a quick search, I found some schematics in datasheet itself (on pg 11, fig 9) which probably you can test out for your application and I believe should work for 3W LED's. And there are many such small current controlling circuits. (for eg CC using 2 transistor)

But for stable and long run purpose the above may not work and drivers specially designed for driving LED's would be a better option. That will help increasing life of your LED's and with better brightness over time.


Put 3 LEDs in serie (3*4V=12V) and then add a 700mA constant current regulator (CCR) or two 350 mA CCRs like this one in parallel (but it's not as efficient): http://www.mouser.com/ds/2/308/NSI50350AD-D-118972.pdf If you can find a 700mA driver it's better. Do two series of 3, or 3 series of 3. As for street compliance, use it only as "extra" light, when no other car comes in front of you. ;)


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