0
\$\begingroup\$

I'm going to be using 2 sets of three 12v 9W leds, purchased from that well known auction site so that's all the info I have, as daylight running lights.

Obviously I could simply connect them up (in parallel) and hope, but I'd like each of them to both last AND output very similar (preferably adjustable) levels of light. I'm aware that a cars voltage can rise to as much as 16v so somehow I'd like to protect them from that too.

Can someone simply advise me on what I should be looking for pls? Somewhere in my head I've got constant current as a prompt but TBH I've no real idea what that means or how it works.

Any pointers would be really appreciated.

\$\endgroup\$
  • \$\begingroup\$ If it says "12V LEDs" then it already has current limiting (well, resistors) built-in. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 18 '15 at 22:24
  • \$\begingroup\$ HI. I appreciate that, but I also understand that vehicle voltages can rise to 15/16 volts. As installation is going to be 'somewhat challenging', what I don't want to do is over drive them and shorten their lives. If they already have current limitation inbuilt - which makes perfect sense - would I be better off using a voltage regulator like an LM317 driving say a 2n3055 to perhaps drive them at a constant 11.5 volts? \$\endgroup\$ – YorkshireDave Sep 18 '15 at 23:01
1
\$\begingroup\$

A simple solution for raw diodes can be the following schematic: Constant current driver example

Credit for this wonderful image goes to Brian Duxbury. He has a nice post about this schematic here.

You regulate the current with the sense resistor, the approximate formula is this: $$R_{sense} = V_{base-emitter}/I_{diode}$$ For ballpark estimates, expect the V_base-emitter to be 0.7V, so if you want 20mA, that's 0.7/0.02 = 35 Ohms.

Some notes for choice of transistor

  • the "Drive" transistor has to have a maximum collector-emitter voltage exceeding the maximum expected voltage of the power supply (i.e. if the power supply max voltage is, say 16V, better be on the safe side and take a transistor with a maximum collector-emitter voltage of at least 20V)
  • same goes for collector-base voltage rating
  • consider the voltage that'll fall on the transistor and thus, calculate the power requirements (i.e. if the LED current is 20mA, the supply voltage is 12V and the LED's forward voltage drop is 3V and there's three LED's, it means the transistor must be able to dissipate at least $$(12-3*3)*(20*10^-3) = 60mW$$

A nice feature of this schematic is that you can control it with PWM via the control pin, so if you know how to generate such a signal, you can just attach the generator to the control pin and off you go. R_control can be around 10K or so.

For multiple strings of LEDs, you can put such a constant current generator on the bottom of each of them (SMD parts would make it a very compact device) and just join the control pins at the PWM signal source to adjust light simultaneously for all strings.


If the LEDs already have resistors, if you can find out the value of the resistor you can calculate the danger of the LEDs suffering from overcurrent (you can find common forward LED voltage drops by colour online) and determine whether you even need such a constant current driver $$(V_{supply max}-V_{forward})/R=I_{diode max}$$

If you find that the LEDs are sufficiently regulated by the resistor, you can simply insert a MOSFET between the LED and the power supply (correct orientation of course) and use a PWM signal on that to control the brightness.

\$\endgroup\$
  • \$\begingroup\$ Thank you for that, but the things come as a package already to be used on 12v - wire and go! I'm more concerned about their longevity (so potentially under driving them) due to installation challenges. Once in, taking them out again is a complete pain! \$\endgroup\$ – YorkshireDave Sep 18 '15 at 23:08
  • \$\begingroup\$ Under driving them wouldn't hurt their longevity, only over-driving would. If you're worried about not using them to their full potential, you'd have to find out the added series resistance and do the current calculation for the diodes. \$\endgroup\$ – Linards Sep 18 '15 at 23:13
  • \$\begingroup\$ Apologies for not making myself clear. What I'm getting at is deliberate under driving and supplying a nice steady voltage to help in their longevity. I tried to over spec - in terms of output - so I could under drive. Then I realised I hadn't got a clue about how to drive them in a manner that delivers the longevity I need! So, would using an LM317 driving a 2n3055 even work? Is it suitable? \$\endgroup\$ – YorkshireDave Sep 18 '15 at 23:30
  • \$\begingroup\$ You definitely can. I don't know if it's necessary though. As I previously said, you should check the LED series resistor value, find out the maximum supply voltage and check what's the worst case current. Also it's important for how long the voltage persists. If you want complete peace of mind, sure you could plop a voltage regulator in there, however keep in mind that it should be a low-dropout regulator, hence LM317 would be a poor choice since it requires at least a 3V difference between Vin and Vout, which would turn 12V to 9V at best. An LM2940 would be a better choice. \$\endgroup\$ – Linards Sep 19 '15 at 19:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.