I didn't check the datasheet for those leds, I just looked at the description on the page and noticed forward voltage = 3.2v and current = 150mA.
If you want to make everything compatible with 12v, the first chip (mic2287) is out. That chip is designed to use a low voltage (2.5-10v with an absolute maximum of 12v which won't work for you, because a 12v car battery may give you up to 14v and would damage the chip).
The chip would then boost the voltage as much as needed until the current going through the led(s) on the output reaches the amount you set. The idea would be to configure the chip to 150mA, you would set two leds with 3.2v in series on the output (so you'd have a 6.4v 150mA led) and then the chip would boost the 6v input voltage to 6.4v and light up the leds.
This chip only works in boost mode, so it's not easy to use such chip to light up just a couple of leds that don't need so much voltage.
The other two chips (bcr320/321) are however designed differently... you power the leds directly from the input voltage and after the leds you put this chip and the chip will only let a particular amount of current to the ground.
So with such chip, it doesn't matter if the input voltage is 6v or 12v (the chip supports up to 25v input voltage), you just put one or more leds in series and then after the leds you put this chip and configure it to only let 150mA (or whatever value you want) go through.
Naturally, with this chip, the input voltage must be higher than the voltage dropped on the leds and the chip itself ... so you can't use a 6v input voltage and put two leds in series (as these would need 6.4v to light up).
1. Would these LED's be adequate for an indicator lamp?
2. Does the LM317 CC source follow voltage?
as example, I might design this for a 6v system, but what design aspects would come into play if he decides to upgrade to a 12v system?
I recently talked to my uncle and he isn't planning to upgrade which makes my life easier.
We do use amber/orange turning signals here in Australia, and the housing has an orange lens so I thought either white or amber/orange would do.
I have next to no experience in lighting so I looked up some information, I saw that there was already a 12v 3W globe in the housing and what I saw from a google search was 16lm/W for an incandescent globe (48lm) so I worked off that :/ I was obviously incorrect
So I'm going to have to aim for 200-300 lumen in orange/amber light with good thermal design.
Could I put an inductor on the input to "simulate" the effects of a incandescent so he wont need to change his blinker can?
Thanks for the pointers Corporate666
I'd be tempted to use relays controlled by the 6V system that switch off and on 12V lights. Though once you get to this point you have to figure out how to keep the 12V system charged.
What if we went about this from the other direction. Replace the generator / magneto (if not part of the engine) with a modern alternator and automotive regulator that charges 2 6V batteries in series. Use the full 12V for the lighting and tap off one of the batteries for 6V. Or use a standard automotive 12V battery with a high current 6V regulator.
Is the move to LED's for extra brightness? Or reduced power draw? Or for the novelty of LED? Or all of the above?
I am surprised the existing bulbs are only 3W - most blinkers in cars here use 1156 or 1157 bulbs which are 25W. If you are concerned about power draw, you could stick with the existing 3W draw, but the most efficient yellow LED's will get maybe 90 lumens per watt, so you would be lucky to get 250 lumens considering driver inefficiency, but that would be MUCH brighter than an existing 3W bulb, especially if the existing bulb is white (since it makes white light but most of that light is blocked by the yellow lens).
So if you are going with the 6V system, I like the MLE LED's since they are still pretty efficient... the higher bins can achieve 90 lumens/watt at 150mA. I don't know how comfortable you are using surface mount parts, but a quick look at Digikey, and I would probably consider something like the Texas Instruments TL4242 boost LED driver.
I'm going to STRONGLY suggest keeping this simple, I would forget about LED drivers or any active circuitry, it's simply not needed.
I'm going to STRONGLY suggest keeping this simple, I would forget about LED drivers or any active circuitry, it's simply not needed.
In general, I have to disagree with this....
Consider a standard 12V system. Many people say "just pick the right value resistor and away you go".
Ok, so let's say you have an LED with a forward voltage of 2.4V and If of 100mA. A car electrical system can vary from maybe 10.4 volts when off to 14.5 volts when running. So if you wanted to run a string of 4 LED's... what resistor value do you choose?
4 LED's is Vf of 9.6, so let's say you choose an 8 ohm resistor. At your lowest voltage, you are getting 100mA and things are good. But when you turn on the ignition and the voltage goes to 14.5, now your LED's are running at 600mA and they blow up. If you choose your resistor value based on your maximum voltage of 15, that would be say 54 ohms for 100mA for your 9.6 forward voltage LED string. But now at 10.4 volts, you're only getting 15mA through your LED's... when you start the engine and the voltage rises to 14.5V, now 90mA is flowing through your LED's. That is a very big difference and will be very noticeable - it will make the LED's look much brighter when the engine is running.
Now I realize you were not suggesting he just use a simple resistor, but in case you were , I have to say I disagree with that.
And when you consider that a chip like an RT8472 is $0.25 and needs only a capacitor, diode and inductor to run and is constant current and offers PWM of voltage level dimming, I think there's little reason to go with that solution. Total parts cost is something like $0.65 exclusive of the LED, probably equal or cheaper than any linear solution, plus you get dimming and it's a switcher so much more efficient and less heat - and heat is always an issue with higher power LED's anyway, so there's no sense in making the heat issue even worse by burning power as heat with a linear solution.
Active solutions are super simple, cheap and easy to implement. Plus it's the right way to drive LED's. There's no reason not to do it that way other than fear of the unknown, IMO.
Considering I've done this on MULTIPLE occasions, I know it works EXTREMELY well. You can do all of the math you want, but in practice the intensity of the LEDs varies little between engine off and running, most people would not notice the difference. If your 12V battery is resting at 10.4 volts it's dead and you need to replace it. A 12V car battery should rest at 12.6V fully charged with no load. In practice it will usually sit about 12.4 to 12.5 due to some parasitic drain for some of the on board electronics, and should never go over 14.4 volts when charging. If the voltage goes higher then the regulator is shot failing or failed and needs to be replaced. But we are talking about a 6V system here., resting voltage should be around 6.3 volts and should not reach much beyond 8 volts when charging, if it ever gets that high, I know my Grandfather's 40 Dodge that has a 6V system barely gets that high.
You want dimming? Add a resistor and diode to the circuit to give dual brightness for running light and turn signal. You really need to look at the available LED replacement lights on the market, very few, if any actually use anything more than a resistor or two and sometimes an inductor (to simulate load), that I have used or seen.
"The right way" is subjective, since a correct way to have this work is to simply use a resistor that will limit current based on maximum voltage the system will attain, works very well as I have said.
I've opened up hundreds OEM LED headlights, tail lights and turn signals .....