Yeah it is very hard to get to them now. Sorry. There are actually 5 red leds wired to each lm317

 

Yeah it is very hard to get to them now. Sorry. There are actually 5 red leds wired to each lm317

Depending on the forward voltage of the LEDs, five LEDs is pushing the limit of operation with the voltage range you are applying.

hgmjr

 

I am a little annoyed that our OP made an issue of the 11.3V minimum voltage, but seems to be totally ignoring it now.

 

the forward voltage is 1.8 per led

 

the forward voltage is 1.8 per led

That is cutting it a bit close.

Oh well, if it works to your satisfaction I would say go for it.

Here is an analysis of the LM317 constant current circuit. It is a bit math intensive.

hgmjr

 

yep. I do not notice a difference when the car is off or when I turn it on. I am still hoping to get the rest of the things done tonight and installed and hopefully it will all be satisfactory and I can get a pic of the whole front dash for you guys.

 

I am still working on the interior lighting. It's taking longer than I suspected, especially tweaking the lights so there is not spotting and whatnot so that I am pleased with the results. I am working hard to get it done and will post pics asap.

 

Here it is, put together and pretty much finished. Thanks for all the help. Hope you enjoy the pics.

There are a couple things I'm going to tweak on it and I need to get the wiper illumination fixed but here it is.

 

You can use this calculator to vary the value of the program resistor (R1) and output set resistor (R2) and calculate the output voltage for the LM317 / LM338 / LM350 family of three terminal adjustable regulators. This calculator will work for most voltage regulators with a reference voltage (VREF) of 1.25. Typically, the program resistor (R1) is 240 ohms for the LM117, LM317, LM138 and LM150. For the LM338 and LM350, 120 ohms is typically used. However, other values such as 150 or 220 ohms can also be used. Refer to the adjustable regulator datasheets below for more information.

 

You can use this calculator to vary the value of the program resistor (R1) and output set resistor (R2) and calculate the output voltage for the LM317 / LM338 / LM350 family of three terminal adjustable regulators. This calculator will work for most voltage regulators with a reference voltage (VREF) of 1.25. Typically, the program resistor (R1) is 240 ohms for the LM117, LM317, LM138 and LM150. For the LM338 and LM350, 120 ohms is typically used. However, other values such as 150 or 220 ohms can also be used. Refer to the adjustable regulator datasheets below for more information.

This thread is about 9 months old. It looks to me like spam - an opportunity to advertise that damned strange invitation letter web site in his signature line.

 

As a completely different alternative, you may consider this:

If you were to use a Voltage Regulator directly off of the battery (through a fuse) you could then power your LEDs through a MOSFET transistor. The gate connected to the Parking lights through a resistor will turn the MOSFET on, and using a regulator will keep the voltage at a constant level. By doing this you may calculate the resistance needed for your LED setups and what not, and the voltages wont change.


Not shown is also a resistor on the gate to GND as a pull down resistor. maybe 10K ohms.

7812 is listed in the pic, but many types are available.

LM7812 + 12 Volts
LM7805 + 5 Volts
LM7809 + 9 volts
LM7806 + 6 Volts

you get the idea...

I make a lot of circuits for cars and I usually implement something like this to be rid of the Voltage fluctuation problem from the alternator. The filter caps on the regulator also help keep the + Power lines clean from noise that can mess with Digital Logic and counters and stuff.



I hope this may help. PArdon the diagram, i was rushing and using MS Paint

 

Corrections!!!


I made a few errors in my last post.

Here is the revised circuit of what i was trying to say.

 

LOL!!!


Sorry, i missed that last post...

im feelin a little dumb here...its friday and i just got out of work! Leave me alone!

hahahaha

 

LOL Metalfan

BTW, you're trying to use an N-ch MOSFET as a high-side switch without a source driver. You should be sinking current from the LED cathodes instead.

You could also use LEDs in series with lower values for the limiting resistors.
[eta]
Correction; it's a P-ch, but you're showing it connected incorrectly; the body diode will cause the LEDs to be on regardless.
Also, the 7809 regulator will put a constant 5mA drain on the battery whether the LEDs are on or not.

 

Corrections!!!


I made a few errors in my last post.

Here is the revised circuit of what i was trying to say.

If the control signal going high is supposed to turn on the LEDs, it won't. It will turn them off. If it is active low, then you're OK, except the transistor source and drain are swapped.

 

Very informative discussion.
I learnt a lot from this.

I am in the process of driving 30 Luxeon Star III,
Using series-parallel matrix on 12V.

Do you thin i am better off using LM 1086 for lower dropout? how much is the difference actually?

LEDs are 3.4V, 700mA
Luxeon Star III, Warm White.

 

I would use either a dedicated LED driver, or a smps chip as a LED driver: it is a lot easier on the LED, and it is a lot more efficient at that kind of current levels.

here is an example of configuring a LT1072 smps regulator into a CCS LED driver. the input voltage is 12v, +/- 2v (see the green trace). The output current is the blue trace, set at Vfb/R2. For LT1072, Vfb=1.25 thus the current is about 400ma.

it is driving 6 LEDs, and the output voltage from the chip is about 27v.

most smps chips are like this, and can be easily configured into either boost (which it is in this case), or buck (where the output voltage is lower than the input voltage). and you can find many chips from many other vendors to do this: the SimpleSwitcher series from National is really good with this.

at 400ma, the chip (to220) runs luke warm without any heatsink.

the newer chips also use a PWM or CE pin that allows you to digitally adjust its intensity. for other chips that do not have such a pin (like lt1072 here), you can apply an analog voltage to the feedback pin to influence the output current.

there is NO proper way to drive high power LEDs without using a constant current source. or they burn out real fast.

 

Many thanks for the input,
you guys are very informative!

BTW, i am very new to electronics, and would appreciate a Simpler circuit.

We can use LM317T as Constant current regulator, but it eats out 3V.

I was just wondering is there are some easily available Lower Dropout regulators Similar to LM317T.

If i manage to drive 3LEDs on 12v, it will be efficient ...and Simple!

 

at 700ma current output, a linear CCS will dissipate massive amount of heat, LDO or othewise. LDO regulators don't help here because it is unlikely you will be able to hold the input voltage steady, or the Vf of the LEDs steady over time either.

so you have to rely on either a resistor or the regulator to take on the variation. That means either the current is very unstable (with a resistor), or the regulation is poor / heat dissipation on the regulator is large.

do a quick math. let's say that you are stringing two 3.3v LEDs together to a 12v power supply (a car battery for example). the resistor needed is (12-3.3*2)/700ma=7.7ohm. the current through the LEDs is 700ma.

when the car is running, the battery is being charged at 13.7v. and the current going through your LEDs become (13.7-3.3*2)/7.7=920ma, or 1/3 higher than your spec.

the situation gets worse as the voltage drop on the resistor / regulator gets lower (more LEDs in serial).

so you either have to derate the current (running the LEDs at 500ma @ 12v so it runs at close to 700ma @ 13.7v), or use fewer LEDs in serial, or stabilize the DC voltage, etc.

Or just replace your burned out LEDs regularly, .

 

Or just replace your burned out LEDs regularly, .

No way!!
These 3Watt Luxeon Stars are Frigg''n Expensive!!

http://www.instructables.com/id/Super-simple-high-power-LED-driver/

I was inspired from there..
Its very cheap,and readily available.

BTW, i will use a Regulted 12V supply.
Yeah its a PC SMPS.

BTW, that smps also has a 3.3V output, Shall i connect the LED directly to that?