Thread: Complete newb, trying to understand Lm317 w/resistor! :-(

Originally Posted by wy2sl0

Hi everyone, thanks for taking some time and helping me out

Im trying to make a new trunk light for my car. I read using 1W LED's with resistors is A) not very smart because of a vehicles voltage fluctuations, and B) the resistors for 1W LED's would create alot of heat and wasted energy.

So I have spent a couple hours reading on how to wire an LM317T with 1, 2 or more 1W LED's off a car system.

I understand you have to wire a resistor somewhere between pins to adjust current regulation. What I dont understand is 1) where to wire this and b) how a resistor to adjust CURRENT output, will somehow also get the lm317 to output 3.3V from 12-14v?

Lets say I used a 3.9ohm resistor, to make 321ma output ( which is close to the 300 required for my 1W LED's ) however although it is now putting out 321ma, at what voltage and how does it magically match the LED's requirement?

And of course one more thing, how would I wire 3 of these onto 1 LM317 so I do not need 3 voltage regulators and heatsinks?

Thanks very much in advance your really helping me! I want to understand.

The circuit below is right off the LM317 data sheet Basically, the resistor shown programs the current. If you want exactly 0.3A, the resistor should be 1.25/0.3=4.166Ω. A 3.9Ω resistor would make the output current 1.25/3.9=0.32A. If you wire three of your 3.3V 300ma LEDs in series with the cathode pointing to ground, the drop across all three will be 9.9V. 1.25V is lost across the resistor, and ~1.7V is required for the regulator to work (Drop Out Voltage), meaning that it will regulate properly if the input voltage is 9.9+1.25+1.7=12.85V or higher, which is the case if your engine is running (alternator voltage is set to 14.2-14.6V) but not if the engine is stopped (resting battery voltage is 12.65V).

As a practical matter, it will work with the engine stopped, but the Leds will be a little dimmer.

The worst-case power dissipation in the LM317 with the engine running will be [14.6-(1.25+9.9)]*0.3 = 1W, which if you use the TO220 package you can get by without heatsinking it. (Not true if you only use one or two leds).

Thank you very much for your help.

I have a few questions though

First thing is, the regulator works but check what the voltage drop is and then regulates that exactly? I ask this because I always thought an LED would take more voltage if given more?

Second thing is, what is C1 0.1uF in that diagram/also voltage dropout pointed to the ground? What does that mean sorry for my ignorance

Third is, the resistor MUST be mounted in one way only, because polarity matters correct?

Finally, it says LOAD after the resistor. For the negative of the LED's and the positives, where do I connect? I ask this because these diagrams only show one line going to the Load of the circuit. Thank you!

P.S Sorry one more thing! On digikey they have this

The C1 thingy is a capacitor and is needed by the regulator for it to work correctly..Dont omit it. It can go in any way round as long as it is not an electrolytic...polyester or similar is ok

The resistor is non polarised..it can go in any way round...

The load is your led(s)...

Connect the led -ve to ground and the +ve to the junction of the resistor and Adj..

Originally Posted by Chippie

The C1 thingy is a capacitor and is needed by the regulator for it to work correctly..Dont omit it. It can go in any way round as long as it is not an electrolytic...polyester or similar is ok

The resistor is non polarised..it can go in any way round...

The load is your led(s)...

Connect the led -ve to ground and the +ve to the junction of the resistor and Adj..

Ok so I am thinking best bet is a 4.3ohm 1W resistor, to run each at 290ma, just to be safe ( 10ma difference in brightness probably nil ).

Some of the LED's though, say forward voltage of 3.2-3.6v. How do I set the voltage output of this regulator, and what to?

Also the C1 comes with an LM317? Or what do I need for it.

HS104-2-ND HEATSINK TO-220 PWR CLR 1.45"10W
4.3W-1-ND RES 4.3 OHM 1W 5% METAL OXIDE
LM317TGOS-ND

These are the parts I have ready to order, does this seem right?

You need to buy C1....Where ever you are buying your parts from, look for a 0.1mfd capacitor(the working voltage needs to be around 25v or more)..

Go for the 4.3 Ohm resistor, everything else looks good...

You dont need to worry about the voltage, see the post above..number 2 by MikeMI...This is a current regulator you are building...Re-reading Mike's post will make this come clear..

It is a decoupling capacitor on the input to the regulator...It 'shorts out' any ac interference on the supply line put simplisticly...

I think for what cost is involved, its not worth leaving the heatsink off...If there's a chance that the reg may overheat, then I'd put one on...Saves frying the reg and mebbe the led(s) too....

5 cents provided...

More than the usual 2 cents due to the current economic climate

The regulator does not fry. It simply turns itself off when it gets too hot. It has "thermal shutdown" to protect itself.

But don't let it heat then cool then turn on and heat again then turn off and cool again too many times because the thermal stress might break it.

ok...fry perhaps isnt the appropriate techie term....destroyed by thermal cycling

When the voltage drop of the regulator is larger then its heat output is also larger.
With a pretty big heatsink and free air around it an LM317 can dissipate about 18W.

Originally Posted by audioguru

When the voltage drop of the regulator is larger then its heat output is also larger.
With a pretty big heatsink and free air around it an LM317 can dissipate about 18W.

Ok thats what I thought.

The largest drop from just 1 LED on a 12v system, lets say at 14.5v (max) would be about 4W of heat, so with that heatsink im clearly fine.

Saying that however, if I did it this way instead

14.5v Source -----> 39 ohm resistor (4 watt capable) ------> 3.3v@320ma

Would this resistor fry in a car out in the open? Or would it work?

Also, how would I connect my LED to a heatsink? I.E the metal on my car chassis/body? Just home depot silicone?