# LM317 Voltage Regulator R1 & R2

Discussion in 'Homework Help' started by shredability, Aug 19, 2012.

1. ### shredability Thread Starter Member

Apr 1, 2012
31
0
I'm building a strobe and I need max current from my voltage regulator. I have 13.56V DC coming in from the 2000uf caps. I need 12V DC & a max of 1.5A. I haven't been able to find anything that will let my control the voltage and set the max current. I have found current regulating setups, like this:

And Voltage Setups like this:

I used this formula:

R1 = 1.25v / Iout
R1 = 1.25v / 1.5A
R1 = 0.8Ω

Then used:

Vout = 1.25(1+(R2/R1))
(((12/1.25) - 1) / 0.8) = R2
R2 = 10.75Ω

So I grabbed the parts and threw it together an or course everything caught on fire. So I'm guessing the power rating of the resistors are way off? I had 1/8W and looking at the circuit I'll probably need something like 2W? Am I on the right track? or way off? should I just go get some higher wattage resistors and use those?

On another note I may be able to mod my circuit to work on 13.56V right from my cap. But i'm guessing this would be a bad idea no?

2. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
What do you mean when you say that you have 13.56V (that's an awfully exact value) coming in from your cap? Where is the power coming from?

You threw what together? You gave two very different circuits. Which one did you build?

What caught fire? One of the resistors? The regulator? Whatever is supplying the power to the regulator?

When you say, "set the max current," what do YOU mean by that. Normally, that would mean to put a current limit so that the current can be less that that value, but cannot exceed that value. In that case, if you are below the current, the voltage is held constant at 12V, but when you reach that current, the voltage drops to whatever value is needed to maintain the current at the max limit. Your regulator can control voltage or it can control current -- one or the other, not both. The regulator determines one and the load determines the other.

If you had 1.5A flowing through a 0.8Ω resistor, then you have 1.2W, or ten times the power that a 1/8 watt resistor is rated for. That would definitely tend to let the magic smoke out.

If you are delivering 1.5A with (13.56-12.00)V across the regulator, then it is dissipating 2.3W of heat. Be sure to heat sink it properly otherwise it, too, will have it's magic smoke purged.

shredability likes this.
3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
At 1.5A, the dropout voltage of the LM317 is typically 2.7V (3V spec'ed maximum). This means that to get 12V out, you need at least 15V in.
You might want to look at low dropout (LDO) regulators.

shredability likes this.
4. ### shredability Thread Starter Member

Apr 1, 2012
31
0
Hey guys thanks! I ended up figuring everything out, basically I thought there was some resistor arrangement that would limit the amount of current or something in the second schematic but it seems that's not the case!

5. ### #12 Expert

Nov 30, 2010
16,671
7,314
This has been the subject of many posts here. You can't control voltage and current at the same time with a single active device (unless you include the idea that most voltage regulator chips contain a non-adjustable high current limit circuit).