# Help with this Led Circuit

Discussion in 'Homework Help' started by kreston, Dec 27, 2009.

1. ### kreston Thread Starter New Member

Dec 27, 2009
4
0

Hi there. I need some help with this circuit. I want to know if there is a problem with the voltage input.

In lm317's datasheet it says maximum voltage difference : 40V
Does this mean the maximum voltage input is 40v? or what?

Because there will be a voltage drop because of led so voltage difference will be around 2-3v max.

If this circuit is problematic,Can you suggest another adjustable voltage regulator that can handle higher voltages and currents?(and cheap also )

2. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
Actually you have the other problem, the minimum voltage a LM317 is around 3VDC. It must have that voltage to power it's internal circuitry. If the LED is 33V then you have no problem (the LM317 is dropping 7V), if the LED is 37V then you might have a problem (the LM317 is dropping 3V), but then again, maybe not.

Doing the math you might be pushing the current limits for the LM317, but I think it can handle it. I calculate a current of 1.56A from this setup.

Be sure you have good heatsinking. This is a requirement no matter what regulator you will use.

Generally you want to use a little less than the part requires, it will extend the lifespan a quite a bit. You're pushing the LED and the LM317 a bit over spec.

You could also use qty 12 10Ω ¼W resistors in parallel and get exactly 1.5A.

Last edited: Dec 27, 2009
3. ### kreston Thread Starter New Member

Dec 27, 2009
4
0
what if i change my 40v voltage source to 42v or 45v?

4. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
You're pushing the LM317 to it's maximum limits. This is generally not a good way to go.

If your LED is rated for 50W, that means 50W/1.5A = 33.333...V. If you exceed the power rating of the LED, you'll burn it up rather quickly.

The LM317 is cheap and simple. However, you'll wind up paying for it over time due to the relatively poor efficiency in comparison to switching regulators.

The minimum voltage drop across the regulator is 3v when used as a current regulator. This goes up as the current through the regulator increases.

Switching regulators are much more efficient. They're harder to understand and more complex, but they will save you money over the long run.

5. ### PRS Well-Known Member

Aug 24, 2008
989
36
For a higher current use the LM317 in conjunction with a high current transistor.
For higher voltage, I'm not sure whether they make ICs that will do this or not. But there is a method of using the LM317 as a reference and using feedback to double its voltage handling abilities. More if you want. Warning: It's not simple.

6. ### kreston Thread Starter New Member

Dec 27, 2009
4
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I know about how voltage effects current in leds. thats why i'm using lm317 as a n adjustable regulator. The regulator limits the current thus led will choose the proper voltage for the limited current. There is no way to be burned up if current is limited to 1500ma

7. ### kreston Thread Starter New Member

Dec 27, 2009
4
0
By the way i found lm317HV and Lm2576. Maybe these could work?