High Power LED Driver Circuits

Step 10 The analog adjustable driver

This circuit lets you have an adjustable-brightness, but without using a microcontroller. It's fully analog! it costs a little more - about $2 or $2.50 total - i hope you won't mind.

The main difference is that the NFET is replaced with a voltage regulator. the voltage regulator steps-down the input voltage much like the NFET did, but it is designed so that its output voltage is set by the ratio between two resistors (R2+R4, and R1).

The current-limit circuit works the same way as before, in this case it reduces the resistance across R2, lowering the output of the voltage regulator.

This circuit lets you set the voltage on the LED's to any value using a dial or slider, but it also limits the LED current as before so you can't turn the dial past the safe point.

I used this circuit in my RGB Color Controlled Room/Spot lighting project.

please see the above project for part numbers and resistor value selection.

this circuit can operate with an input voltage from 5V to 28V, and up to 5 amps current (with a heatsink on the regulator)

Comments

[csepzol]

Can anyone help me to calculate the R1, R2, R3, R4, Q1, C1 with an lm317?

[Spuzzum]

The LM317K is rated at a "minimum" 1.5A, "typical" 2.2A, and a "maximum" 3.4A.

[ac-dc]

This is another very lossy circuit, not what one would want to use to power an LED except as a good learning exercise about linear regulator control. As xsmuft mentioned other regulators could be substituted but I would suggest using an LDO, Low DropOut type as you then don't need as high a supply input voltage. However there is a problem with this circuit. Suppose 3 x white LED at 3.6V forward and 1A current. That's 10.8V total and at up to 28V input to the regulator we have 17.2W of heat!! That is a very unrealisticly sized heatsink and in fact any use with linear regulators driving 1A LEDs would require a larger assembly and more costly heatsinked cooling strategy. In other words, it's just too lossy. It might be a better way to hack together a circuit for driving a few encapsulated 100mW (~ 20mA) LEDs instead of high powered 1-3W LEDs.

[dan]

if you want to maximize efficiency you need to use the correct number of LED's based on desired input voltage. for example if you have 24V input, then use 6 LED's in series to minimize loss.

[ac-dc]

Agreed, but in most situations this is the reverse of the way things work out, that one has X # of LEDs they need, and Y PSU voltage to work with to drive them.

[cahillicus]

 ac-dc, you regularly chime in about loss and efficiency.  What would you recommend as an alternative?  I was looking into the STCS1 from SMT Electronics used in another instructable.  That is until I realized the IC itself was 2 x 3mm.  I can't work with components this small.  All the other IC drivers I found suffer the same flaw.  I need something I can build with my hands and a soldering tool, not a commercially fabricated PCB, a microscope, and re-flow oven.  Seriously, I am killing myself to find a do-able driver.  

I am trying to run 12 1 watt LEDs with 3.7Vf off of a 12v motorcycle @ 900 mA. I realize I will probably have to have 4 drivers each running 3 LED's.  Please, any suggestions for an electronic noob?

[Peregrine7]

Hi Dan,
Thank you very much for so much good info here. However, I'm still at a loss on how to proceed with my project. I was wondering if you could help me fill in the gaps. I think the analog adjustable driver would be the best option in my case.

I'm doing a circuit that will use LEDs in car taillights. So the input power will be a standard automotive 12V battery, which usually has a voltage that varies between 11V -14V. The LEDs will have 2 stages: full brightness when in stop light mode, limited brightness when in parklight mode. There'll be 4 identical lights, so let's concentrate on only 1:

The driver for each light will accommodate 2 series (in parallel) of 4 LEDs in each serie. So 8 LEDs total. The following are specs and parts I already have on order:

Regulator LM317
Red 1W LED, 2.1v-2.3v, 300mA

4 of the above LEDs in serie is (2.2 x 4) + 3 = 11.8v necessary to support the LM317 regulator.
Since there will be 2 of those series in parallel, it will require 1.25v / (0.300A + 0.300A) = 2.08ohms R1 resistor (I believe closest is 2.1).

1) Can you help me fill in the gaps for the rest of the parts in your circuit (R2, R3, R4, Q1, C1) ?

2) Do you think this would be the best solution based on my above specs? The 2 stage brightness is what complicates my circuit. The R1 would be enough if I only wanted to run them at full brightness. But since I also need them to switch to limited brightness, I think that's what your circuit can accomplish. Initially, I thought I was gonna just do 2 inputs on the regulator separated by diodes, and one input would be with some resistor that would cause the LEDs to be dimmer, but I'm not sure if that would work because I don't know how the regulator would affect that.

Thank you in advance!

[nukte]

Can't find the specified voltage regulator anywhere. Do you have the specs for it or do you have a substitute number? Thanks

[xsmurf]

I believe you can use any voltage regulator. An LM317 for example will handle up to 1.5A. Just find one that matches your specs. Linear and TI both make some.

[Jugfet]

If using a an LM317 be sure to get the right one ie LM317T as there are 'L' and 'M' versions that have significantly lower ratings.