LED Brightness Control

Thread Starter

phoebe_

Joined Jul 1, 2012
13
I'm trying to control the light intensity of an HPLED without using PWM. Would a simple circuit using LDR in series with the HPLED and a resistor work?
 

bertus

Joined Apr 5, 2008
22,266
Hello,

Do you have more detailed information on the led, like a datasheet?
A LDR is probably not capable of dimming the led directly.

Bertus
 

Wendy

Joined Mar 24, 2008
23,408
I understand, have one that looks very similar.

There are devices called buck pucks. They are SMPS (switching mode power supply) constant current regulators.

I have one build I have yet to test out.

Say you want 0.7A for the LED (typical for 1W). If you use one of these devices with 12VDC it will draw 0.3A, give or take. This is part of what I mean about efficiency.

The LED will get very hot, heat sinking is a must. If you use a resistor it will likely have to be a high wattage. A buck puck does not get hot.
 

bertus

Joined Apr 5, 2008
22,266
Hello,

That led probably needs 350 mA to light up.
The voltage may be between 3 and 4 Volts.
A LDR will never manage the 350 mA.
As Bill said PWM will be the correct method to dimm the led.

Bertus
 

SgtWookie

Joined Jul 17, 2007
22,230
I'm wondering what your aversion to PWM is all about?

High-power LEDs really need a buck or buck-boost type of current regulator instead, as other methods are inefficient (such as purely resistive). You'd need a physically very large resistor, and it would make a good room heater.

Have a look at BuckPucks:
http://www.ledsupply.com/buckpuck.php?gclid=CO7Hncve-LACFU2htgod7hXzlQ
Not particularly cheap, but compact, reliable, and relatively easy to wire up.
 

Thread Starter

phoebe_

Joined Jul 1, 2012
13
Hello,

That led probably needs 350 mA to light up.
The voltage may be between 3 and 4 Volts.
A LDR will never manage the 350 mA.
As Bill said PWM will be the correct method to dimm the led.

Bertus
Thank you very much! I'll start reading about PWM and hopefully I can make my own circuit.
 

Thread Starter

phoebe_

Joined Jul 1, 2012
13
I'm wondering what your aversion to PWM is all about?

High-power LEDs really need a buck or buck-boost type of current regulator instead, as other methods are inefficient (such as purely resistive). You'd need a physically very large resistor, and it would make a good room heater.

Have a look at BuckPucks:
http://www.ledsupply.com/buckpuck.php?gclid=CO7Hncve-LACFU2htgod7hXzlQ
Not particularly cheap, but compact, reliable, and relatively easy to wire up.
Thanks a lot for the quick responses! Sorry for my ignorance, I'm a newbie to electronics and I just want to make things convenient for me. Anyway, this seems to be a great community.
 

Wendy

Joined Mar 24, 2008
23,408
If you need help with the circuit we can help, or I can show you how to do it with a LM317 (which will get very hot, as it is the same as a resistor).

Wookie caught me out on one detail, his 350ma figure is correct. I have a 3W LED.

What is the power supply voltage you are powering this with? If it is not stable (such as a car would be) we need to know.

Most of these circuits are pretty simple. Don't sweat it.

Since I brought it up automotive is not allowed. The exception is the cigarette lighter. That is never my first assumption though.
 

SgtWookie

Joined Jul 17, 2007
22,230
If you need help with the circuit we can help, or I can show you how to do it with a LM317 (which will get very hot, as it is the same as a resistor).
An LM317 has a rather large dropout when used as a series current limiter (~3v), and since the LED is probably dropping ~3.2v-3.6v across itself, you're just about guaranteed to waste at least half as much power across the LM317 & resistor as you would in the LED itself.

Wookie caught me out on one detail, his 350ma figure is correct.
It was Bertus who mentioned 350mA, and I agree that Bertus is most likely correct.
 

Thread Starter

phoebe_

Joined Jul 1, 2012
13
If you need help with the circuit we can help, or I can show you how to do it with a LM317 (which will get very hot, as it is the same as a resistor).

Wookie caught me out on one detail, his 350ma figure is correct. I have a 3W LED.

What is the power supply voltage you are powering this with? If it is not stable (such as a car would be) we need to know.

Most of these circuits are pretty simple. Don't sweat it.

Since I brought it up automotive is not allowed. The exception is the cigarette lighter. That is never my first assumption though.
I would really appreciate it if you could teach me how to use a current regulator. I'm using a power supply with voltages up to ~5V.
 

SgtWookie

Joined Jul 17, 2007
22,230
If your power supply is only 5v, then you are just about out of luck.
That won't leave enough "headroom" for either the BuckPuck or the LM317.

If your LED drops 3.6v, you will need at least 2v more; 3.6v+2v=5.6v.
With an LM317, you would need at least 6.6v.
 

Thread Starter

phoebe_

Joined Jul 1, 2012
13
If your power supply is only 5v, then you are just about out of luck.
That won't leave enough "headroom" for either the BuckPuck or the LM317.

If your LED drops 3.6v, you will need at least 2v more; 3.6v+2v=5.6v.
With an LM317, you would need at least 6.6v.
Oh, what I meant was I was only using around 5V for this project. I actually have a 15V power supply.
 

bertus

Joined Apr 5, 2008
22,266
Hello,

Perhaps a bit odd use of a powerfet, but it will drive the led.



The current through the led will be about 0.7 / Rbe = 0.7 / 2.2 = 0.318 A.

Both the led and fet must be cooled.

Of course the others may comment on my circuit.

Bertus
 

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Wendy

Joined Mar 24, 2008
23,408
That would work.

Where is this 5V coming from? I ask because we need to know how regulated it is.

At that low voltage a buck puck or LM317 won't work. If you could use the 15V more options open up.

I suspect I know the answer on this one, have you ever soldered electronics before?

We'll probably have to adjust our responses to your skill level.
 

Thread Starter

phoebe_

Joined Jul 1, 2012
13
That would work.

Where is this 5V coming from? I ask because we need to know how regulated it is.

At that low voltage a buck puck or LM317 won't work. If you could use the 15V more options open up.

I suspect I know the answer on this one, have you ever soldered electronics before?

We'll probably have to adjust our responses to your skill level.
I tried getting the characteristics of the LED by creating a simple circuit with the LED, a resistor, and the voltage supply. The peak voltage across the LED is ~3V. The current on that circuit was ~370mA.
 

Wendy

Joined Mar 24, 2008
23,408
Shoot, if the power supply is stable, like a computer PSU, you are there. I would consider 370ma close enough. Does the resistor get hot?
 
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