svtiss

What is the best way to drive power LED's ?
Can I use just a voltage regulator , lets say 3.3 v and feed the LED's
Thanks for any ideas. I mean a switching regulator .

Papabravo

Regardless of the voltage you should use a current limiting resistor based on the forward voltage of the LED and the available voltage being monitored. A separate regulator just for the LED seems unecessary and wasteful. You might want to make a distinction between the input of your main regulator and the output of the main regulator.

__________________
We never have time to do it right; but we always have time to do it over.

svtiss

I have a 12v power supply( its a 12 v battery) for the leds , if I use resistors for the current limit, that would be a lot heat , is that right ?

crutschow

Yes, a separate resistor for each led would generate significant heat. To miniminze this you can connect the leds in series to better match your supply voltage. The led voltages are then added together. For example if each led had a 3V drop at the rated current, you could use 3 in series to give a 9V drop (you need to leave some voltage for the current regulating resistor). Then pick a resistor value to drop the final 3V at the rated current. For example, if the current is 20mA the resistor would be 3V/20mA = 150 ohms and the resistor power would be 20mA x 3V = 60mW.

For maximum efficiency you could also use a switching regulator configured to deliver a constant current (which the leds require). But obviously that's more complicated and expensive.

svtiss

thanks
I have 2 leds and current around 100-150 ma for both , and the voltage ~3.2

crutschow

So for 125mA and the 2 leds in series, the resistor value would be (12.6-6.4)/125mA = 50 ohms. The resistor dissipation would be 125mA x 6.2V = .775W, thus you would need at least a 1W resistor.

If you don't want to waste that much power you would need to go to a switching regulator configured to output a 125mA constant current.

svtiss

yes that was my question , use a v regulator or there are led drivers on the market ? and whats better to connect leds in series or parallel?
thanks

crutschow

You want to drive them in series. In parallel, one of the diodes may hog more current then the other.

An example of a switching regulator for leds from Linear is at http://www.linear.com/pc/productDeta...4,C1768,P13452. National, Maxim, and others make similar devices.

Ubergeek63

Shoot I thought he said power LEDs. I usually think of 350-700mA plus when I think of power LEDs

Papabravo

Seems like an excessive amount of current for an LED to show that the power is on or off. I've got whole products that consume a tenth of that power. How much current does the rest of your circuit consume?

__________________
We never have time to do it right; but we always have time to do it over.

Hero999

I dissagree, a switching regulator wouldn't be wasteful, it would save energy and increase the battery life.

Ideally you want a constant current mode switching regulator. Failing that, just one that reduces the voltage to as close to the LED's forward voltage but leaves adequate headroom for the LED.

For example if you have a red LED and power it from a 3.3V switching regulator, it will drain far less power from the battery than if it were just connected via a series resistor.

However, as I said before, a constant current mode switching regulator would be a better choce.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

Papabravo

A whole switching regulator just for an LED to tell you that the power is on? I must be missing something here. Why would such an indicator take several hundred milliamps? Wouldn't a super high efficiency RED at 2 mA do the job just as well?

It makes no sense to me to be talking about batterties and efficiency when your LED takes several hundred milliamps. This is like Bizarro World.

__________________
We never have time to do it right; but we always have time to do it over.

Hero999

I'm confused, a 125mA is a power emitter not just an inducator and whether it's worth using a switcher depends on the size of the battery. If it's running from a car battery, then I wouldn't bother but if it's running from a small sealled lead acid battery then I might consider it.

As far as price is concerned, switching regulators don't need to be expensive, a current limited Black regulator could power a high power LED and it's cheap to build too.

Official Home Page of Roman Black

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez

Papabravo

An LED is now a power emitter. Things just keep getin' curioser and curiouser. This is without a doubt like the deaf and the blind trying to communicate; I can't interpret ASL (American Sign Language) and you can't hear me screaming. Why does an LED, used to incicate that the power is on, require several hundred milliamps and a switching regulator? This great long thread has completely failed to articulate a coherent answer, or even explain the purpose of such a device.

__________________
We never have time to do it right; but we always have time to do it over.

crutschow

Have we established that the LEDs are only to indicate that the power is on?