High-power LED's
but...how do you use them? where do you get them?
1-watt and 3-watt Power LED's are now widely available in the $3 to $5 range, so I've been working on a bunch of projects lately that use them. In the process it was bugging me that the only options anyone talking about for driving the LED's are: (1) a resistor, or (2) a really expensive electronic gizmo. now that the LED's cost $3, it feels wrong to be paying $20 for the device to drive them!
This instructable will give you a blow-by-blow of all the different circuits for powering Big types LED's, everything from resistors to switching supplies, with some tips on all of them, and of course will give much detail on my new simple Power LED driver circuits and when/how to use them (and I've got 3 other instructables so far that use these circuits). Some of this information ends up being pretty useful for small LED's too
Overview / Parts
There are several common methods out there for powering LED's. Why all the fuss? It boils down to this:
1) LED's are very sensitive to the voltage used to power them (ie, the current changes a lot with a small change in voltage)
2) The required voltage changes a bit when the LED is placed in hot or cold air, and also depending on the color of the LED, and manufacturing details.
so there's several common ways that LED's are usually powered, and I'll go over each one in the following steps.
This project shows several circuits for driving power LED's. In order to avoid much duplicated content this project only discusses specific circuits and their pros and cons. To learn more about assembly techniques and to find out LED part numbers and where you can get them (and other topics), please refer to one of my other power LED projects.
Power LED Performance Data - Handy Reference Chart
Below are some basic parameters of the Luxeon LED's which you will use for many circuits. I use the figures from this table in several projects, so here I'm just putting them all in one place that I can reference easily.
Luxeon 1 and 3 with no current (turn-off-point):
white/blue/green/cyan: 2.4V drop (= "LED forward voltage")
red/orange/amber: 1.8V drop
Luxeon-1 with 300mA current:
white/blue/green/cyan: 3.3V drop (= "LED forward voltage")
red/orange/amber: 2.7V drop
Luxeon-1 with 800mA current (over spec):
All colors: 3.8V drop
Luxeon-3 with 300mA current:
white/blue/green/cyan: 3.3V drop
red/orange/amber: 2.5V drop
Luxeon-3 with 800mA current:
white/blue/green/cyan: 3.8V drop
red/orange/amber: 3.0V drop (note: my tests disagreed with spec sheet)
Luxeon-3 with 1200mA current:
red/orange/amber: 3.3V drop (note: my tests disagreed with spec sheet)
Typical values for regular "small" LED's with 20mA are:
red/orange/yellow: 2.0 V drop
green/cyan/blue/purple/white: 3.5V drop
Direct Power!
Why not just connect your battery straight to the LED? It seems so simple! What's the problem? Can I ever do it?
The problem is reliability, consistency & robustness. As mentioned, the current through an LED is very sensitive to small changes in the voltage across the LED, and also to the ambient temperature of the LED, and also to the manufacturing variances of the LED. So when you just connect your LED to a battery you have little idea how much current is going through it. "But so what, it lit up, didn't it?". ok sure. Depending on the battery, you might have way too much current (led gets very hot and burns out fast), or too little (led is dim). The other problem is that even if the led is just right when you first connect it, if you take it to a new environment which is hotter or colder, it will either get dim or too bright and burn out, because the led is very temperature sensitive. Manufacturing variations can also cause variability.
So maybe you read all that, and you're thinking: "so what!". if so, plow ahead and connect right to the battery. for some applications it can be the way to go.
- Summary: only use this for hacks, don't expect it to be reliable or consistent, and expect to burn out some LED's along the way.
- One famous hack that puts this method to outstandingly good use is the LED Throwie.
Notes:
- If you are using a battery, this method will work best using *small* batteries, because a small battery acts like it has an internal resistor in it. This is one of the reasons the LED Throwie works so well.
- If you actually want to do this with a power-LED rather than a 3-cent LED, choose your battery voltage so that the LED will not be at full power. This is the other reason the LED Throwie works so well.
The Humble Resistor
This is by far the most widely used method to power LED's. Just connect a resistor in series with your LED(s).
pros:
- This is the simplest method that works reliably
-only has one part
- costs pennies (actually, less than a penny in quantity)
cons:
- not very efficient. you must trade off wasted power against consistent & reliable LED brightness. If you waste less power in the resistor, you get less consistent LED performance.
- must change resistor to change LED brightness
- If you change the power supply or battery voltage significantly, you need to change the resistor again.
How to do it:
There are a lot of great web pages out there already explaining this method. Typically you want to figure out:
- what value of resistor to use
- How to connect your LED's in series or parallel
There's two good "LED Calculators" I found that will let you just enter the specs on your LED's and power supply, and they will design the complete series/parallel circuit and resistors for you!
If you are using the resistor method with power LED's, you'll quickly want to get a lot of cheap power resistors! Here's some cheap ones from digikey: "Yageo SQP500JB" are a 5-watt resistor series.
Switching regulators, aka "DC-to-DC", "buck" or "boost" converters, are the fancy way to power an LED. They do it all, but they are pricey. what is it they "do" exactly? the switching regulator can either step-down ("buck") or step-up ("boost") the power supply input voltage to the exact voltage needed to power the LED's. Unlike a resistor it constantly monitors the LED current and adapts to keep it constant. It does all this with 80-95% power efficiency, no matter how much the step-down or step-up is.
Pros:
- LED performance for a wide range of LED's and consistent power supply
- High efficiency, usually 80-90% for boost converters and 90-95% for buck converters
- can power LED's from both lower or higher voltage supplies (step-up or step-down)
- Some units can adjust LED brightness
- Packaged units designed for power-LED's are available & easy to use
Cons:
- complex and expensive: typically about $20 for a packaged unit.
- Making your own requires several parts and electrical engineering skillz.
High-power LED's
but...how do you use them? where do you get them?
1-watt and 3-watt Power LED's are now widely available in the $3 to $5 range, so I've been working on a bunch of projects lately that use them. In the process it was bugging me that the only options anyone talking about for driving the LED's are: (1) a resistor, or (2) a really expensive electronic gizmo. now that the LED's cost $3, it feels wrong to be paying $20 for the device to drive them!
This instructable will give you a blow-by-blow of all the different circuits for powering Big types LED's, everything from resistors to switching supplies, with some tips on all of them, and of course will give much detail on my new simple Power LED driver circuits and when/how to use them (and I've got 3 other instructables so far that use these circuits). Some of this information ends up being pretty useful for small LED's too
Overview / Parts
There are several common methods out there for powering LED's. Why all the fuss? It boils down to this:
1) LED's are very sensitive to the voltage used to power them (ie, the current changes a lot with a small change in voltage)
2) The required voltage changes a bit when the LED is placed in hot or cold air, and also depending on the color of the LED, and manufacturing details.
so there's several common ways that LED's are usually powered, and I'll go over each one in the following steps.
This project shows several circuits for driving power LED's. In order to avoid much duplicated content this project only discusses specific circuits and their pros and cons. To learn more about assembly techniques and to find out LED part numbers and where you can get them (and other topics), please refer to one of my other power LED projects.
Power LED Performance Data - Handy Reference Chart
Below are some basic parameters of the Luxeon LED's which you will use for many circuits. I use the figures from this table in several projects, so here I'm just putting them all in one place that I can reference easily.
Luxeon 1 and 3 with no current (turn-off-point):
white/blue/green/cyan: 2.4V drop (= "LED forward voltage")
red/orange/amber: 1.8V drop
Luxeon-1 with 300mA current:
white/blue/green/cyan: 3.3V drop (= "LED forward voltage")
red/orange/amber: 2.7V drop
Luxeon-1 with 800mA current (over spec):
All colors: 3.8V drop
Luxeon-3 with 300mA current:
white/blue/green/cyan: 3.3V drop
red/orange/amber: 2.5V drop
Luxeon-3 with 800mA current:
white/blue/green/cyan: 3.8V drop
red/orange/amber: 3.0V drop (note: my tests disagreed with spec sheet)
Luxeon-3 with 1200mA current:
red/orange/amber: 3.3V drop (note: my tests disagreed with spec sheet)
Typical values for regular "small" LED's with 20mA are:
red/orange/yellow: 2.0 V drop
green/cyan/blue/purple/white: 3.5V drop
Direct Power!
Why not just connect your battery straight to the LED? It seems so simple! What's the problem? Can I ever do it?
The problem is reliability, consistency & robustness. As mentioned, the current through an LED is very sensitive to small changes in the voltage across the LED, and also to the ambient temperature of the LED, and also to the manufacturing variances of the LED. So when you just connect your LED to a battery you have little idea how much current is going through it. "But so what, it lit up, didn't it?". ok sure. Depending on the battery, you might have way too much current (led gets very hot and burns out fast), or too little (led is dim). The other problem is that even if the led is just right when you first connect it, if you take it to a new environment which is hotter or colder, it will either get dim or too bright and burn out, because the led is very temperature sensitive. Manufacturing variations can also cause variability.
So maybe you read all that, and you're thinking: "so what!". if so, plow ahead and connect right to the battery. for some applications it can be the way to go.
- Summary: only use this for hacks, don't expect it to be reliable or consistent, and expect to burn out some LED's along the way.
- One famous hack that puts this method to outstandingly good use is the LED Throwie.
Notes:
- If you are using a battery, this method will work best using *small* batteries, because a small battery acts like it has an internal resistor in it. This is one of the reasons the LED Throwie works so well.
- If you actually want to do this with a power-LED rather than a 3-cent LED, choose your battery voltage so that the LED will not be at full power. This is the other reason the LED Throwie works so well.
The Humble Resistor
This is by far the most widely used method to power LED's. Just connect a resistor in series with your LED(s).
pros:
- This is the simplest method that works reliably
-only has one part
- costs pennies (actually, less than a penny in quantity)
cons:
- not very efficient. you must trade off wasted power against consistent & reliable LED brightness. If you waste less power in the resistor, you get less consistent LED performance.
- must change resistor to change LED brightness
- If you change the power supply or battery voltage significantly, you need to change the resistor again.
How to do it:
There are a lot of great web pages out there already explaining this method. Typically you want to figure out:
- what value of resistor to use
- How to connect your LED's in series or parallel
There's two good "LED Calculators" I found that will let you just enter the specs on your LED's and power supply, and they will design the complete series/parallel circuit and resistors for you!
If you are using the resistor method with power LED's, you'll quickly want to get a lot of cheap power resistors! Here's some cheap ones from digikey: "Yageo SQP500JB" are a 5-watt resistor series.
Switching regulators, aka "DC-to-DC", "buck" or "boost" converters, are the fancy way to power an LED. They do it all, but they are pricey. what is it they "do" exactly? the switching regulator can either step-down ("buck") or step-up ("boost") the power supply input voltage to the exact voltage needed to power the LED's. Unlike a resistor it constantly monitors the LED current and adapts to keep it constant. It does all this with 80-95% power efficiency, no matter how much the step-down or step-up is.
Pros:
- LED performance for a wide range of LED's and consistent power supply
- High efficiency, usually 80-90% for boost converters and 90-95% for buck converters
- can power LED's from both lower or higher voltage supplies (step-up or step-down)
- Some units can adjust LED brightness
- Packaged units designed for power-LED's are available & easy to use
Cons:
- complex and expensive: typically about $20 for a packaged unit.
- Making your own requires several parts and electrical engineering skillz.
No comments:
Post a Comment