Intro: Using LED's and Laser Diodes
There are a lot of instructables about using LED's in cool and interesting ways, but not often is discussed how you should use LED's on a electronical level.
Since laser diodes are effectively a LED (Light Emitting Diodes) this will work for those as well.
There are a few things you need to know that will stop you from blowing up LED's which can be a pain, especially on the more expensive high power ones.
I'm specifically not going into which LED types produce more light, etc. This is just about hooking up LEDs.
Step 1: Which Specs Do You Normally Care About.
If you look at any (high power) LED or laser there are a few numbers that are almost always listed:
- Operational or Forward voltage.
- Operational or Forward current
- Peak, pulse or 'absolute maximum' forward current
- Reverse voltage
Each of these will get a small chapter explaining what is it and why would you care.
Step 2: Forward Voltage
A very common misconception is that LED's just need a voltage, just like a normal light bulb.
This is not the case, not matter how precise your voltage is, just supplying a LED with it will probably break it over time.
Also, providing a much higher voltage will not necessarily break it.
Forward voltage simply means the voltage that will drop over the LED when the normal operating current is applied.
Effectively this is the minimum voltage the LED can operate on.
Step 3: Forward Current
Here is where it gets interesting, the forward current of a LED is the current the LED can operate at.
You should never* go over this current if you want your LED to survive for any decent period.
Any LED needs a current limit to make sure that it never exceeds this current, which can take many shapes:
- A special LED driver board, useful for high power LED's and just a few bucks on ebay.
- A simple resistor, plenty of 'LED resistor calculators' out there that can help you select the correct one based on your power supply voltage, forward voltage and forward current.
- A 'parasite' current limit, a current limit made by the components around it.
- A battery or power supply that can't supply enough current, an example of this are LED throwies.
- Current limit on microcontroller pins, some microcontrollers have a 'build in' current limit if the VCC becomes low.
- LED driver chips, plenty chips that have internal current limiting for LED's.
Varying this current is also a good way to dim a LED, although PWM (turning it on and off really fast) allows for greater flexibility.
* Except with pulse currents, see the next chapter.
Step 4: Absolute Maximum Current.
This is useful value if you are driving a lot of LEDs.
For example if you are driving a LED matrix display you LED might not be on all the time, but only 1/10th ot the time for example.
At that point this current becomes interesting, if you have a 20mA LED and its maximum current is 0.2A you can drive the LED up to 200mA without a chance of breaking the LED.
This will make the display appear as bright as with the LED at 20mA continuously.
Step 5: Reverse Voltage
Not often a important value, but an get important when battery powering or using a LED on an AC voltage.
This is the maximum voltage the LED can have in reverse before it will break.
Most high power LED's this will be a very low value and will break instantly then exposed to that voltage.
Something to keep in mind when designing your battery holder or circuit.
There are several ways to protect against reverse voltages in a circuit.
A diode in series with the LED is an option, but for high power LED's this has to be a very powerful diode and will waste a lot of power.
A better idea is a diode anti-parallel on your LED, this diode will protect only your LED.
You should add a fuse that will break before the diode breaks otherwise the diode will just break and directly after that your LED will still suffer.
Step 6: What Can Go Wrong?
Therein lays the problem... Unless you grossly fail at getting it right the LEDs will supply light.
You will just notice that the LED banner spec of 10 000 or 50 000 hours is suddenly only 100 or 200 hours.
You start questioning the quality of the LED's when you should be questioning the LED drivers.
This is one of the main causes of death of LED's, either by accident* or on purpose** supplying too much current to LED's causing them to fail prematurely.
The other main cause is overheating, LED's and LED drivers like it very cool, if they are too hot to touch there is something really wrong.
* Sometimes because the inherent current limit in a system is removed, for example but putting better batteries in a cheap flashlight or using an AC adaptor instead of batteries.
Other causes can be swapping LED's by types with a lower forward voltage and forgetting to change the circuit accordingly.
** Often done by crap companies that just want to boast with '10W led' or 'high power' when they aren't but will work for long enough to question yourself and not the LED.