So lets talk about Constant Current Drivers. What does that mean exactly? What is the difference between Constant Current and Constant Voltage? What is the difference between the two anyways?
Wikipedia says: “Electric current means, depending on the context, a flow of electric charge or the rate of flow of electric charge. This flowing electric charge is typically carried by moving electrons, in a conductor such as wire.” Voltage is a difference in electrical potential. For example, a charged battery has two terminals, and Postitive and a Negetive (denoted by the + and the -). The negative contact on the battery has an excess of free electrons, and they are looking to go somewhere to restore balance in the world, whereas the positive side has a deficiency in electrons, and therefor has a positive charge. This is the fundamental of Voltage, it is simply the theoretical potential difference in charge between two substances. In a perfect world, the only way to get the electrons from the negative end of the battery to the positive end, you need some pathway for the electrons to flow, lets call it a conductor. Some materials conduct electricity better than others, Copper is an especially good one, having an excess of electrons that like to travel around and socialize. So lets say you attach this conductor (a piece of copper wire) between the positive and negative terminals on a battery, what do you get? Physical electrons flowing from one place to another. This is Current. If you put something in between the Negative and Positive terminals, say a light bulb (which is in reality just a resistor (a conductor that doesn’t conduct very well and produces a lot of heat in the process) in a special atmosphere so it doesn’t combust), you have a flashlight.
Now that we know what Voltage and Current are, we can start thinking about how all of this applies to LED’s, because that is all I care about. LED’s are special in some ways. With a normal resistor, as you increase the voltage across it, the current increases in a linear fashion as well. Mathematically: V(voltage)=I(current)xR(resistance); if R is constant, V is directly proportional to I. Diodes are significantly more complicated, but a picture can explain it better.
As you can see, as the Voltage increases by a pretty insignificant portion (.2-.4v), the current it draws increases significantly. Thus the importance of having a current limiting regulator. But how is that achieved? Well, allow me to explain…
Below is the circuit schematic of the Constant Current driver I am playing with currently.
The main current flow travels through R3, Q2, and the LED’s. Q2 is used as a variable resistor in this application.
When too much current flows through R3, Q1 will begin to turn on. Q1 is acting as a switch, when its Base (B) pin gets a certain amount of current in begins to allow electrons to flow between the Emitter (E) and its Collector (C ), which in turn applies current to the Ground (G) pin of Q2. This causes Q2 to clamp the current flowing through it, reducing current through the LED’s and R3. This will turn off Q1, eliminating the negative charge on Q2′s G pin, unclamping the current flowing the LED’s and starting the whole process over again. R1 is there to keep a positive bias on Q2′s G pin unless Q1 is on. The current through the system is controlled through the resistance of R3, because it controls Q1, which controls Q2.
Simple, right?