Hi all, got time for a quick question, please?
if I am understanding the basic principle of LED lighting...would this be a complete setup:
battery(xV)-->on/off switch(xV)-->LED Driver(xV)-->LED on PCB(xV)
where 'x' = a voltage amount
Electrically, yes. That's a complete setup. Of course you need body, reflector, lens, heatsink, etc.
the idea is that each component's input voltage matches the output from the previous component, right?
Yes, that's right. However, the driver has an output current, not a voltage. The output voltage of the driver is determined by the LED.
For example, suppose we have a "14.8V" battery, which when fully charged is actually 16.8V, and when discharged is only 12V (this is pretty typical of a 4-cell LiIon pack). Now let's say we have an ideal buck driver with a 3A output. With a buck driver, the output voltage has to be between 0V and the battery voltage. That's the nature of a buck driver. Now say we connect an SST-90 LED to that driver and turn on the switch (which I conveniently haven't discussed). The driver wants to put out 3A, and the LED says 'at 3A, I need a voltage of 3.7V'. So the driver output will be 3.7V at 3A.
Now say someone points out that at 3A, an SST-90 is really overkill, and more expensive than we need, that we should be using an XM-L2. We swap it out. The XM-L2 says 'at 3A, I need 3.35V'. So now the driver output is 3A at 3.35V. As the LED heats up, it needs less voltage, so after a while the LED may only need 3.2V. Then the driver output would be 3A at 3.2V.
If we wanted more light out of this thing, we can use 2 XM-L2's in series. Initially the driver output would be 3A at 6.70V (2x3.35V), and after warmup it would be 3A at 6.4V. Running 3 LEDs is a no-brainer at this point, but it gets interesting at 4 LEDs.
Initially, the driver output would be 3A at 13.4V. The driver has no problem with this when the battery voltage is 16.8V. But when the battery voltage drops below 13.4V, something interesting happens. The driver can't put out 13.4V any more. So what happens? It puts out the maximum voltage it can, which is the input voltage. As the battery voltage continues to drop toward 12V, now the LED string is saying 'you can't supply 13.4V, so I won't accept 3A any more'. At, for example, 13V, the LEDs might only draw 2.7A. At 12V, they might only draw 0.5A.
A discussion of non-ideal drivers will be left for later.
How important is a heatsink in this illustration?
That depends on how much current you are putting into the LED. For a '1 watt' LED, the current is usually set at about 350 mA (0.35A). If mounted on a 25mm star, a 1W LED probably doesn't require much if any heatsinking. But as you go higher and higher in power, heatsinking becomes continuously more important. Above a few watts (1A or more of drive current), heatsinking becomes a critical part of the design.
I read a thread that talked about making a round heatsink, so that it fits just so, then mount the driver on one side and the LED on the other. Would you use thermal compound on the heatsink? Or back of PCB driver or LED PCB?
At this point it becomes important to understand the true meaning of 'heatsink', as well as the difference between 'heat' and 'temperature'. In common usage, the terms 'heat' and 'temperature' are used pretty interchangeably. That's all well and good, I even do it myself (though probably less than other people do). But when you want to design heatsinks, it's really important to understand the difference.
Imagine you are blowing up a balloon. If you have a constant flow of air into the balloon, and no exit, what happens? The pressure in the balloon continues to rise, and eventually the balloon exceeds its maximum pressure, and it bursts.
Now imagine you are pumping heat into a heatsink at a constant rate (say 2 watts of heat). If that heat can't exit the heatsink the temperature continues to rise, and eventually something fails.
Now imagine an exit orifice in the balloon. The higher the pressure in the balloon, the faster air exits the balloon. Initially, air is going in faster than it goes out, and pressure builds up. Eventually the pressure builds up to a point where the air flow out of the balloon equals the airflow in. At this point the pressure will stabilize (stay the same), as the flow in equals the flow out. This is called equilibrium.
Sorry, it's getting late and I can't make sense any more. To translate this into heat and temperature is a task that I need to take up later. Here's a teaser: airflow into the balloon is like heat into a heatsink. Pressure in the balloon is like temperature in the heatsink.
i guess that may not be just a quick question...but I really appreciate it!!
