# LED Lighting 101 Question

#### Dawsona2

##### Newly Enlightened
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

the idea is that each component's input voltage matches the output from the previous component, right?

How important is a heatsink in this illustration?

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?

i guess that may not be just a quick question...but I really appreciate it!!

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#### DIWdiver

##### Flashlight Enthusiast
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!!

#### Dawsona2

##### Newly Enlightened

So, I understand heat/temp...I am more curious in the realm of homemade flashlights, per this forum category, specifically if a heatsink is needed in this application?

#### DIWdiver

##### Flashlight Enthusiast
What application? You haven't said anything at all about what you want to do. With no specifics, nobody will be able to answer your question.

We need to know the LED and how hard you intend to drive it, or at least how much light you want. The host, what you plan to do to it, how long you plan to use it, the environment, and what are your expectations, skills, and risk tolerance.

It's quite possible to build a light that needs no special heatsinking, or one that can't possibly be operated for minutes at a time without destroying something.

#### TEEJ

##### Flashaholic

So, I understand heat/temp...I am more curious in the realm of homemade flashlights, per this forum category, specifically if a heatsink is needed in this application?

Yup....look at the data for the LED etc, and if you are going to generate too much heat, then you need a heat sink, and/or another way to get RID OF the heat you are generating.

As you don't have any info for us to know what these factors are, its like saying you are building a car, and want to know if it needs a live axle or not.

So, if you are building a flashlight that will be generating enough heat, you need to be able to shed the heat.

The heat sink soaks up heat, but, as explained above, it can only hold so much, and then your LED etc, fries.

You have no way of knowing any of this right now...as you will need to learn a lot more before you can properly engineer a flashlight...let alone do thermodynamic calculations.

Plan B - Add a massive heat sink and cooling fins to help shed heat. If you don't need it, the light is merely heavier and bulkier than needed. If you do, you have it.

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#### Dawsona2

##### Newly Enlightened
Yup....look at the data for the LED etc, and if you are going to generate too much heat, then you need a heat sink, and/or another way to get RID OF the heat you are generating.

As you don't have any info for us to know what these factors are, its like saying you are building a car, and want to know if it needs a live axle or not.

So, if you are building a flashlight that will be generating enough heat, you need to be able to shed the heat.

The heat sink soaks up heat, but, as explained above, it can only hold so much, and then your LED etc, fries.

You have no way of knowing any of this right now...as you will need to learn a lot more before you can properly engineer a flashlight...let alone do thermodynamic calculations.

Plan B - Add a massive heat sink and cooling fins to help shed heat. If you don't need it, the light is merely heavier and bulkier than needed. If you do, you have it.

Thanks!

So....to take your lead....I am building a flashlight...not a car, and as such...with an LED light...how [in the illustration I put forth] important is a heatsink?

examples of answers are: unless you are building a monster flashlight, you do not need a heatsink; 98% of LED lights do need a heatsink; etc

Specifically, would a XML LED need a heatsink?

You have no way of knowing how much experience I have with thermodynamic calculations. But, I can tell I better not ask you.

#### TEEJ

##### Flashaholic
Thanks!

So....to take your lead....I am building a flashlight...not a car, and as such...with an LED light...how [in the illustration I put forth] important is a heatsink?

examples of answers are: unless you are building a monster flashlight, you do not need a heatsink; 98% of LED lights do need a heatsink; etc

Specifically, would a XML LED need a heatsink?

You have no way of knowing how much experience I have with thermodynamic calculations. But, I can tell I better not ask you.

Sigh.

How hard are you going to drive that XML2 (I recommend the XML2 over the XML)?

If not hard (So as to maximize run time) no, you don't.

If harder, (So as to maximize brightness), then, yes if hard ENOUGH.

Ironically, so far, all I know is you're building that car, and you mentioned it WILL have an engine, but, still want to know if it needs a live axle.

As we at least know the light engine, the answer would be analogous to that engine pulling a truck, or, a go cart....and, we need to radiate the heat from it.

Soooooooo

Are you pulling a light load, a heavy load / going for run time or lumens?

mpg or mph?

#### Dawsona2

##### Newly Enlightened
Thank you for your help! I apologize for my rudeness....been a long few days here.

So, I was being vague and non-specific for a reason...I was looking for specifics and principles rather than exact answers. That is why the thread was titled "101", just the basics

It appears that the answer is....it depends!! Great, I can now dig deeper!