![]() The easiest thing to do is add a potentiometer in series with the LED. Rather than control the voltage, it’s better to control the current passing through the LED directly. Both have a 3 V potential difference but the amount of current supplied by either is different and consequently, the brightness is different. The brightness will be different if you connect a coin cell as opposed to two alkaline batteries. Well, there is just one small hiccup that the curve of the forward current is so steep that a small increment in voltage will have a larger change in current. Given that the LED follows Ohm’s Law, the current should be directly proportional to the voltage and hence we can vary the voltage to control the brightness. The second curve is the relative LI vs forward current which shows that the current controls the amount of light output (the straight line stretching up to the “4” mark). The current rises ohmically after that and at around 3 V, it is reported to draw around 200 mA. ![]() The first is the forward current and voltage graph which shows that a voltage of around 1.8 V is enough to forward bias the LED. Two more important pieces of information are used which are represented as graphs. The values are 350 mA and 500 mA respectively and should not be exceeded. The datasheet has some pretty important information starting with forward current(continuous) and peak forward current. Here is a quick look at some of the parameters of for the LED. This one is rated at 90 Lumens and comes with an aluminum PCB as a heat sink. Let us start with a simple 1 W SMD LED like the one available from Adafruit. This means that for a 5-degree spot the source needs to increase by a factor of 8 to seem twice as bright and a point source, needs to increase by a factor of 4 to seem twice as bright. For a 5 degree spot the exponent is about 0.33 but for a point source, it is about 0.5. Perception of light intensity follows Stevens’ Power Law with an exponent that depends upon the amount of your field of view occupied by the light. In reality, human eyes are logarithmically sensitive to intensity change which means that doubling the intensity will be perceived as a small change. ![]() So how is brightness perceived? Logically speaking, when you have two LEDs lamps of 100 lumens each, the result should be double the brightness. On the contrary, a disco light will require fluctuating intensities of various colored LEDs. A lighting application such as a work bench light will seldom require a romantic mood light control. In any case, it is essential to have a clear understanding of the end application. A little further down the line, it comes down to brightness control and then mixing of colors to produce any shade from the color picker. Most newbies will be interested in making an LED glow without blowing it up. Let’s take a look at the problem and then discuss the solutions. An efficient driver can make all the difference if you plan to deploy them for the long-haul. The idea is to be able to effectively control the brightness of the LED and prolong their life while doing it. There is no “one size fits all” but I will try and generalize as much as possible. In this write-up, I want to give some examples of driving LEDs and comparing a few of the most commonly used methods. In a previous article, I discussed LEDs in general and their properties.
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