Computing stuff tied to the physical world

Easy Electrons – LEDs

In Hardware on Dec 19, 2010 at 00:01

This is the second installment of the Easy Electrons series. This isn’t a “course” in electronics. Just a grab bag of topics, and an attempt to convey my somewhat intuitive grasp of them. That’s also why I’m not going to systematically cover all the usual types of components, not in the traditional order anyway.

Today’s topic is low-power signaling LEDs, or Light Emitting Diodes.

Heres a very nice exposed diagram of a standard low-power red LED, from HowStuffWorks:

Led Diagram

Schematically, a LED is shown as an arrow (same as a diode), with light coming out. Here’s the Blink Plug, with two LEDs and two switches:

Screen Shot 2010 12 12 at 14.42.29

Here is the way you hook up a single LED:

Screen Shot 2010 12 12 at 14.47.06

The arrow has to point from + to – for the LED to work. It’s a diode, meaning that in the other direction it just “blocks” (no current, nothing happens). Also note that, since this is a series circuit with LED and resistor connected one after the other, it doesn’t really matter in which order you hook them up. This circuit will work just as well if the resistor comes first. As long as + “flows” to – through the LED.

So what’s that resistor doing in there?

Well, LEDs (like any diode) are very peculiar in terms of voltage and current. They behave in a specific way:

  • below a certain voltage, nothing happens, i.e. no current flows
  • when no current flows, the resistor makes no difference at all (E = I x R, i.e. 0 x anything is still 0!)
  • above a certain threshold, the LED starts conducting and letting all current through
  • the LED only emits light above that threshold
  • the brightness of a LED is determined by the amount of current through it
  • LEDs only support a certain amount of current, any more will damage them

To use the water analogy, an LED is like a dam which will not let water through before a certain level has been reached. Once that happens, everything more spills over:

Screen Shot 2010 12 12 at 15.09.35

The threshold depends on LED type: with red LEDs, it’s usually around 1.7V, with blue LEDs, it’s more like 3V.

You can’t just connect an LED directy to a power source. Think about it. LED behavior can be graphed as follows:

Screen Shot 2010 12 12 at 14.47.16

In other words: if the voltage is too low, nothing will happen. And if it’s too high, the current through the diode will be immense (and destroy the LED). We could try to regulate the voltage just right, but it varies slightly per LED and also depends on temperature, for example. There’s no way we can adjust our power supply once and get just the right level, day in day out. Besides, often we can’t even adjust the voltage at all, such as with a battery.

So how do we solve this?

Well, that’s where the resistor comes in. This is an example of another major use of resistors: current limiting.

Again, go back to Ohm’s law, i.e. E = I x R (memorize it, please!). The more current flows through a resistor, the higher the voltage over it. Or equivalently, the higher the voltage placed on it, the more current will flow.

In the above LED circuit, the resistor will always have the input voltage minus the LEDs threshold voltage over it (once the input is higher than the threshold). The effect on the voltage and current in this circuit changes in an important way due to the extra resistor:

Screen Shot 2010 12 12 at 15.09.44

The vertical “knee” in the original LED graph has turned into a more gradual slope, due to the added resistor. As more surplus voltage is handed over to the resistor, it starts to gradually use more current. This effect is completely linear, btw. An LED with a 1.7V threshold will be twice as bright with a 4.9V power supply as with a 3.3V supply.

Ok, well, so much for the pictures. What this tells us is that we can now calculate exactly what resistor we need.

For example, say we want to light a LED using a 3.3V power supply and let 10 mA (0.01A) current flow through it, which is a good value for standard LEDs. What resistor do we need?

  • the LED “takes” a fixed 1.7V
  • that leaves 1.6V for the resistor when the supply is 3.3V
  • we want to get 10 mA flowing through the LED
  • since the resistor is in series, it’ll get those same 10 mA
  • E = I x R can also be written as R = E / I (same law, different usage)
  • so R needs to be 1.6 V / 0.010 A = 160 Ω

Take a LED, add a 160 Ω in series, and the LED will work great on a 3.3V power supply. A more easily available 150 Ω resistor will work fine too, BTW. Almost the same currrent.

What if you don’t have a 150 Ω resistor, but only a 1 kΩ one? No problem, the current through the LED then becomes: (I = E / R), i.e. 1.6 V / 1000 Ω = 1.6 mA – usally still enough to turn the LED on, but not as bright.

Easy Electrons!

  1. Great introduction!

    I was wondering about why you use the letter “E” for Voltage in your articles? This doesn’t change anything about the use or correctness of Ohm’s law. Just that I, as most people in Europe (?), learned the letter “U” for voltage. And most other people use the letter “V” for voltage.

  2. In physics class we used E (“electromotive force”). In electrodynamic fields it was E and H for electric and magnetic fields, respectively.

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