  LED driving and controlling methods AuthorGiorgos LazaridisFebruary 5, 2012

PAGE 1 of 6 - LED voltage control with limiting resistor      This is not an LED theory page, therefore i will not go into details about the LEDs. You can read the LED Theory for more extensive info. There are some things though that you need to have in mind:

LEDs are current-depended components. People usually confuse voltage and current, because current depends from the voltage. Same happens with LEDs, the current that an LED will draw depends on the voltage that is applied. But the brightness of the LED depends on the current that it will draw. Ideal LEDs will have the same brightness for the same voltage, since they will draw the same current. But real-life LEDs will draw different current even at same voltages due to slight differences in the P-N materials, so they may appear having slightly different brightness.

LEDs have different forward voltage. We use to say that the LED forward voltage is 3 volts, but this is totally wrong. Different LED colors have different forward voltages and this depends on the chemical that is used. Moreover, LEDs with higher power rates have different supply voltage. So you always need to refer to the manufacturer for the correct voltage.

LEDs have polarity. Just to remind that LEDs have positive and negative, anode and cathode...

The maximum forward voltage is critical. The voltage-to-current characteristic of an LED is similar to a diode. If the maximum forward voltage is exceeded by little, the current is increased logarithmically. So, it may be that the LED is off at 2.5 volts and max brightness at 3.2, but at 3.8 the LED may be in danger. Never exceed maximum forward voltage.

LEDs love being connected in series and hate being connected in parallel. Theoretically, you can connect LEDs both in series and parallel. If you connect them in series, you will need double the voltage to turn them on, and they will draw the same current as one LED. Avoid connecting LEDs in parallel, unless you are absolutely sure that they are completely identical. If not, then all current may go though one LED and it can be damaged! To safely connect LEDs in parallel, then you need to add one resistor for each LED. This is called balancing resistor and balances the current that flows within each LED.

LED voltage control with limiting resistor
This is the most commonly used method to control low-current LEDs, because of the low price (can't get any lower) and the simplicity of the circuit (can't be any simpler either). I'm talking about the limiting resistor method. A resistor is connected in series with an LED. The resistor causes a voltage drop across its leads. With proper calculations the LED gets the proper forward voltage to operate. Here is a typical circuit: A very common mistake that people do when they try to calculate the resistor R, is that they use the Ohm's law to calculate the R for given V (Vdd) and required I (LED's forward current). This leads to wrong results though. Let's see an example. Suppose that the LED has forward voltage 3 volts and at that voltage it draws 20mA (0.02 Amps). The power supply is 5 volts.

A WRONG calculation is;

R = V / I => R = 5 / 0.02 => R = 250 Ohms [WRONG CALCULATION]

Why is this wrong? As we said above, LEDs are current depended components and they regulate the current themselves from the forward voltage they're provided. You cannot calculate their current directly! Instead, you can calculate the voltage that needs to be dropped across the limiting resistor R, so that the LED will operate to the desired Vf and If (Forward Voltage and Forward Current).

To do this, you first need to calculate the desired voltage drop across the resistor. Simply subtract the LED's forward voltage from the source:

VR = VDD - LEDVf => VR = 5 - 3 = 2V

So, we need to drop 2 volts on the resistor, and the rest will be delivered to the LED. Now we can use the Ohm's law to calculate the resistance of the limiting resistor. We know that we want to drop 2 volts, we know that we want to operate the LED with 3 volts, and we know that the LED draws 20mA at 3Volts. Since the resistor is in series with the LED, the same amount of current will flow through. Here is the formula to calculate the resistor:

R = VR / LEDIf => R = 2 / 0.02 => R = 100 Ohm

The proper resistor value is 100 Ohms. But this is not the end. Resistors have also a maximum power dissipation rate. We say 100 Ohms 1/4 Watt, which means that this resistor can dissipate maximum 250 mWatts. To calculate the dissipated power on the resistor, we will use the I2R formula:

PR = IR2 x R => PR = 0.022 x 100 = 0.04 = 40 mWatts

For this example, a 1/4 watt or 1/8 watt resistor is enough. You need to make sure every time that the resistor wattage is sufficient for the application, otherwise you will end up with a black smoking resistor.

• On-Line LED resistor calculator

• Cheapest method - can't get any cheaper
• Simplest method - Can't get any simpler or smller
• Perfect for small low-current (<100mA) indicating LEDs on circuits

• Very bad efficiency factor - Lot of power is dissipated on the resistor as heat
• Cannot be used for high-current LEDs due to the high power dissipation
• Unstable - A small change on the supply voltage will cause a large change on the LED current

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