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What makes LED lights dim up and down automatically when on Is it a capacitor or some other thing built in to the circuit?
Wiki User
∙ 13y ago
LEDs are current based devices in that they will emit more light as more current passes through them. Light emitting diodes are like other diodes in that they require a certain voltage across them before they will pass a current. Typically, an LED will require between 2 and 4 volts.
LEDs need a controlled current in order to prevent damage. Often, the current is controlled using a fixed voltage and a series resistor. The resistor will fix the current at a calculated level. If the voltage across the LED and resistor varies, so the current will also vary and hence the brightness as well.
Although capacitors are often seen around LEDs in circuits, they do not directly play a part in controlling the light intensity.
Many variable brightness LED circuits use a technique of pulse width modulation. A series of pulses at high frequency (1KHz upwards) are used to drive the LED. When the pulse in on, it will generate a voltage sufficient to turn on the LED at full brightness. When the pulse is off, the LED will also be off. With a very short pulse, the LED will be off far more than it is on so the LED will look dim. As the pulses get longer, the LED will be on for a greater proportion of the time so it will look brighter. Eventually, each pulse can be made so long that it will meet the next pulse in the stream of pulses. That means that the LED is on all the time and will be at maximum brightness.
In some cases, the current switching may not be desirable so a capacitor might be added into the circuit. When the pulse is on, the capacitor will be charging so the voltage across the capacitor will increase. The LED and resistor will be drawing current all the time so as soon as the pulse is turned off, the voltage across the capacitor will reduce over time. If the width of the pulse is small, the average voltage across the capacitor and therefore across the LED and resistor will be small, so the brightness will be low. As the width of the pulse gets wider, the capacitor is charged more and the average voltage will rise as a result, so the LED glows brighter.
The stream of pulses and the width of the pulses can be generated in several ways but it is common to use a processor such as a micro-controller rather than more traditional hardware.
Wiki User
∙ 13y ago
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