Series Resistor
Series resistors are a simple way to stabilize the LED current, but energy is wasted in the resistor.
Miniature indicator LEDs are normally driven from low voltage DC via a current limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficiency tends to reduce at low currents, but indicators running on 100 μA are still practical. The cost of ultrabright LEDs is higher than that of 2 mA indicator LEDs.
In coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type.
LEDs can be purchased with built-in series resistors. These can save printed circuit board space and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. Alphanumeric LEDs use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. Seven-segment and starburst LED arrays are available in both common-anode and common-cathode form.
Read more about this topic: LED Circuit
Famous quotes containing the word series:
“Every Age has its own peculiar faith.... Any attempt to translate into facts the mission of one Age with the machinery of another, can only end in an indefinite series of abortive efforts. Defeated by the utter want of proportion between the means and the end, such attempts might produce martyrs, but never lead to victory.”
—Giuseppe Mazzini (1805–1872)