How do you power a 7 segment display?

Answer 1

Each LED in the display must be powered separately with a current source. The required current depends on the ratings of the LED and the desired brightness level. The usual grubby 7-segment display will accept 10-20mA and be adequately bright over that range. Most 7-segment displays have 8 or 9 LEDs (one for each segment, one for the decimal, and sometimes one for a high left-side decimal). Most displays have common cathodes and independent anodes.

Each anode will typically be powered independently of the others, often by the output of a microcontroller or converter/driver chip (BCD to 7 seg or binary to 7 seg). As these outputs are often un-limited voltage sources, a limiting resistor must be used in series with the output powering each LED.

Calculating the resistor value is simple. Use the formula R = (Vsrc-Vdrop)/Iseg, where Vsrc is the source voltage of the output, Vdrop is the forward voltage drop of the LED (Typically you can approximate this to 2V for most LEDs.), and Iseg is the current desired. Assuming you have 12V outputs and want around 15mA driving each LED, the resistors should be (12-2)/0.015, or 666ohms. (680ohms is the nearest standard value.)