# How to connect a seven segment LED

#### Roger at CCCC

Joined Jun 8, 2009
17
Very simple question, I know, but I can't find the answer:
I bought a 4 digit 7 segment LED Part Number LTC-4727JS made by LiteOn. It's the first time I am trying to use one of these.
The datasheet at
http://optodatabook.liteon.com/DataBookFiles/12826/C4727JS.pdf
doesn't seem to say (or I can't understand) whether each segment needs a resistor to limit current. I want to know this before I connect it so I don't blow it up the first time. Can anybody tell me whether each segment needs a resistor, and if so, what size? And if I could figure this out myself from the datasheet, how should I have done that? If it DOES need a resistor, what's the best way to handle the 28 resistors that would apparently be needed?

#### beenthere

Joined Apr 20, 2004
15,819
The display is strobed by applying ground to each of the segments for a period of time. You only need 7 resistors in the common anode lines.

#### Audioguru

Joined Dec 20, 2007
11,249
The datasheet shows the Absolute Maximum allowed peak momentary current and the continuous current at room temperature.
Your circuit must limit the current so the current ratings are not exceeded.

#### davebee

Joined Oct 22, 2008
540
The current does have to be limited, but there are several ways to do this.

You can sometimes get by with using only a single resistor in the common line per seven segment unit.

Some controllers turn individual segments on, one at a time, at a very fast repitition rate, so at any given instant, only one segment's current is being pulled through the resistor. With a fast enough repitition rate, the display looks to the eye as if it were on steadily. This design works great with just one resistor per seven segments.

But if instead your controller simply turns on the desired segments without pulsing, then this method won't look so nice, because varying amounts of current will be pulled through the resistor depending on the digit being displayed, resulting in a dimmer display when there are more segments lit. That design should have one resister per segment.

The value of the resistor is whatever is needed to allow the current needed to light the displays as bright as you want them, based on whatever driving voltage you use. It wouldn't hurt to experiment, starting at a relatively high resistance, and see how the display looks. Lower the resistance until it looks as bright as you want.

A good place to start would be by picking some midrange current from the datasheet chart, like 5 or 10 mA, wire up a test circuit, and see how the display looks at that drive level.

You can buy resistor arrays in DIP packages that combine 8 or more resistors to simplify the parts count if you do wind up wanting a design of one resistor per segment.