# seven segment display

Discussion in 'Homework Help' started by terrakota, May 28, 2006.

1. ### terrakota Thread Starter Well-Known Member

Feb 8, 2005
67
1
Hi,
i need to connect a seven segment display, without any kind of ic just a power supply and resistors, my question is :
i need 1 power supply for every segment???? how can i turn on the leds with only one power supply without burning the device?

im just learning how to use it, thats why i dont need any special ic, i just want to control wich segments turn on with only simple switches.

please excuse my poor english and

Aug 11, 2005
105
0
This depends on the details of your actual 7-segment display - is it just 7 LEDs, or does it have extra features?

You can run 7 LEDs in parallel off one power supply. Every LED needs a resistor in series with it.

In the case of 7 normal LEDs, you can split the power supply into 7 parallel branches, and put one LED, one resistor and one switch on each branch.

(I think I might have it right this time, but since I wrote nonsense on the first two attempts, take this with a pinch of salt...)

3. ### terrakota Thread Starter Well-Known Member

Feb 8, 2005
67
1
and the value of the resistors muts be ?
i need to know the led resistance? how i know this?

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

The hookup all depends on the display. They come in common anode or common cathode packages. You need to determine this, as well as the pinout. There should be a number on the display that will let you get a datasheet.

Each segment will require a current limiting resistor. The value will depend on the voltage you have to drive it with. You make the calculation as in any led device.

5. ### terrakota Thread Starter Well-Known Member

Feb 8, 2005
67
1
oh yea i just treat it like any led

May 30, 2006
17
0
A LED segment needs at least 2volts of forward current or more to operate. a current of about 5MA should be availible to light each section.

So if you have a 5volt DC supply it will need at least 5ma. X 7 segments = 45ma.

the resistor for each segment controls the current so it will not allow the diode to go into thermal runaway and short out. (Flash-poof)

680 ohms should be ok for each segment.