7 segment displays

rr

Hello,
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If I wanted to use seven segment displays to display static numbers (non changing) can I just tie all the necessary pins together to ground and the common anode to positive 3 volts. I know this will light but I am wondering if 3 volts is too much current for the displays (most are red and I know I usually power red leds with less than 3 volts). I am having a hard time finding some data sheets. I am also wondering if it is ok to wire these thing up in parallel like this, I usually wire led in series but there is no way to do this with a common anode. Basically I am just wondering what the best way is to wire up 7 segment displays for displaying static numbers???
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Thanks
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rr
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Phil Pilgrim (PhiPi)

You'll fry your display doiing that!

You've got to use current-limiting resistors: one for each segment that gets lit. And this will only work for a group of single-digit displays. Displays with multiple digits in one housing have to be time-multiplexed.

-Phil

RDL2004

For a static display you can probably get by with a single current limit resistor for each digit. However, when each digit has a different number of segments lit, the brightness will vary from one digit to another if you use the same value of current limit resistor for each digit. You could use a pot instead of fixed value resistor to ajust brightness of each digit so that they all match. You could also look at using a current limiting power supply for all the digits.





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- Rick

Phil Pilgrim (PhiPi)

The problem with using a single resistor for each digit is that the forward voltages of the individual LEDs are going to vary a little. What will happen is that the LED with the lowest forward voltage will dominate and the others in parallel with it will either be much dimmer or not light up at all. One resistor per lit segment is the best way to go.

-Phil

rr

ok new plann...

i need to wire up about three hundred of these displays and resistors add a lot of work. what if i regulate my voltage to·2 volts·using some thing like this.

http://focus.ti.com/lit/ds/symlink/pt6461.pdf

i know that i really need to get my multimeter out and make some mesurements. but i just want to make sure that as long as give the displays 2 volts i dont need resistors. i guess i could give them even less if i need (1.8 volts or something)...

do think it is·safe to think that as long as each segment sees less than 20 ma i am ok???





Thanks

Mike Green

One reason for the resistors is that there's a little variation from LED to LED and from transistor switch to transistor switch and you really want to hold the current to within a narrow range for each segment (for relatively uniform brightness) and the resistors will accomplish that. There are LED drivers that can also provide the "per segment" current limiting using a current regulator, but that's more expensive and probably as much work as using the resistors ... They're intended for multiplexing.

Paul Baker

rr, no that still wont work. LEDs are diodes and for every change in voltage across thier terminal the current increases exponentially. And due to variations of the process of creating them there is a variation in the slope of the curve, leading to varying brightness between displays using the same voltage. It is possible to do a no-resistor setup by using a current supply rather than a voltage supply, but it is far from a trivial task to do so.

It is possible to cheat a little by using a single resistor on the common pin, but you must calculate it's value based off of each display and the number of segments lit, and if you use really cheap displays, the variation in Vf (the voltage drop across the resistor when illuminated at target current) can cause some segments to become the main conduits of current in the display causing one or more segments to burn more brightly than other segments.

You are best served by purchasing SIP bus resistors, these are a series of·resistors whose one end are all tied together, and the other end you tie to each segment you illuminate. They take very little space and are easy to incoporate and wire into a design.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.