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Topic: 7 segment tutorial calls for 8 resistors...can't I just do 2 (or 1?) (Read 1 time) previous topic - next topic

3z33

This tutorial

http://www.hacktronics.com/Tutorials/arduino-and-7-segment-led.html

says "Connect LED pins 3 and 8 to GND.  Use a resistor between each of the other connections to your Arduino"

I wired it up that way, and of course it works...but just got me thinking:

since its all in series couldn't I just connect pins 3 and 8 to GND using a resistor (for each) as the jumper?  Doesn't that effectively do the same thing as the 8 resistors?

also, is it ok to send both pins 3 and and 8 to the GND rail on my breadboard, and then jump that to GND using only 1 resistor?

I thought that when wiring in series, you can put the resistor wherever you want and it effects the whole circuit?

johnwasser

The segments are in parallel so it DOES make a difference.
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westfw

If you connect only the common pin through a resistor, the segment brightness will vary depending on how many segments are on.  The best you can hope for is that the segments would equally share the current limited by the resistor, so brightness of all segments would depend on how many segments were on at one time, but other scenarios are possible if some segments use more than one LED...

3z33


liudr

Besides varying brightness with amount of LEDs on and on different LEDs as explained, you also should watch out for excessive current through the resistor. A typical resistor is 1/4 W rated. With say 2.5 V dropped across it, it takes a maximum of 100mA, which may not be enough when many of the LEDs are lit. Say each segment needs 10mA then you're getting pretty close to max. You may need two resistors parallel in this case. If I were to do this, I would go for separate resistors any day of the week.

floresta

Quote
A typical resistor is 1/4 W rated. With say 2.5 V dropped across it, it takes a maximum of 100mA, which may not be enough when many of the LEDs are lit.
Keep in mind the fact that the 1/4 watt rating is based on a resistor in free air, not near anything else and not inside any kind of small enclosure.  It's based on how much heat the resistor can withstand without harming itself.  Running near full rating the resistor may not get hot enough to destroy itself but it may overheat nearby components and you certainly wouldn't want to touch it.

Don

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