Dumb Question: 7-Segment LED - Why 8 resistors ?

bdb

In the What's a Microcontroller, Chapter 6, the 7 Segment LED is set up with 8 resistors (1 k-ohm).· 1 resistor for each LED on the 7-segment LED unit.

Of course, I understand that each LED in the LED unit is in parallel.· And, of course, you want to provide a resistor to throttle the current for each LED.

However, instead of using 8 resistors, it seems like you could just add one kilo-ohm resistor to the common ground of the unit and have the equivalent circuit.· So why not go with the 8-resistor setup ?

The only thing I can possibly see is that in the books use of 8 resistors, the resistors are on the high voltage side of the unit, so if one·LED is high and another LED is low and should they short out, this would still protect the Basic Stamp.· But on the other hand, using 8 resistors close together with a breadboard, it seems like the 8 resistor setup increases the possibility of the resistor leads touching.

Thanks for any help.

InSilico

Actually you would not get an equivalent circuit with the single-kilo-ohm resistor setup you propose.

The reason why is that not all the segments on the 7-segment display is going to be on all the time. Displaying an "8" consumes more current as opposed to a "1".

Because the current consumption varies, so will the voltage across the single resistor (remember Ohm's Law: V=IR), which causes the voltage across the LEDs to vary as well. So what you get is that the LED segments will vary in brightness depending on how many segments are on at once.

With the parallel network of resistors, each LED will be the same brightness no matter how many segments are on at once.

When constructed properly, the resistor leads should not touch. If you're concerned, you can be·like some of my friends who put insulation on the resistor leads themselves. Or you can obtain a resistor networks, which have multiple resistors in a single SIP or DIP package like a 4116R-1-102LF you can get at Mouser: (http://mouser.com/Search/ProductDetail.aspx?qs=ehdiChDUEdUNHnqeuqk8Ew%3d%3d) which is 8 1K-ohm·1/4W resistors in a 16 pin DIP package.

Hope this helps!

Post Edited (InSilico) : 1/4/2009 5:37:03 AM GMT

bdb

Thanks InSilico - you're right - I was forgetting about the voltage drop across the LED - so the circuit with one resistor would work but have strange fluctuations in brightness - more than enough reason to spend a few cents for resistors. Or it looks like if I was going to do this "for real" the DIP would be even easier.

Again, thanks