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Old 08-29-2009, 05:17 PM
jlon jlon is offline
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Thumbs down common cathode LED display limiting resistor?

I'm want to design a circuit with some 7-segment LED displays driven from a 4511 or some variant. Now, I haven't burned up anything in quite some time. So before I burn anything up, I'm wondering:

Is it okay to put a single current-limiting resistor on the common cathode, or is it a necessity to have a separate current-limiting resistor on each LED segment? Can someone experienced in this give me a hint?

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Old 08-29-2009, 05:18 PM
Mike2545 Mike2545 is offline
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You will need a separate resistor for each segment.
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Old 08-29-2009, 05:43 PM
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hgmjr hgmjr is offline
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I too recommend you go with a resistor per segment. If you try to get away with one resistor, you will find that the brightness of the segments will change as a function of the number of segments you have illuminated at any one time.

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Old 08-29-2009, 05:45 PM
jlon jlon is offline
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Default thanks!

That's going to be lot of resistors, but I don't want the display to change in intensity. So thanks to all!

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Old 08-29-2009, 08:41 PM
rjenkins rjenkins is offline
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If you are multiplexing several digits, you only need one set of segment resistors as only one display is actually drawing current at any time.
Robert Jenkins.
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Old 08-29-2009, 11:03 PM
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Bill_Marsden Bill_Marsden is offline
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If you research it they make 7 resistor arrays, in a 14 pin DIP package. Resistors are cheaper though.
"Good enough is enemy of the best." An old engineering saying, Author unknown.

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If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
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Old 08-30-2009, 01:32 AM
jlon jlon is offline
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Yes, I've seen the resistor arrays. I'd rather just solder some 0805 packages. Not hard at all and much cheaper, lots of flexibility in positioning.
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Related Site Pages
Section Title
Worksheet Digital display circuits
Textbook 7-segment display : Digital Integrated Circuits
Textbook Special-purpose diodes : Diodes And Rectifiers

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