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Topic: 7-segment display and resistors question  (Read 733 times) previous topic - next topic

firashelou

hello guys,
i have a small question about resistors used with the 7 segment (common anode and cathode):

let's start by common anode, so i connect the common anode to +5, my question is, is it better to use 1 resistor on the common or many resistors on each segment ?

same question for the common cathode
Aiming for the unreachable !

MarkT

#1
Aug 07, 2015, 11:29 am Last Edit: Aug 07, 2015, 11:29 am by MarkT
One for each segment or you'll get the digit '1' much brighter than the '8'.  With a single
resistor you are also depending on the segments being voltage matched or current will
distribute unequally between the segments (sometimes the output impedance of the driver
is enough to help balance this, and usually the segments are identical LED dies from the
same batch so they are as well matched as you can get)

There is an exception to this, if you are multiplexing such that only one cathode at a time
is active, one resistor will be fine.   Normally though you would drive all the cathodes
with one anode active, then step to the next anode.


Common cathode is exactly the same issues.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

billhowl

Better to use many resistors on each segment, if use 1 resistor on the common, the digits light up with different levels of brightness depending on how many segments are lit.

Because this way of connection will make it the more segments that light up the more current the resistor carries and less current to each segment and the duller it gets.


firashelou

One for each segment or you'll get the digit '1' much brighter than the '8'.  With a single
resistor you are also depending on the segments being voltage matched or current will
distribute unequally between the segments (sometimes the output impedance of the driver
is enough to help balance this, and usually the segments are identical LED dies from the
same batch so they are as well matched as you can get)

There is an exception to this, if you are multiplexing such that only one cathode at a time
is active, one resistor will be fine.   Normally though you would drive all the cathodes
with one anode active, then step to the next anode.


Common cathode is exactly the same issues.
aha yes you're right i forgot about that  :-X  :smiley-zipper:

about the multiplexing, you mean if i have a 2 digit anode and i want to use 1 segment only from each ?
Aiming for the unreachable !

MarkT

There's no such thing as a two-digit anode.  Two digits each with a common anode, yes, exactly.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

firashelou

There's no such thing as a two-digit anode.  Two digits each with a common anode, yes, exactly.
ok thanks
Aiming for the unreachable !

Paul__B

Does it clarify things to say that however it is arranged, each LED that may be illuminated at the same time as another, must have its own resistor?  The same resistor may be common amongst LEDs that will be lit by the multiplexing at different times.

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