Suggestions to improve this 7 segment display circuitry

Thread Starter

chrisjsmith

Joined Nov 12, 2016
41
As Dave says, ditch R2, R4, R6, R8.
Install resistors in A-G and DP as Dave says. 470Ω (220Ω if the digits are multiplexed).
Ok understood. Justification for swapping the resistor locations would be to limit current entering the display segments. Looking at my transistor datasheet the Vce rating is 40 V so having those resistors at the collector is pointless. Is my line of thinking correct
 

AnalogKid

Joined Aug 1, 2013
10,971
IF it is a multiplexed LED display (any particular reason why that detail is a secret?)
AND IF it is common-cathode
THEN the four pullup resistors R2-R8 are not necessary.

IF the segment drive signals are not current-limited externally
THEN you need to add 8 resistors, one for each segment.

ak
 

AnalogKid

Joined Aug 1, 2013
10,971
Is my line of thinking correct
Unlike a logic gate of opamp unused input, an LED can have a completely floating pin with no problems. Since only one digit is energized at a time, the other three digit's anodes are floating when their drive transistors are off. That is ok; the 1K pullup is not needed.

ak
 

Thread Starter

chrisjsmith

Joined Nov 12, 2016
41
IF it is a multiplexed LED display (any particular reason why that detail is a secret?)
AND IF it is common-cathode
THEN the four pullup resistors R2-R8 are not necessary.

IF the segment drive signals are not current-limited externally
THEN you need to add 8 resistors, one for each segment.

ak
Unlike a logic gate of opamp unused input, an LED can have a completely floating pin with no problems. Since only one digit is energized at a time, the other three digit's anodes are floating when their drive transistors are off. That is ok; the 1K pullup is not needed.

ak
Thanks Analogkid it is multiplexed i forgot to mention
 

dannyf

Joined Sep 13, 2015
2,197
to improve efficiency
depends on what you meant by "efficiency".

The general solution is to put resistors on the segment lines.

Putting resistors on the digit lines saves a few resistors (4 exactly in this case) but it can cause variable brightness.

so the best solution depends on your design objectives.
 
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