[sdiy] 7-segment LED displays with 4511 driver chip - max current?

[Richie Burnett]

Provided you're not multiplexing the three displays on one 4511, then 7mA 
per segment should be more than enough current.  Make sure you choose decent 
high-efficiency 7-segment displays though.  Some of the old 
standard-brightness LED displays can be quite poor brightness wise, and need 
a bit more current.  I wouldn't recommend exceeding the total package 
current rating for the driver.

Personally, I don't use 7-segment decoder-driver chips, I usually do the 
7-seg numeral decoding in whatever microcontroller is running the show. 
That way you can display non-standard "characters" too, like a dash "-" or 
the hex digits "abcdef" etc...  Sometimes this is useful.  The micro also 
handles all of the LED multiplexing to use the minimum number of I/O lines. 
>From that perspective the 7-seg displays aren't any different to any other 
panel LEDs.

-Richie,



-----Original Message----- 
From: Rick Jansen
Sent: Wednesday, August 05, 2015 5:32 PM
To: Synth-Diy
Subject: [sdiy] 7-segment LED displays with 4511 driver chip - max current?

I need three 7-segment digits for a small project, and am looking at the 
common 4511 driver to drive them. There are numerous examples around of 
circuits to do just that, but I'm a bit puzzled. The specs for the 4511 say 
that it can source 25 mA max current, per pin. However, with a max total of 
50 mA for the entire package. So, if you light ALL segments, excluding the 
dot, that would be 50/7 = 7 mA max current per segment.

Only TI says 7.5 mA explicitly : 
http://www.ti.com/lit/ds/symlink/cd74hc4511.pdf
NXP: http://www.nxp.com/documents/data_sheet/HEF4511B.pdf

A very common value in available circuits for the current limiting R is 
470Ω, which gives ca 10 mA, but which would exceed the total current for the 
package. Even 220Ω is common, which indeed delivers 20 mA to one LED 
segment, but may fry the 4511 if you light 7 segments with 140 mA.

How do you use the 4511?

thanks

rick







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