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

[Tim Ressel]

Hi,

Some thoughts:

Modern displays don't need ads much current as older ones. You might be 
able to get away with just a few mA, especially if you use a color 
filter to enhance contrast.

My go-to for this would be a 74HC595 shift register. I would multiplex 
the displays, either with 2 '595s or with a single and use 3 proc lines 
to drive the display common pins. Multiplexing might be off-putting if 
you haven't done it before, but it is really quite easy.  '595s are 
groovy because you can clock them fast, and you can use SPI or 
bit-banging to drive them. With an Atmel AVR at 20 MHz and SPI you 
should be able to get in and out of the display routine in a few micro 
seconds. Also with multiplexing only one display is on at a time, thus 
reducing total current draw.

--TimR

On 8/5/2015 9:32 AM, Rick Jansen wrote:
> 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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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com