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

Brian Willoughby brianw at audiobanshee.com
Wed Aug 5 19:34:48 CEST 2015


In the old days, a TTL chip like the SN74143 would drive up to 22 mA on each of its 7-segment driver outputs, and had what seems to be plenty of headroom since the chip could dissipate 1.4 W total. But that's an obsolete chip that only operated on a 5 V supply.

I agree with Richie: handle the BCD conversion in your code, and wire the individual LED segments so that you can create any pattern you want. Even old product designs from the pre-surface-mount decade of the eighties had direct drivers for the LED segments to save money on parts like special BCD decoder chips. Have fun.

Brian


On Aug 5, 2015, at 9:50 AM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 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.
> 
> -----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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