4511 BCD and common anode 7-segment display

[NikitaB]

Hi all,

I purchased 2 of the 4" Kingbright SA40-18SRWA 7-segment modules, and two 4511 BCDs to interface the displays to a LabJack. I discovered however, that the Kingbright displays are common anode, while the 4511 BCD is meant to drive a common cathode display...

As I am fairly new to electronics, I am unsure of what I need to add (or if it is hopeless) in order to make the common anode displays compatible with the 4511 BCD. I read on the SparkFun comments for the MAX7219CNG driver that one could use a 16 bit hex inverter in order to make the chip compatible with a common anode display, but I'm not quite sure how to implement this. Ideas, suggestions, schematics, plans, websites, part numbers, anything helps!

Thanks!

 

[poor mystic]

Yes, if you were stuck on a desert island with only a crashed airplane for parts you could use an inverter but I recommend going back to the shop and getting the common cathode display the chip wants.
The conversion of the o/p of the 4511 to suit your display will complicate and possibly uglify your project, which besides will end up costing more in circuit board real estate and wiring effort.

 

[NikitaB]

Hmmm, alright. Unfortunately, the displays that I am using are 4" tall, and as I am no longer abroad (where I got the common anode displays) I have to go through an online store. But there just seem to be no large common cathode 7-segment displays out there.

Even though an inverter would be archaic, I would still appreciate some schematics for that setup as I think that is what I will have to use.

Thanks

 

[poor mystic]

How much current does the display want to drive each segment please?

 

[NikitaB]

Per the data sheet, the forward current per segment is 60mA. Here is a quick link to the data sheet as well: http://tinyurl.com/7-segment

Thanks for the help

 

[(*steve*)]

As per the datasheet, the 4511 isn't actually capable of driving these anyway as it is limited to 25mA.

The datasheet does suggest a method to drive common anode displays -- it requires an additional transistor and resistor per segment.

You could possibly use an inverting line driver in place of the base resistor and transistor. Alternatively, there are a number of other driver chips. One of the many ULNxxxx chips may be what you want. Some of them even have the base resistors incorporated.

edit: Note that you will still require series resistors to limit the current to the segments.

 

[NikitaB]

Quite true. So that is a double mistake on my part. I would like to use only a few digital out pins on the LabJack, so I don't think that the ULNxxxx will do the job, as I believe it will use ~8.

What is the family of shift registers that can handle 60mA? I've been looking online, but can't find much besides the basic +/-30mA shift registers.

 

[(*steve*)]

Digital output pins? Use the 4511 to drive the ULNxxxx.

In this case the fact that you need a ULNxxxx or something to get the extra current means that a common anode display is a feature and not a bug.