Common Anode vs. Common Cathode 7-segment Display

Clint

I'm using an SX28 to·multiplex a three digit 7-segment LED display. Is there any preference for using a common anode versus a common cathode display?

There is an example in the SX/B help file for using a common cathode display. Would the 1s and 0s just be switched when using a common anode display instead? Does it make a difference which type of display I use?

Thanks,
Clint

Phil Pilgrim (PhiPi)

Clint,

It's sometimes easier to find N-type transistor drivers that can handle high current levels than it is to find equivalent P-type drivers for the same price. For this reason, common cathode displays may be very slightly less expensive to construct than common anode displays. Other than that, there's no reason to favor one over the other, except possibly for the availability of the requisite LED modules.

-Phil

Clint

Thanks for the response Phil. I currently have some common anode displays, so I'll give 'em a try.

Is there any way to use the SX to drive LEDs without using external resistors?

Phil Pilgrim (PhiPi)

You need a series resistor for each LED to limit the current through it. Without it, you risk damaging the LED, the output pin driving it, or both. If space is an issue, you can always use resistor arrays containing isolated (as opposed to bussed) resistors.

(Okay, in theory, you could use the SX's internal pullups in a common cathode configuration to "drive" the LEDs. But at 20K ohms, the room would have to be pitch dark to see the glow. )

-Phil

T&E Engineer

If you had Common-Cathode·displays you could use a max·MAX7219 which can drive up to (8) 7 segment displays with only 1 resistor (or 64 LEDs). I beleive the SX/B Help has an example showing how to connect a display to an SX-28. It·can probably·be modified to add more than 1 display.

Clint

T&E Engineer said...
If you had Common-Cathode·displays you could use a max·MAX7219 which can drive up to (8) 7 segment displays with only 1 resistor (or 64 LEDs). I beleive the SX/B Help has an example showing how to connect a display to an SX-28. It·can probably·be modified to add more than 1 display.

I have used that driver successfully and it works very well, but since I only have three digits and I'm trying to cut costs, I think it might be better to have the SX do the multiplexing.

I plan on using a resistor array as Phil suggested.

For now I only have a common anode display to use for testing, but it shouldn't be hard to transition to a common cathode display with some minor reprogramming, right? Basically a switch of 1s and 0s.