Common Anode SSD vs Common Cathode SSD

Hi!
This is regarding common Anode seven segment displays and common Cathode displays which works with PIC microcontrollers.

I have seen many PIC circuits they use common Anodes in multiplexing circuits.

When using common Anode SSD displays the PIC acts like DRAIN path.
When using common Cathode SSD displays the PIC acts like a SOURCE.

But I like common cathode SSD. Because I have used to this method.

I want to know when using in continuous run (24HRS) which is better? What you all using with PIC's?

Thankx

PICs can sink and source the same amount of current, but the voltage drop between sourcing and sinking is less when sinking. For example they spec the output low voltage to be 0.6V at 8.5ma but the output high voltage is Vdd-0.7 at only 3ma per pin.

I use Common Cathodes for the hell of it lol.

The ULN2003 makes mince meat of multiplexing multiple CC displays

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kchriste said:

PICs can sink and source the same amount of current, but the voltage drop between sourcing and sinking is less when sinking. For example they spec the output low voltage to be 0.6V at 8.5ma but the output high voltage is Vdd-0.7 at only 3ma per pin.

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I can verify this after experimenting with Charlieplexed or N(N-1) multiplexed displays a couple years ago.

Displays are brighter when PIC pins are sinking current. My current direct drive designs sink current.

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If you have more than a few digits, or if you need a "full brightness" display, you really shouldn't be driving it/them directly from I/O pins anyway.

Once you have segment and column drivers that can supply "peak" current for your duty cycle you can use whatever display type you want. You can use transistor or IC drivers on segments and columns.

I prefer common anode displays simply because of the way I drive my displays and the choice of driver ICs available.

Thanks for the replies.

Oh I see! The voltage drop between sourcing and sinking is LESS when SINKING

But that will not effect too much if I use common cathode SSD’s isn’t it? Earlier there was a myth that the PIC is getting hot when using the CC Displays than CA Displays.

Thanks kchriste

For Mike
Ya few digits of course ok. Because most of the time I’m using 4 digits. Very rarely connecting more than 4 digits.

For the 4 digits I’m not using row drivers. Only column (transistor) drivers.

Here I used high speed switching transistors. Earlier I used GP transistors for this but I got very bad results like resetting problems, display brightness etc….so I shifted to 2N 2222 series it worked well with standard SSD’s.

Actually ULN series good for LED matrix systems with more columns. For segment circuits, only the column driver transistor method working perfectly.

But that will not effect too much if I use common cathode SSD’s isn’t it?

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To a point, you could compensate by using lower ohmage current limiting resistors.

myth that the PIC is getting hot when using the CC Displays than CA Displays.

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With the increased voltage drop (while sourcing current) this is a possibility though I haven't actually experimented with this myself.

kchriste said:

To a point, you could compensate by using lower ohmage current limiting resistors.

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The lower ohmage resistors you mean the base resistor or the Row resistors?

The Row resistors that go direct to the anodes when using common Cathode SSD displays.

Oh I see I’m using 150 ohm row resistors for two seven segments(CC).

For the base resistor in the column driver I ‘m using 1K. (with less ohmage resistor will produce heat much in the PIC)

Thanks kchriste

Just use led drivers like DS75492 MOS-to-LED Hex Digit Driver what got me to using them is a sign It had 1050 leds that's where I got my 7x15 led Matrix the old cpu die in it. but the thing is bright. It was sinking the commons and sourcing the rolls I use the drivers with seven segments they work great