Electronics > Beginners

Interchangeable 7 segment display - CA and CC

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Zero999:
Ok, I see the problem now, sorry I got confused with PNP and NPN transistors. :palm:

I can see why this isn't working now and no, there is no short circuit which will damage the transistors.

I Googled the data sheet for the  BC817 and it became obvious to me why the common cathode works, but not the common anode. I was on the right track with the base-emitter diode issue.

The common anode is an emitter follower configuration, which needs to have the full LED voltage on the base, plus about 0.6V to work.

The common cathode is a common emitter configuration. The diode junction in the BJT is clamping the voltage to about 0.7V, which is preventing the high side from turning on.



You need to change it the base connections, so the lower BJT's base, is not directly connected to the upper BJT's base. The common anode BJT doesn't need a base resistor, because it's configured as an emitter follower, the base current is self-limiting.

james_s:
Could you not just use a totem pole follower stage to drive the common pin of the displays? I don't think I've ever tried making an interchangeable board like that but it shouldn't be too hard to do.

Peabody:
Depending on the segment current needed, it may be possible to provide for both CA and CC parts (but not mixed) without using any transistors.  The choice of CA or CC would be set in software.

You would set up the 8 segment drivers just as you have them, but the CA or CC pins would be driven by GPIO pins with only a current-limiting resistor.  The key to making this work is to multiplex by segment instead of by digit.

In traditional multiplexing, each segment driver provides current to only one segment at any one time, but the CA or CC pin must source or sink current for as many as 8 segments including the DP - hence the need for the transistor.

But if you multiplex by segment, each segment driver must supply a maximum of 4 segments at one time, but the CA or CC sources or sinks current for only one segment at a time, and therefore needs no transistor, just a resistor.  The question is whether a PIC can drive 4 segments at once, and that depends on how efficient your displays are.  But it appears the absolute maximum rating for GPIO pins for the PIC is 50ma, so if you can use 10ma per segment or less, this may work.

You would have to refresh twice as often as the traditional method because each segment is ON only 1/8 of the time  versus 1/4 of the time when multiplexing by digit, and this will make the digits appear a bit dimmer.  But your software would define what GPIO state is "ON" for both the segment lines and the CA/CC lines.  They would just be opposites for CA and CC parts.  It's even possible that your software could autodetect which displays are being used (I had no luck finding a way to do that, but perhaps your high-voltage-drop parts would provide a way.)

This is all explained in detail in this video and the related Github repo.


David Hess:

--- Quote from: james_s on June 18, 2019, 04:30:11 pm ---Could you not just use a totem pole follower stage to drive the common pin of the displays?
--- End quote ---

In the past totem pole output stages would not meet the current requirements for the common signal of a multiplexed display but today high efficiency displays might make this feasible.


--- Quote ---I don't think I've ever tried making an interchangeable board like that but it shouldn't be too hard to do.
--- End quote ---

I have seen it done before but I never studied exactly how.  The common terminal drive requirements are much greater than the segment terminal drive requirements.  So use a discrete driver for the common terminal and a gate output for the segment terminals as is commonly done anyway.

Yaroooo:

--- Quote from: Zero999 on June 18, 2019, 03:41:14 pm ---Ok, I see the problem now, sorry I got confused with PNP and NPN transistors. :palm:

I can see why this isn't working now and no, there is no short circuit which will damage the transistors.

I Googled the data sheet for the  BC817 and it became obvious to me why the common cathode works, but not the common anode. I was on the right track with the base-emitter diode issue.

The common anode is an emitter follower configuration, which needs to have the full LED voltage on the base, plus about 0.6V to work.

The common cathode is a common emitter configuration. The diode junction in the BJT is clamping the voltage to about 0.7V, which is preventing the high side from turning on.


--- End quote ---


Thanks, this solved my problem!

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