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Interchangeable 7 segment display - CA and CC
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Yaroooo:
In my application, I should control 4 display composed of 7 segments. I want to use a classic method with 4 BJT and segments all driven by a Micro. An update rate will show up each value on the powered display.

Particularly is that I'd like to have the possibility to interchange between Common anode screen and Common Cathode.

Is there an optimized or right way to do that?

Here's my BJT tests, power supply is a single buck regulator that regulates form 24v to 5v. There's a resistor on each port that goes from my Micro (PIC16F15354) on a single segment bus. Display models are SC15-11SRWA and SA15-11SRWA.




In the Test 1, all were fine, I've tested both CA and CC displays without problems, One were brighter than other because a voltage drop on BJT when used with LEDs between emitter and ground.

On test 2, only CC display works, but no light on CA.

What is wrong?

I've suppose that is something related to voltage drop on BJTs and Forward Voltage, but on my first board (3 in total) all worked perfectly. On a new board (exactly the same but with new BJT configuration to reduce jumpers), I've 2 assembled two of them but only CC screens are working.
Zero999:
I can't see any of the images because they're blocked by the firewall. Please attach them here.

No, common anode and common cathode LED displays are not interchangeable. For common anode, the four transistors need to switch the anode, connected to the positive supply voltage and the logic of the MCU output to each segment needs to be reversed.

Trace the current flow, from the MCU output, though the current limiting resistor, the LED and transistor and you'll see what I mean. Look up the schematics for the common anode and cathode LED displays, if needs be.

You could use eight transistors (four NPN and four PNP) controlling the anode/cathode of the LED display and reverse the logic of the output, in software, but you'll need a way to identify what type of display is connected to the output.
Yaroooo:

--- Quote from: Zero999 on June 18, 2019, 10:09:17 am ---I can't see any of the images because they're blocked by the firewall. Please attach them here.

No, common anode and common cathode LED displays are not interchangeable. For common anode, the four transistors need to switch the anode, connected to the positive supply voltage and the logic of the MCU output to each segment needs to be reversed.

Trace the current flow, from the MCU output, though the current limiting resistor, the LED and transistor and you'll see what I mean. Look up the schematics for the common anode and cathode LED displays, if needs be.

You could use eight transistors (four NPN and four PNP) controlling the anode/cathode of the LED display and reverse the logic of the output, in software, but you'll need a way to identify what type of display is connected to the output.

--- End quote ---

I've changed images link. Now you should see them.

I know the display type mounted, in this case. And the FW will adapt based on which display will be mounted, so CA or CC will be driven by a sink or source pin output configuration. Jumpers on BJT, that is fixed, will interchange wires to VCC or GND.

Worked till now with TEST 1 board. But on TEST 2 board doesn't work anymore. What I'm asking to myself is if I was lucky in the first case. Since both schematics are same. The only difference is pin on my gate resistor that have two BJTs instead a single one. Already performed a cut test to exclude one, but nothing.
Zero999:
If you've fitted both the PNP and NPN transistors, then they will both turn on, short circuiting the power supply and most likely be destroyed as soon as power is connected to the circuit. The base connection is a low impedance diode. Connect a PNP and NPN transistor like this and a very high current will flow directly from the PNP transistor's base, through the NPN transistor's base.

The solution is to only fit either the PNP or NPN transistors.

If you can redesign the board, configure the transistors as emitter followers: NPN to the positive and PNP to the negative, reduce the LED current limiting resistor values slightly (I assume they're on another schematic) and it should work.

Another possibility is to replace the NPN BJTs with N-MOSFETs and PNP BJTs with P-MOSFETs (gate to base, drain to collector and source to emitter, connections), which have a high impedance gate connection and won't blow up like this, but you need to find parts with the correct pin-out to avoid redoing the board.
Yaroooo:

--- Quote from: Zero999 on June 18, 2019, 01:01:55 pm ---If you've fitted both the PNP and NPN transistors, then they will both turn on, short circuiting the power supply and most likely be destroyed as soon as power is connected to the circuit. The base connection is a low impedance diode. Connect a PNP and NPN transistor like this and a very high current will flow directly from the PNP transistor's base, through the NPN transistor's base.

The solution is to only fit either the PNP or NPN transistors.

--- End quote ---
On schematics I've SOT (with double bjt package) NPN connected to a common side of the diode through a jumper. But only one will be closed. So I should never have a short circuit.

This give me an idea, use the same configuration and package but with a component that have a PNP-NPN BJTs inside, maybe in this case I'll reduce Vce since I'll use my BJTs in correct way.

My supposition is that, since SC version have a lower Vf, I don't have any problem.

In case of SA, SA15-11SRWA have this specifics:
Vf min 3.7 - Vf max 5.0
(I've tried also to unsolder and resolder a working display from test 1 board to test 2, but nothing)
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