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Posts: 3
Hello everyone
I have a problem with the chain of 8-Digit 7-Segment Digital Display Module.
The first and second modules work without problems. The third works from time to time. Fourth and fifth do not work.
I have Arduino connected to a computer via USB, without additional power supply.
What may be the problem?
2020-03-22 22:09
From: EDDG, Germany
Posts: 1446
Hi marmar,

this is a known problem. It's up to the boards of the MAX7219. There is a protective diode D1 thereon.
Usually the board is supplied with 5V. The diode on Board 1 drops the voltage to 4.3V. This is usually not a problem. The problem only occurs if these boards are connected together as a daisy chain.
If you pass on the output voltage (4.3V) from board 1 via the plug-in connection to board 2, board 2 again has the protective diode D1 and thus the MAX7219 on board 2 is only supplied with 3.6V.
Exactly what you have described occurs. Boards 1 and 2 run perfectly, there are problems from board 3, because the 3.6V from board 2 now come to board 3 and also there via the protective diode D1. Since the voltage is reduced by 0.7V due to the diode, the 3rd board only works with 2.9V!
According to the data sheet, the MAX719 requires at least 3.4V. So the problems start with board 3.

So much for the description and reason why only boards 1 and 2 run perfectly.

What can you do?

1. Unsolder the diodes on the boards and replace them with a wire bridge. Then 5V come in and go away from the board.
Disadvantage: the protective diode is missing. If the voltage is turned upside down on the board, this will immediately destroy the MAX7219. So if you think you can, remove the diodes and replace them with a wire bridge.

2. Supply each MAX board individually with the 5V voltage and do not unsolder the diode. Then all boards are only operated with 4.3V, but that is enough to ensure perfect operation. The 3 data lines can / must be looped through the boards so that the chain works as such.

Which variant you choose is up to you. In any case, your 5 boards will then work as a chain.
2020-03-22 23:15