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I am controlling a 6x6 matrix of LEDs using a PLC with sourcing outputs and sinking outputs (24V). However, there is an issue with current leaking through to other lights.

For example, in the schematic below if I turn ON L25 (Source) and L30 (Sink), LED 16 will illuminate, but LEDS 4,10,22,and 28 will also slightly illuminate. Additionally, LEDs 13-18 will slightly illuminate. It appears that current could be using the open sinking as a ground or that the LEDs are not acting as perfect diodes.

The leakage LEDs are very dim and initially, I could not observe the problem. However, since running the matrix over several months, the leakage lights have become brighter. I checked using a multimeter and found that 1-3V are leaking across to other LEDs.

My question: what is a possible issue here? How can I resolve this most easily as I do not want to make significant changes to the matrix.

Thanks in advance.

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  • \$\begingroup\$ Show oscillograms. \$\endgroup\$ – winny Jun 20 at 15:24
  • \$\begingroup\$ Show your drivers also, that may be part of the problem \$\endgroup\$ – Voltage Spike Jun 20 at 15:24
  • \$\begingroup\$ @laptop2d I added the PLC schematic \$\endgroup\$ – plc1121 Jun 20 at 15:49
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"For example, in the schematic below if I turn ON L25 (Source) and L30 (Sink), LED 16 will illuminate, but LEDS 4,10,22,and 28 will also slightly illuminate. Additionally, LEDs 13-18 will slightly illuminate."

In using PLC on an LED matrix, you need to make sure your PLC source goes to ground when off and your PLC sink goes to Dcc when off (these are called pull-up and pull-down resistors).

In this circuit design and if you only want one LED on at any given time, you can ONLY have one source and one sink on at any given time; otherwise, you will get multiple LEDs energized.

L25 (ON), all LED's on L25 will have source.

L30 (ON), all LED's on L30 will have sink.

Only L16 will light.

Now, turn on L31 also - LED's 16 and 22 will light.

Now, turn on L24 also - LED's 15, 16, 21 & 22 will light.

You may want to use a multiplexer depending on your application.

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  • \$\begingroup\$ Thanks for the suggestions! One question: how should I ensure the PLC source goes to ground and the PLC sink goes to +24V? Would this involve adding a resistor (sized properly) as shown in my latest edit? I discovered that there is some voltage on the OFF pins. \$\endgroup\$ – plc1121 Jun 21 at 15:48
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Most likely your column (sink) drivers are leaking some current in OFF state.

Since I don't have more information on the type of drivers you are using, here are a few possibilities:

  1. The column driver has push-pull outputs as opposed to open drain or open collector outputs and the high voltage is lower than the voltage provided by the source driver minus some drop across the LED;
  2. The ESD protection diode inside the sink drivers is creating a leakage path to VCC even if the output is high impedance;
  3. The reverse current leakage of the LEDs is too high what create other possible current paths in your matrix. If this were the case you would see LEDs in other rows also lighting up, what you don't report.

Start troubleshooting the problem by measuring the voltage and the current going into a sink driver when in OFF state using a digital multimeter. This will provide clues to isolate the problem.

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