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TL;DR: LEDs when 'on' are great, however, they're too bright when they should be 'off'.

I'm trying to build a custom, large 7-segment LED display driven by a Raspberry Pi. I'm a coder so the bit twiddling is the easy bit (although I'm very willing to believe I've screwed that up too :)), electronics ... not so much.

I'll eventually be using 21 LEDs per segment of the digit, split into 3 strings of 7 LEDs, so I need about 24V DC to power the LEDs.

I thought of using an MIC5891 in each of the digits because it promises latching, easy serial input with 5V logic supply, chaining and 'will sustain at least 35V in the on-state'.

Here's what my test board looks like:

schematic

simulate this circuit – Schematic created using CircuitLab

All 'off' and the LEDs are very dim (about 1V) but acceptable.

If any of the 'segments' are lit, the ones which should be off are showing and I can record about 6v across them, so they're pretty bright :(

Just for my sanity here's the 21 LED segment setup.

schematic

simulate this circuit

I wonder if there's a grounding issue here, I keep seeing mention of pull-down resistors but always in the context of inputs.

Any advice very welcome -- it's driving me nuts :(

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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. OP's first schematic rearranged for clarity.

I wonder if there's a grounding issue here, I keep seeing mention of pull-down resistors but always in the context of inputs.

There's a grounding issue alright, but it's not to do with pull-down resistors. You have no ground connection between the two circuits. Connect the 24 V negative to the ground of the rest of the circuit.

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  • \$\begingroup\$ Thanks @Transistor! I was too tentative plugging in -24V into my Pi :) Works like a charm. \$\endgroup\$ – Maxitone Jun 28 at 8:15
  • \$\begingroup\$ Good. Just checking: do you understand why the absence of the ground link caused a problem? Some of the circuitry had no return path. To see it clearly you'd need to draw out one channel of the MIC5891 but the datasheet shows a common ground between the logic circuits and the outputs so that's what we've done here. Note the few simple improvements on the schematic. The GND symbol eliminates a bunch of wires / clutter and allows you to see the referenced voltage instantly. In general current should flow from top to bottom, hence the orientation of the LEDs and 24 V PSU. \$\endgroup\$ – Transistor Jun 28 at 9:03
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TL;DR

The drivers are NPN Emitter Followers or high-side drivers that require common returns with logic ground.

-1st cct draws about 25mA which you can measure across series drop R using Ohm’s Law ... is ok but bright near abs. Max.
- 2nd cct draws (24-21)V/330 Ohms=12mA for a 3V drop. After you connect the shared grounds.

  • IC is rated (while hot) for 800mA max continuous on all ports added up so , or 100 mA per port or 33mA per array or 11 mA per string of 3.

Since only 6 of 7 segments max will ever be driven, you can use the 12 mA you selected.

Coincidence or good plan? But pushing IC to max temp rise , so have convection air flow over it, not sealed up.

  • or use JB Weld epoxy thin layer to a heatsink to IC

YMW

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  • \$\begingroup\$ Thanks for your feedback. I'll bump the resistors up to bring the LED current down. >> IC is rated (while hot) for 800mA max ... I couldn't find the 800mA you mentioned, but my calcs are based on Ic = 500mA. So for 7 segments => 71.4mA per segment => ~24mA per 'string' of 7 LEDs. Now I've got the basics working I am now considering pulsing the device to reduce the load on the IC and putting a fat capacitor (technical term I'm sure) across the LEDs to smooth the pulse out (size yet to be determined). >> Coincidence or good plan? I'd like to think it was planned :) \$\endgroup\$ – Maxitone Jun 28 at 8:31

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