5V external power strangeness
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I am powering the Duet with an external 5V PSU – It is a regulated 5V 5A supply which powers the Duet and the a Pi which is used to control the relays to turn on the 24V supply and feed power to the bed SSR. I have one of those little (intended for USB) display units connected inline with the 5V line to monitor voltage and current use (it is also rated for 5A)
I tuned the 5V PSU using this LCD to output exactly 5V. Recently I've been having issues with the 7" LCD -- it is flickering goes of, on -- if I power cycle (unplug the LCD) -- it starts working properly. I check the current draw on the 5V supply and under normal conditions the Pi and Duet + LCD etc do not exceed 1.5A -- so 5A is underutilized, but I figured good to have headroom. However when the LCD starts freaking out -- I see current draw jump up to 1.8A. For curiosity sake I measured the voltage sent to the LCD on the 5V rail and it's 4.5V -- I measured the input voltage to make sure it is what the USB display says it is -- and it's 5V exactly, so .5v is being lost somewhere -- I measure the voltage going to the BL-Touch and it's also 4.5V. I tuned the 5V PSU to get to where the LCD 5V output actually says 5V and I notice that the PSU has to output 5.5v for that to even out.What is going on here...is this expected? I assume 5V for LCD should be 5V -- so tuning the PSU to get it to level out at 5V is correct, but why does it need to feed 5.5V to get the 5V rail on the Duet to register 5V?
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There is a diode between the 5V external power connector ands the 5V bus on the Duet, to prevent the internal 5V regulator back-feeding power to anything else driven from the external 5V supply and to provide reverse polarity protection. There is another diode on the PanelDue controller board between the 5V input from the 4-pin connector and the output to the LCD, again to provide reverse polarity protection. So it's normal to see about 0.5V drop between the external 5V input and the Duet 5V bus, and between the Duet 5V bus and the LCD backlight input.
The Duet can work quite happily with the 5V rail at 4.5V, however it may be that the drop caused by the two diodes is reducing the backlight input voltage too much (to about 4V) for the backlight inverter to work reliably. You could bypass one or both diodes, but then you will lose the protection that they provide.
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I guessed about the diode on input – I didn't think that there'd be another diode on the PanelDue -- belt and suspenders -- impressive. I increased the input voltage to 5.5 -- and the strangeness has gone away. Everything else is tolerant enough so there is no issue. I recommend you add a little note to the documentation that this is likely -- I am pretty sure I was borderline because before I got the little in inline volt/amp meter the PSU was set to 5.25 -- then I lowered it to get the readout to be 5 -- that's when the weirdness started to happen. Now everything is happy at 5.5.
