You can insulate the diode. It probably isn't really used as it is for blocking inductive kickback (http://www.falstad.com/circuit/e-inductkick.html) and the bed isn't inductive or only parasitically inductive.
Note that without the LED, the polarity of the connection to the bed doesn't matter. However, it will when the LED is added,
Also note that the PWM output for the fan on the UM print head is actually ~19.5 volts. The UM1 does use a 12V fan which is apparently OK with the higher voltage. However, in my experience, other 12V fans do NOT tolerate the higher voltage and immediately go "POOF".
I didn't even see those, diodes they are really tucked away under those Darlington transistors.
Thanks for the further explanation about low side and high side. I will go with your original setup. I almost had it put together last night accept I ran out of 14 gauge wire. I appreciate the tip on the silicone wire, that stuff is a lot easier to work with then 14 gauge auto wire.
Can the diode be insulated or have heat shrink applied to it or PTFE tubing without effecting its operation? I have like 24' 8" of PTFE 1/32" ID left. The diode seems like it could be a shock hazard or worse with it's exposed leads.
I soldered the tiny surface mount LEDs to the PCB now just waiting for the surface mount thick film 1k resistor to arrive so that the LEDs do not get fried. It is the last piece I need to finish the PCB, hoping it arrives today. I ordered two new fans the same day I ordered the resistor, the fans came from China and got here snail-mail before the resistor which is only coming from Texas, also snail-mail.
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