Strange Tool-board failure V1.1
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@arnold_r_clark we have seen a small number of failures of the temperature input reference resistor on tool boards. We believe this is caused by static discharge to the temperature sensor on the hot end.
It's possible to replace the reference resistor using a soldering iron with a selection of bits.
To guard against future static discharge we recommend grounding the hot end metalwork.
In the next tool board revision we are switching to a larger and more robust resistor.
Duet WiFi hardware designer and firmware engineer
Please do not ask me for Duet support via PM or email, use the forum
http://www.escher3d.com, https://miscsolutions.wordpress.com -
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@dc42 Hi David, when you suggest grounding the hot end, is this to mains ground, 0V (or possibly both)?
Sort of related is it generally bad/good to connect 0V to mains ground? The reason I ask is that I have a e3d toolchanger and I'd like to be able to detect if the tool is currently attached to the tool carrier. I could of course use a microswitch to do this, but I had wondered about using the "electrical" connection between the two tool carrier plates. The one on the tool itself is isolated from the rest of the frame/tool so it would be easy to detect contact between this and the plate on the toolhead. However the toolhead plate is connected (via the frame) to mains ground and so the "simple" solution would require connecting this to 0V (at the moment 0V is not connected to the frame/mains ground). I think I've seen setups that make use of a circuit being formed between things like a tool and the frame/bed of a machine (possibly in cnc setups)? If connecting 0V to ground is not a good thing to do is there a "safer" way to detect contact with a mains grounded frame?
Sorry for the potential thread hijack, but it is sort of related.
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@gloomyandy Thanks! I think that covers the question of how to connect 0V to the hotend/frame in the case of there not already being a connection to "ground", but I'm curious as to if it is safe/a good idea to connect 0V to a frame that is already connected to mains earth (with or without the resistor). A quick google generates a lot of articles indicating that it may not be a good idea (though I don't remember seeing anything that covered doing so via a resistor - which would be fine for my use case I think).
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@gloomyandy Grounding can be complicated, and most publications about that are it as well - or quite indeterminate and vague. You can imagine the grounding line of mains as a big cage which shields its tiger (AC power) against the outside (which we are part of). General rule: AC leads may never touch the cage.
– Note of caution: there are regions on earth without this third grounding line: I don’t cover this case here. Stop reading. –
In contrast to mains power, low DC voltages are dinky cats, they need not to be caged, but - as is the case with computer equipment or our printers - there are plenty of them: 3.3V, 5V, 12V, 24V, or even negative voltages. These DC voltages often need a common reference point: ground. Ground is different from the AC mains cage, but may be connected to this.
As a rule of thumb, within a printer, there should be one single connection to the tiger’s cage (the mains grounding line), and all DC Power supplies should reference this point, i.e. should explicitly connect their ground to that. As it's bound to earth, this is a good thing.
All metal parts of our printer should also be connected to this single reference point, by this building a kitty cage of our own. This implies that we keep all further DC cables separate from this, both poles. The reason is that we can’t really trust our cage: our playful kitties churn lots of amps, jump around with voltage spikes, handle sensitive signals and provoke static charges with their balls of wool …
So, the second rule is: don’t save a line and connect your load (controller, sensor, motor or heater) to the metal nearby - that’s the cage, not ground power. Always use a separate line, keep it distinct from the kitty cage.
To protect against static charge, we need a third rule: all metal which might be electrically isolated from the frame, e.g. steppers on printed mounts etc., need to be wired to some part of the cage (metal framework of our printer) nearby.
That’s as short as I can put it - three simple rules. – Oh, I forgot one last cat in my zoo: Shielding. For our purposes, a forth simple rule is good enough: connect the cable’s shield to GND of the source (usually a Duet), not to the destination’s ground and not on both ends. Then, all kitties should be happy, and our printers, too.
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