Experimental 'Wireless' Heatbed

Hi all

When I started designing my printer I wanted to keep the wiring tidy and had an idea to eliminate the wiring completely from the Y Carriage.

I have fed power (24V) to the carriage via the rods and picked it up using pairs of silver plated RF screening fingers on each side.

The Heater is controlled from an on-board analogue PID controller, the front panel has a temperature setting knob and a small digital DVM shows either the set point or actual temperature.

The Heater is built from 20 4.7R/15W resistors, a total of 3.76R, giving 153 Watts.

It has been successful, I only lose about 100mV on each pickup (at 6amps) and the supply to the Heatbed is constant when the bed is moving.

Its not perfect though, it weighs 2.3Kg ! and I've noticed that the plating is worn through on the pickups so don't know how much longer it will last, its probably done around 50 hours of printing.

Chris

PS Apologies for the pictures being attachments, I couldn't figure how else to add them ?

Maybe you could use brushes from dc motors instead?
You just have to find a clever way to shape them to match the rods.

ps: if you don't have a cloud for the pictures, you can only attach them. Or you start a thread in the "sandbox" section here, attach the pics there and link to this attachment in the real thread.

Edited 1 time(s). Last edit at 11/20/2016 01:18PM by o_lampe.

That's certainly an ingenious idea. Did you check the resistance without the wipers? It's possible you get a reasonable connection through the bearings. Overall, I suspect you may have solved something of a non-problem though; flexible wires in a suitable gauge for 24V are not much drag to trade for the extra moving mass.

I did carryout a quick test with the bearings but the volts drop was quite high, 2-3 volts, and they started to warm up so that was aborted quite quickly !, I imagine the plastic carriers in the bearings would have melted soon after.

Chris

Although the idea sounds good I'm going to be the devils advocate here: 24 volts dc at those currents used could be fatal, dc kills faster than ac.

Lubrication of the rods will affect conductivity and proper protection will add weight.

24 volts dc at those currents used could be fatal, dc kills faster than ac

Where did you get this idea?

I use to work on cars and the occasional big rig with 24 volt batteries (hundreds of ams available) and I would often grab hold of both posts and could not even feel it. In fact with DC I need around 100volts to even feel it in my hands and then I can barely feel it. Also I use to have RC race cars that had 36v batteries that were capable of 50a and I use to touch the end to my outer lip to see if they were charged. I could hold one there for several minutes.


MDFChris:

You might try some tungsten contacts as that is what we used in Pinball machines for 50V high amperage flippers.

[www.electricalcontactrivets.com]

There is no safety risk with 24 Volts, the current is irrelevant, at least as far as electrocution is concerned, and whether its AC or DC is not an issue either, see the US current wars in the late 19th century.

Thanks for the suggestions of alternative pick-ups but I'm planning on reverting to a 'traditional' wired carriage and running 48 Volts to the heater to reduce the current and wire size (and which is still entirely safe to touch !).
I intend to use a 5mm aluminium plate for the heater and directly attach a larger sheet of PrintBite (I hate bull dog clips !)

Chris

Yeah at uni I did a bunch of physiology experiments including external nerve stimulation. Using electrodes tipped with saline soaked cloth directly above the ulnar nerve you would need around 60 volts to overcome the skin's resistance, even with the salt water to improve conduction.

Quote from wikipedia

"The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 Ohms."

Edited 1 time(s). Last edit at 11/21/2016 11:54AM by DjDemonD.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions