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I am building a whiteboard CNC pen plotter for my classroom out of some old 3D printer parts, a Melzi board running Ramps 0.92 and Inkscape to generate the Gcode. I have the machine up and running but I need a solution to actuate the pen in and out. I want to use an electromagnet or solenoid to do this and I was thinking of running the solenoid off of the cooling fan header. I can use Gcode to turn the fan circuit off and on or control its speed via PWM. The system is a 12 volt system and I think the old 40mm fan was rated at 0.18 amps, 2 pin. I am wondering if a small solenoid could be powered by the fan circuit? If so, what ratings should I look for? Would a solenoid be a bad choice? Could I buy some magnet wire and wind my own electromagnet which could just pull the marker into the whiteboard? I don't need much movement out of the pen, just a few mm.

I know there are better solutions out there like using a servo or another stepper motor. I am avoiding the stepper motor because I am trying to keep the mass of the moving parts as low as possible. I would use a servo, but I really don't know a whole more than basic circuits. Is there an easy/cheap way to convert the two pin pwm fan signal to a servo signal?

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    \$\begingroup\$ I'd use a relay stripped down to remove the contacts and attach the pen to the armature for a little bit of movement control. But, remember this is an EE website and hobby type engineering is not going to get you direct answers without a whole load of description and numbers. What you will get are opinions and, unfortunately (or not) questions seeking opinions are closed. \$\endgroup\$
    – Andy aka
    Nov 22, 2020 at 17:26
  • \$\begingroup\$ A solenoid works slightly differently from a motor. In particular, a solenoid is like a stalled motor. Your solenoid controller should have high initial current to actuate the solenoid, but then drop to a much lower holding current. \$\endgroup\$ Nov 22, 2020 at 17:47

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Looks like the power on the fan connector on the Melzi board is controlled with a RFP30N06LE power transistor...

enter image description here https://reprap.org/mediawiki/images/7/7d/Melzi-circuit.png

This is a very beefy transistor that can switch up to 30 amps...

enter image description here https://www.mouser.com/ProductDetail/ON-Semiconductor-Fairchild/RFP30N06LE?qs=mdiO5HdF0KgGbixxeJqneA%3D%3D

...so your solenoid should be no problem.

You will want to add a flyback diode across the solenoid so that it does not generate a high voltage across the transistor when the power is turned off. You can google about how that works.

enter image description here

Pick (or make) a solenoid that is rated for 12V so that it does not get too hot and burn out. You can use a lower voltage solenoid too, just do not leave it on long enough for it to get hot.

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