Post by thread:[EE] INVENT Musing about high power, near-zero ins

On Fri, Jun 10, 2011 at 5:53 AM, Tony Smith <spam_OUTajsmith1968TakeThisOuTgmail.com> wrote: > Zero force and high amps probably aren't good friends. I can certainly imagine a connection made by spring-loaded contact pads and held in place by neodymium magnets. Neodymium magnets do not need to be very large before a substantial amount of force is required to separate them. It is a neat idea, but I imagine it would be substantial amount of work to develop for self-use. It would be a great exploration space for industrial design, but I am certain there must be multiple COTS solutions, if it is just a solution the OP is after. -p

> > Zero force and high amps probably aren't good friends. > > I can certainly imagine a connection made by spring-loaded contact > pads and held in place by neodymium magnets. Neodymium magnets do not > need to be very large before a substantial amount of force is required > to separate them. > > It is a neat idea, but I imagine it would be substantial amount of > work to develop for self-use. It would be a great exploration space > for industrial design, but I am certain there must be multiple COTS > solutions, if it is just a solution the OP is after. Powerful magnets on tools? Might have a few downsides... Tony

On Fri, Jun 10, 2011 at 4:52 PM, YES NOPE9 <.....yesKILLspam@spam@nope9.com> wrote: > Yes I would like to make this into a FLOSS product. > Yes , I think magnets are the way to go for holding force. > Yes, I expect users to unplug the tool while under power ( I can't control what users do ) > > It would be great if the extension cord is "sucked" onto the tool connector and held with great force. > It would be great if this force could be "turned off" easily to allow disconnection. Rare earth magnets will definitely give you the force you are looking for. Turning off that force is difficult, but you could use a lever/wedge to separate the magnets. If you wanted to avoid arcing when the device is disconnected under load, a combination of spring-loaded pins, a sensor to detect the start of physical disconnection, and an electronic cut-off could be used. Such a system is eminently do-able, and the designer in me would love to work on such a project. The practical side says to just use twist-lock connectors or (as someone else mentioned) air tools. -p.

> The problem is movement. Â If it's zero-force to insert, then it probably > won't take much for it to fall out. Â If you do the MacBook thing - > http://en.wikipedia.org/wiki/MagSafe then you could always just use a really > big magnet. > > I have seen mains-rated stuff like that, I can't remember where though. > > Tony > > IVe seen a deep fryer with a magnetic connector, 2 pins for the mains, recessed a good ways inside of the cord connector, and there was also a lip on the connector so you would rock it into place, and the magnet would hold it down. -- Jonathan Hallameyer

If you go for magnetic holding you could perhaps use very strong magnets to hold it in during use and get over the excessive removal force problem by having an eject button that fires up an electromagnet that briefly overwhelms the holding magnets to spit the connector out. There might be some interesting back EMF problems from the ejection coil as the power gets abruptly cut off though... and you don't want to go permanently demagnetising your holding magnets either ... might work though :) Cheers, Robin. {Original Message removed}

> If you go for magnetic holding you could perhaps use very strong > magnets to hold it in during use and get over the excessive removal > force problem by having an eject button that fires up an electromagnet > that briefly overwhelms the holding magnets to spit the connector out. > There might be some interesting back EMF problems from the ejection > coil as the power gets abruptly cut off though... and you don't want to > go permanently demagnetising your holding magnets either ... might > work though :) The is a fully mechanical version of that - http://www.tengtools.com.au/images/D/119081008.jpg. The knob rotates or moves the magnet so it loses it's holding power, not exactly zero-force though. No-volt safety switches work like you describe, although they are really a variation on a relay. You can sell your electromagnet version as a safety feature, when the mains power goes off the cord pops out, thus the tool won't start up when the power comes back. I'm not sure I'd trust it over a no-volt switch box, but anyway. The problem with magnets in the context of the OP's question (power tools) is metal grit getting into everything. With the indicator holder linked it's not much of a problem as the grit can be brushed off, or at least moved around to the side when it won't get in the way. There is a bit of grumbling with this happening with Apple's MagSafe connector, but not many people tend to use their Macbooks next to a chopsaw, so it's rare. Tony