[TriEmbed] Need simple 1A controlled current power supply

[Rodney Radford]

For the Southeastcon 2017 hardware robotics competition, one of the
requirements is to be able to switch on and off a magnet field.

The field is defined as number of turns, diameter, core, and the current -
1A - through it.  I need a simple (and cheap) way to implement this.  I
will be building 8x of these for the competition and each school will be
building one for their trial runs, so keeping it cheap and simple is better
than elegant and higher cost.

Initially I was planning on a very simple solution - a MOSFET switching
current to the coil in series with a power resistor to drop the voltage.
At first this was a 5 ohm, 5W power resistor (since the coil is below 0.1
ohm), but then that changed to a 12 ohm, 12W power resistor as I switched
over to using 12v to power everything since I needed that for the relays
anyway (for another part of the design).

This works, and is cheap and easy to build and if the coil is only on for a
few seconds at a time, this is a great solution. However, a change in the
contest stated that the coil is initially powered on (so robots can use it
to navigate to the coil) and if the robot fails, it could be on for the
full duration of the 5 minute competition.

I tested this last night and it does work, but 12w through even a metal
power resistor is HOT after about 30 seconds.  I would hate to use this for
5 minutes.  The cost is good - below $2 for the MOSFET and power resistor -
but I am looking to see if there is a better solution.

Some ideas considered:
* use a lower voltage - would work, but I would need a lower voltage 1A
supply
* use a switched regulator - get the lower voltage *and* ability to control
it
* others?

Any thoughts / suggestions?

Btw, I will be bringing this in to the next meeting (next R&A/TAR and next
Triembed) to show the progress on the hardware and software design. Most of
it is done now and I hope to finish it in the next few days.

On another issue - I watched a couple Eagle videos this weekend and was
able to complete the schematic capture for the board and even routed a PCB.
The routes were done with the auto-router and they were ugly, but
functional. Not sure if I will just route it myself, or route some and let
the autorouter finish, etc, but I need to investigate the design rules for
the fab house first to verify I have spacings, hole sizes, trace widths,
etc are correct before I dump too much more time into it.

Thanx,
Rodney
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