computer-controlled electromagnet?

I'd suggest using a motor driver, like one of these:
  http://www.pololu.com/products/elec.html#motocon

These have a serial interface rather than USB, but you can just add a
USB to serial adapter if your machine lacks a serial port.

In a way, I suppose this is overkill, but it does deliver a lot of
current (which an electromagnet is likely to need) and even gives you
PWM if that should prove useful (for example, if you want to make one of
those magic floating balls).

HTH,
- Joe

I don't know much about the USB port, but since you haven't received a
lot of replies, I will jump in with a bit of informaton.

I don't think you have on-off control of individual pins in the USB
port. It used to be that with Parallel ports, it was very easy to
control those pins, and then connect a circuit to turn other things on
and off. You would definitely need a much more elaborate circuit to do
what you want using USB.

Finally, you need to be aware that electromagnet coils resist the flow
of electricity when they are first powered up as they build up the
magnetic field.  Then when you turn off the electricity to the coil,
the opposite happens.  As the magnetic field collapses, it pushes the
electricity down the wire with enough force to damage many electronic
components.  There are ways to protect against this, such as a diode in
the circuit.  In other words, don't go just connecting a coil to
anything directly connected to your computer unless it is designed to
handle a load that uses a coil. (I.e. a motor driver circuit)

Joe Dunfee

All motors used in robotics are computer controlled electromagnets.


USB ports deliver very limited amounts of current to power the attached
devices.  Unless you have an extremely odd low power application in mind,
it's unlikely the USB port will deliver enough power for you.
Electromagnets tend to take a lot of current.

Any of the USB based motor controllers will work fine to drive an
electromagnet (they are probably overkill but might be your cheapest bet if
you don't know how to design these things for yourself).  You just have to
figure out what voltage and current range you need for your application.

--
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

He is correct...between the excessive current draw and the inductive
spikes the coil would produce, your USB interface/computer would not
last long....today's computers do not have the quality of I/O
protection older machines did (it costs money).

I would recommend implementing voltage/current protection on whatever
interface you end up building.

TMT

cadcoke3@yahoo.com wrote:

<bad engineering>
buy a usb-to-rs232 converter.  (about $10.)  slice open the usb cable
on the converter and tap the power lines.  connect one of the control
lines from the serial port to the gate of a low-voltage high-current
n-channel mosfet.  use that as a low-side switch to power the
electromagnet.  place a diode over the coil in whatever direction won't
cause a short.

keep in mind that a usb device is only supposed to draw 100ma.  it may
draw 500ma if it makes the appropriate reservation with the host, which
you won't be doing.  you want to plug this contraption into its own usb
port so you can cheat on that count and not suffer too much.  if you
don't trust yourself with electronics, you'll want that usb port to not
be one of the ones on your motherboard, so when you cook it, it's
cheaper to replace.

-chris

On Oct 11, 5:11 pm, bigw...@gmail.com wrote: