Hello.
Just wondering if anyone has ever seen any computer-controlled electromagnets? I'd like to turn it on/off via USB or serial cable. It only needs to be powerful enough to hold a few ounces of weight, so it doesn't have to be big. I could make one and use the power from the USB cable, I suppose, but is it possible to write a driver to turn on/off the power on USB or is it always on?
- Steve Webb
I'd suggest using a motor driver, like one of these:
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.
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
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
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