# current control for a magnetic coil

Discussion in 'General Electronics Chat' started by llg74, Mar 16, 2011.

1. ### llg74 Thread Starter New Member

Nov 25, 2010
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Dear all,
this is the first time I post a thread. I hope I do it the proper way.
So here is my pb : I want to drive current into a coil so as to generate a magnetic field. This current should be quite strong (up to 5A) and I would like to drive it with various waveforms (square pulses or sinusoidal, up to 1kHz) provided by some external device (output board on a computer). It seems to me that I need a power transconductance circuit. But that's pretty much all I know because I'm not so good in electronics.
Could anyone give me a schematic on how to buid this, or a link to a webpage that describes how to build it and explains the ins and outs?

2. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
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Curious as to what it's for?

3. ### John P AAC Fanatic!

Oct 14, 2008
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Depending on the inductance of the coil, you might have quite a challenge there. The issue would be the energy stored in the magnetic field, if you require very rapid changes. In fact I'd look at that first--the maximum voltage you'd need to apply would be equal to the inductance multiplied by the rate of change of current (Henries * amps/sec). Are you proposing a reasonable system? And then, what is the power required over some period of operation? Where does the energy end up going (now cooling becomes an issue)? Engineering is fun.

4. ### llg74 Thread Starter New Member

Nov 25, 2010
2
0
THanks for your feedback. Here is a bit more detail on what I want to do :
In fact I have 2 projects. The first one is the priority :

1-one coil will have approximately L=6mH and R=3ohms (so the charact. time is 2ms) with a metallic core at the tip of which a high gradient of B field will be generated (I plan to move superparamagnetic microspheres so as to mechanically stimulated biological cells with it).
For this I need a power amplifier that will act as a current source (up to 5A). I want to command this current source with a voltage. Thus, if I understand well, what I need is a power transconductance circuit (that uses whatever feedback to output a current that is proportional to the command voltage, whatever the load of the coil).

2-one will be a bigger coil with L=80mH R=4ohms (char. time = 40ms). I want to use temporal variations on B field to move whiskers of mice (on which magnetic paint has been put). I have seen someone who just crudely fed the the coil with the 50Hz, 220V mains, with the possibility to change the voltage with a Variac. I thought that It would be better if I could choose myself the type of current waveform that I impose. This is why I guess having a voltage to current source would be best.