Coil load

Discussion in 'General Electronics Chat' started by Hochberger, Oct 10, 2011.

  1. Hochberger

    Thread Starter New Member

    Mar 15, 2011
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    Hi.
    I started my first op amp project and I am starting to get lost :(

    I use propeller chip to control coil (actuator) with PWM. I have to be able to switch current (voltage) from possitive to negative and also back. I also have to amplify the signal. For that I use LM675 power op amp but I dont know whether I use it right way.

    I use two propeller output pins connected to + and - of LM675 connected as difference amp for switching +/- voltage. I have only single supply so I connect supply pins to +12V and GND and half voltage parallel to pin + (found this in Texas inst. ebook).

    Now the problem. I have the coil connected to power amp output and to GND. The coil has resistance somewhere around few miliOHMs (dont ask why). SO HOW COULD I CONTROL CURRENT GOING FROM OP AMP OUPUT TO COIL? HOW TO GET THE MAX CURRENT OUT OF IT?

    Thanks for every reply
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is this coil you're attempting to drive? And why is it only a few milli Ohms? I have to ask, as if it's that low in resistance it will mean that your LM675 opamp will be terribly stressed trying to source current to it; as the coil will appear as a dead short.

    If it really is only a few milli Ohms in resistance, applying 12v across it will result in hundreds of Amperes of current flow. The LM675 will protect your coil by acting as a fuse. :eek:

    If one end of the coil is connected to ground, you won't be able to reverse the direction of current flow unless you also have a -12v supply on top of the +12v supply.
     
  3. Hochberger

    Thread Starter New Member

    Mar 15, 2011
    4
    0
    Hi, thanks for reply.

    OK, the coil is DIY. It has only about 100 turns and it will be used as an actuator with moveable ("possitionable") neodym magnet inside - so it will need much less current or less turns than with only an iron inside. The coil is in fact a multicoil - number of small coils each one is controllable separately (some Coil Guns principle).

    I know that it has really tiny resistance but there is no other way for me (my teacher demand - if think it is possible much elegantly but cant do anything :( )

    OK, so the only way to do this is to insert resistor between coil and op amp to set the current flow? For me it is wasting of energy and expensive electr. parts.

    For this purpose the op amp is connected as difference op amp. The direction of current flow is substituted by providing positive or negative current from op amp which should reverse the induction direction inside coil. According to my teacher it will work ... I dont have any experience with driving bi-directional solenoids.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I didn't see your reply until just now.
    That's somewhat unfortunate. The opamp has an absolute maximum of 3A, but you really should limit it to less than that; 1 to 2A. So, with a 12v supply, and assuming a ~4v loss in the output from 12v into an 8 Ohm load, that's 1A current and 4 Watts power dissipation in the opamp - which is a fair bit of power to get rid of. If you went to 2A current (a 4 Ohm load) and the output could keep at a 4v drop, that's 8 Watts of power dissipation in the opamp. You'd need a big heat sink to get the heat away from it.

    The power of an electromagnet/solenoid is related to ampere turns; basically the current flow through the coil times the number of turns in the coil. If you need a higher resistance in the coil, you need to use more turns of a smaller diameter wire that has a higher resistance. It would help a lot to know what the diameter of your spool is, what width you have available for each winding, and how large in diameter the windings can be. If you can give me that information, I can tell you how many turns of what gauge wire you'll need to get an appropriate resistance. 8 Ohms would be a likely number; that would be 1A current.

    Well then you'll have to use a resistor.

    Yes.

    If you only have a single supply (12v and ground) and one end of the coil is grounded, you will only be able to have current flow in one direction.

    If you have ±12v supplies (dual rails) OR you can lift the ground from one end of the coil and use a 2nd opamp, then you can reverse the direction of current.
     
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