Op-Amp help

Discussion in 'General Electronics Chat' started by Adco, Mar 14, 2012.

  1. Adco

    Thread Starter New Member

    Mar 14, 2012
    4
    0
    Hello

    I need a solution please.
    I have a Hall Effect sensor that I want to use in a magnetic lock to sense when the armature( strike plate) has mated with the stator. There is a change in the magnetic field when the armature couples with the stator. I want to make use of that property to show that the lock is secure.

    The Hall Effect sensor, A1302, from Allegro is what I have in mind. It is a linear ratiometric device. The quiescent output sits at half supply voltage, in this case 2.5V with a 5V supply. The output varies up and down as the magnetic fields change. This change is very small, typically 1.35mV/G. I can see the variations on my scope but I need to make the changes bigger. This is where I need an op-amp expert.

    I want to make the 2.5V offset move down to 0V and only amplify the positive movement so that I have a range that goes from 0V to about 5V for the small change from the sensor. I cannot use an AC amplifier as this is a changing DC voltage only.

    I am stumped by this and would appreciate some help if you are prepared to do that.

    Thank you.
    Adrian Carboni
    Adco Electronics
    South Africa
     
  2. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    The op amp solution is relatively easy, but lets first state the issues that must must be dealt with:
    1. Power supply voltage must be stable in order to keep the sensor Vcc/2 output from changing with power supply variations.
    2. Op amp must have low input offset voltage so this does not contribute to the error--or null must be adjustable.
    3. Adjustable null is still required because stray flux from the earth's magnetic field will alter the sensor's Vcc/2 output.
    4. Temperature stability of all components must be good to minimize effect of changing ambient temp.
     
  3. Adco

    Thread Starter New Member

    Mar 14, 2012
    4
    0
    Thanks jimkieth

    I will use a regulator like a 78L05 so voltage will be stable. Offsets shouldn't be too much of a problem.

    I am really looking for an op-amp configuration that will loose my Vcc/2 offset and bring it down to 0V level and then amplify the smallish hall effect signal.

    I need to be pointed to the layout, for example, differential op-amp or instrumentation op-amp etc. Once I have the clue as to what configuration I should use, the rest should be ok.
     
  4. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Try this simple circuit--after the offset is adjusted, the gain may be adjusted without further affecting offset.

    I have shown the cheap LM358 op amp--its max input offset voltage is poor (±7mV)--it may do the job, or may not--I'll let other readers suggest a better device because there are many, many better devices available with input offset voltages that go down to ±10uV or so.
     
  5. Adco

    Thread Starter New Member

    Mar 14, 2012
    4
    0
    Hi Jim

    Thanks for the circuit. I was hoping to not use a dual supply voltage. If I have no choice, I can generate a split supply from the incoming 12VDC using 2 regulators but would rather not.

    At least I have something to work with now.

    Thanks
     
  6. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    The negative rail need not be regulated--so if you move common 5V away from the incoming +12V via a 79L05, it does it all

    Then translating:
    +12V incoming.. = +5V
    output of 79L05 = 0V or common
    0V in............... = -7V

    Tricky, but may not work if common must be common throughout
    Add at least another 1K load to the output of the 79L05 to help make it more stable--sometimes do poorly with light loads.

    Another solution (perhaps better) would be to make an unregulated negative rail via a simple voltage converter IC--there are a number on the market--do not remember off hand any specifics, but I have used them with good results.
     
  7. Adco

    Thread Starter New Member

    Mar 14, 2012
    4
    0
    Thats what I had in mind...making a psuedo split supply. There is no common ground so no problems there. The micro that makes the main decision, based on the op-amp output, will energise a 5V relay and the potential free contacts go out to the real world as a status indication of whether the maglock is secure or not.

    Thanks.
     
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