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

    Well-Known Member

    Oct 26, 2011
    541
    100
    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

    Well-Known Member

    Oct 26, 2011
    541
    100
    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

    Well-Known Member

    Oct 26, 2011
    541
    100
    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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