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Op-amp behaviour confusing for an amatuer

Discussion in 'General Electronics Discussion' started by MikeyH_0001, Apr 21, 2012.

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  1. MikeyH_0001

    MikeyH_0001

    5
    0
    Apr 21, 2012
    Hello,
    I have been trying out a voltage comparator circuit using a few LM1458 (dual op-amp) ICs. I am a little confused by the behaviour I have been getting in my circuit.

    I have a 5v input to the V+ pin and the V- pin is grounded. The output runs through a 100ohm resistor and then is grounded.
    I have two parallel lines from the 5v supply, one with two 100ohm reistors (1), and the other with a 100ohm followed by a switch followed by a 10kohm resistor (2) where both are grounded.
    The inverting input is connected between the two resistors on line 1, and the non-inverting input is connected between the switch and a 10kohm reistor on line 2.

    My understanding is that this circuit should output 5v (ideal) from the op-amp when the switch is closed and 0v while open.

    What I actually see is 1.4V while closed and 1V while open. This is from an actual power supply of 4.1V (from a 5V regulator w/ 6V battery source).

    Can anyone see any issue with my circuits that would cause this behaviour, or is my understanding of the behaviour of an op-amp off here?

    I'm not sure how tolerant these ICs are to static, but I have been handling them with a grounding strap on. However, I cannot say the same for the staff of the store where I brought them. They handled the chips and then put them straight into a plastic carrier bag (I don't mean an anti-static bag). Being the amatuer, I assumed they knew what they were doing.

    Any help would be greatly appreciated here.
    - Mike
     
  2. davenn

    davenn Moderator

    13,813
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    Sep 5, 2009
    Hi Mikey
    welcome to the forums

    I think you will find that the 458,1458 series of Op-Amps are supposed to run off a DUAL RAIL supply, not a + and GND so you should have a +5 and -5 supply that also has the 0V rail.

    cheers
    Dave
     
  3. davenn

    davenn Moderator

    13,813
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    Sep 5, 2009
    ok found a circuit for a dual supply that I drew for some one else
    you could use a couple of 9V batteries for this and a 7805 and a 7905 regulator

    [​IMG]

    cheers
    Dave
     

    Attached Files:

  4. MikeyH_0001

    MikeyH_0001

    5
    0
    Apr 21, 2012
    Interesting,

    Thank you for that Dave.

    I have never used a dual rail supply for anything I have ever made (not that I've made a lot of circuits). So if I were to run off the dual rail in my desired application, I would use the +5V and the 0V rails for the V+ and V- pins? I was using the term 'ground' loosely in my original post, I was meaning that the circuit returned to the negative terminal of the battery. Is this different than a 0v rail? I will have a read up on this anyway and see what I can find.
     
  5. davenn

    davenn Moderator

    13,813
    1,945
    Sep 5, 2009
    no, you use the +5 and -5V rails for the Vcc pin8 and Vee pin4 respectively, the 0V rail doesnt get connected directly to the chip, unless needed on one of the inputs

    Post the circuit you were working on and I or one of the others can check it out and recommend changes :)

    Dave
     
  6. davenn

    davenn Moderator

    13,813
    1,945
    Sep 5, 2009
    ok... here's an example of an OpAmp using a dual rail
    The Ground symbol denotes connection to the common 0V rail

    the OpAmp is a TL071

    [​IMG]

    cheers
    Dave
     

    Attached Files:

  7. MikeyH_0001

    MikeyH_0001

    5
    0
    Apr 21, 2012
    The goal is to use the op-amp to make a comparison against a fixed-level voltage and the output will be used as an input to an OR gate (74LS32 TTL), so the idea was to have a 5V output if the non-inverting input was greater than the fixed level, otherwise 0V.

    Should I have chosen a different op-amp for this job?

    If I used a dual-rail supply and had the op-amp output -5V if the fixed level input was greater, is it possible to use this as a logical 0? (Never heard of this before, but the datasheet has no minimum voltage for the LOW level input).
     
  8. davenn

    davenn Moderator

    13,813
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    Sep 5, 2009
    You can use an opamp for that job, or you can use a comparator IC
    say a LM311 voltage comparator

    here's a datasheet for it

    the circuit at the bottom of page 13 is for driving a TTL cct, that may help your cause :)


    again post a pic of your circuit diag of what you have done so far :)

    cheers
    Dave
     
    Last edited: Apr 21, 2012
  9. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    exactly what I was thinking. You want a comparator, don't forget the pull-up resistor.

    still want to use an op-amp? Maybe this will help.
    http://www.ecircuitcenter.com/circuits/op_comp/op_comp.htm
     
  10. BobK

    BobK

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    Jan 5, 2010
    I have yet to see an op amp that requires dual supplies. Since they typcially have only V+ and V- power connections, how would they know whether you are using dual supplies or not? The only consideration is what range of votages you need at the output. If you need 0V, you will need dual supplies or a rail-to-rail opamp.

    That said, the LM1458 is only characterized in the datasheet with supply voltage of -15 and +15V, so it probably will not work from either 0 and 5V or -5 and +5V.

    Bob
     
    Last edited by a moderator: Apr 22, 2012
  11. davenn

    davenn Moderator

    13,813
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    Sep 5, 2009
    WOW seriously!! look at the tone control cct I posted up the page, a classic example, I could show dozens more :)

    you read the data sheets ;)


    many ( would hazzard a guess that most ) of the op-amps these days are designed for dual rail supplies... its just the norm now and its almost rare to see single rail supplies on op-amps unless its portable battery operated gear

    Dave
     
    Last edited: Apr 22, 2012
  12. MikeyH_0001

    MikeyH_0001

    5
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    Apr 21, 2012
    [​IMG]

    Here is the circuit as it is desired, sorry it's not as 'clean' as the ones you've shown me.

    So this is my understanding of how I would run this off the dual rail supply you posted earlier. Where Bob said that the datasheet specifies +-15V supply, it also notes that for supply voltages less than +-15V the max output voltage is dictated by the supply voltage. So my understanding here is a lower voltage is fine as long as it is say (a)V and (-a)V. My output on the op-amp in this case would be either +5V or -5V (ideal).
     
  13. MikeyH_0001

    MikeyH_0001

    5
    0
    Apr 21, 2012
    I have had success with the new circuit. My op amps behave correctly on the dual rail supply and the -ve output of the op amps is fine for a low logic signal to the OR gates.

    Thank you for your help.
     
  14. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    You clearly did not understand my post. I did not say that no op amp circuits require dual supplies, I said that no op amps require dual supplies. With only 2 power connections, how can the op amp tell the different between a dual 5V supply and a single 10V supply?

    The reason dual supplies are often needed in a circuit is because the output needs to go to zero or below, and the op amp is not capable of pulling the output all the way down to the V- rail. If the output is always a volt or two above ground and lower than the V+ supply, you do not need dual supplies. I have built AC amplifiers from op amps that are "designed for" dual supplies, simply by using capacitive coupling for the in and out, and setting either the + input to a resistive divider at half the supply voltage. This works just fine.

    Newer rail-to-rail op amps have simply made it possible for the output to get closer to the V+ and V- rails, which makes them usable in a wider variety of circuits with a single supply. For the OPs purpose a rail-to-rail op amp would be ideal since it would work off a single supply and allow him to use the output to swing far enough to use as a logic signal.

    Bob
     
  15. gto_ron

    gto_ron

    7
    0
    Oct 5, 2011
    single supply

    If you are determined to use an op-amp with a 0-5V supply the NS LM358 works fine in that mode. The output will track almost to either rail and it's a low power device so battery operation is fine.

    regards

    ron
     
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