Need help with op amp design, pls help!

Discussion in 'General Electronics Chat' started by bck1990, Apr 12, 2011.

  1. bck1990

    Thread Starter New Member

    Apr 11, 2011
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    Hi everyone i am using an op amp as a voltage follower for a circuit to control some actuators as the output current from my controlled voltage output (from a device connected to my computer and controlled by LabVIEW) is not high enough. basically i need the output voltage (to the actuators) from the op amp to be equal to the input voltage.

    i also need an LED to indicator that the op amp is working. i have connected an LED that way (shown in the schematic diagram below). output A,B,C and D goes to each of the actuators. the led was connected to the Vss pin of the MAX4234 op amp to ensure that current is following through the op amp and that it is functioning well.

    however i read that the Vss supply is supposed to be connected to ground. from my circuit, when i vary the input voltage (eg input A), the voltage across the V+(Vdd) and V-(Vss) varies (it changes from 3.5V to 3.1V when i change the input at A from 0.1V to 1.5V) and the voltage across the LED varies, does this affect the performance of the op amp? My actuators have a resistance of about 50-60 ohms. should i redesign the circuit to place the LED at a different place without affecting the op amp's function? thank you!

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  2. SgtWookie

    Expert

    Jul 17, 2007
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    You can't put the LED in either the Vcc or Vss power supply path without encountering big problems. You will have to put it somewhere else, and you will need to provide current limiting so that it's rating will not be exceeded.

    The specs say 200mA output drive capability, but you have to read the applications information portion of the datasheet that begins on page 8 and runs through half of page 9.

    You're showing +6v for the supply, and you say your actuators are 50 to 60 Ohms. 6v/50 Ohms = 120mA; you are going to run into problems with package power dissipation if all of your outputs are sourcing/sinking current at the same time.

    You probably won't be able to use an LED, even on a single output - as it will add too much more load.
     
  3. bck1990

    Thread Starter New Member

    Apr 11, 2011
    27
    0
    thank you very much for your reply! i am really a noob at circuit design. do you have any advice for using an LED as an indicator to show the amplifier is working? I am only going to input a voltage of 0 to 1.5V for controlling the actuators as the actuators will break down after 1.5V is exceeded, so the max current across each actuator will be about 25mA (so maximum of 100mA in total for 4 actuators). I put 6V in the power supply as the voltage across the Vdd and Vss pin is only 3.5V (is this equivalent to a 3.5V supply for the op amp?) and the voltage across the LED is 2.5V. however the voltage between Vdd and Vss drops to 3.1 when input A is increased from 0.1 to 1.5V, and the voltage across LED increases correspondingly from 2.5V to 2.9V.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Not offhand.

    1.5v/50 Ohms = 30mA. If your actuators are really 60 Ohms, then it would be 25mA.

    You really need to go by the worst-case scenario, otherwise you can "get bitten".

    You're probably exceeding the current rating of the LED.

    If the LED were on the Vdd side, it would have up to 4*30mA = 120mA current flowing through it, which would be far more than a typical LED could handle.

    You can tell if the output of the opamp is working by measuring the input voltage, and the difference between the input and the output voltage. There should not be more than 8mA difference between the two.
     
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