setting voltage cap after opamp

Discussion in 'Homework Help' started by Macabra, Jul 23, 2008.

  1. Macabra

    Thread Starter Active Member

    May 31, 2008
    49
    0
    I'm curious as to what would be the best way to set a voltage cap after an amplifier. What I mean is that if the input to opamp is 300mV and the output is 3.3V how would you set it so it is always 3.3V regardless if the input of opamp increases to like 1V or .5V, etc.

    I was thinking of using regulated zener diode? IN4733 i think is the part..
     
  2. weije1986

    Member

    Apr 19, 2008
    14
    0
    voltage regulator ? try and search about LM1117T V3.3
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    When you say you want to "set" the voltage, I think you want to clamp the voltage so that it does not rise higher than 3.3V.
    The 1N4733 is a 5.1V zener diode. It is tested at 49mA.
    Most opamps limit their output current to 20mA.
    A 3.3V/20mA zener diode is a 1N746A or 1N5226B.

    Low voltage zener diodes are poor. Their voltage changes when their current or their temperature changes.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    What are you trying to protect from overvoltage?
     
  5. Macabra

    Thread Starter Active Member

    May 31, 2008
    49
    0
    I'm trying to protect input to microcontroller :p

    and yes, thank you, I need to clamp it to 3.3V so it does not go higher..or even 3.2V.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    If your microcontroller is a PIC, you can just use a series resistor.
    Is your input a digital signal, or are you driving an A/D converter? The clamp can be sloppy if it's for a digital signal. If it's for the A/D, you need a precision feedback clamp.
     
  7. Macabra

    Thread Starter Active Member

    May 31, 2008
    49
    0
    yeah, it's a PIC. Since it only needs 3.3V to be set to "high" i don't want it to have output coming from the opamp higher than that and damage the PIC. The input of the PIC is A/D, can you please explain to me what a feedback clamp is?

    Much thanks
     
    Last edited: Jul 24, 2008
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Here is an active (feedback) clamp. As you can see, it clamps to 3.3V and 0V. You don't actually need a 3.3V battery, of course. Use the PIC power supply voltage instead. You should add an RC filter between the PIC 3.3V and the op amp positive input.
    If you don't need to protect against negative voltage, you can omit the lower op amp and diode.
     
Loading...