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Negative Voltage on Breadboard with no negative voltage input

Discussion in 'General Electronics Discussion' started by ArisAlexiou, Oct 5, 2013.

  1. ArisAlexiou

    ArisAlexiou

    1
    0
    Oct 5, 2013
    Hello, I am new to the forum and i was seeking help. I have a take home lab to do and the objective is to make a few op amps. (Summing amp, difference, voltage follower and proportional amp). My question to you guys is, how can I get a negative voltage on a breadboard which only contain positive voltage and ground. I need +4.5V and -4.5V to power the integrated circuit. (LM 124 N)
    Thanks for reading and thank you for your time
     
  2. duke37

    duke37

    5,231
    718
    Jan 9, 2011
    It depends on the current you want.
    A two transistor or 555 multivibrator can produce an AC source which can be capacitively coupled to a rectifier. There are chips made for this job.
     
  3. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    You could use two identical surplus 5V cell phone chargers with a diode drop to reduce the voltage, two 3-cell battery holders with AA cells in them, a 9V supply with a rail splitter, a 9V supply with a resistive divider or any of a number of other methods.

    Try searching "virtual ground" and "split supply" on this forum or Google.
     
    Last edited: Oct 5, 2013
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    The ICL7660 is the old standard for generating a negative supply rail from a positive supply. It uses a switched capacitor arrangement and it can't supply much current but this is usually OK for op-amp circuits. There are other similar devices such as LT1054 and MAX1044. These are good if you need to generate a negative rail that is more negative than the negative terminal of your power source. Use pi filters on the input and output to avoid switching noise.

    If you only need to split the supply, I recommend the Texas Instruments TLE2425 and TLE2426. The 2426 splits a supply in half, i.e. it generates a "virtual ground" rail half-way between the positive and negative supplies, so you could split a 9V battery into +4.5V and -4.5V; the 2425 splits the supply at 2.5V above the negative rail, which would give you +6.5V and -2.5V which is not what you asked for. These are suitable if your supply is isolated - for example if you have a 9V battery supply that you want to split up, and it's OK for your circuit's "ground" rail to be part-way between the positive and negative rails from the battery.
     
  5. BobK

    BobK

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    Jan 5, 2010
    Or, if dealing with small signals, you can just use an op amp to split the rails. Just tie the output to the - input and place a 1:1 divider on the + input. The output is then at 1/2 the supply. Use about 10K resistors.

    Bob
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    Or if there's no significant current into the ground rail, you don't even need the op-amp.

    The op-amp is a good idea if you're already using three op-amps from a quad package.
     
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