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A question about 555

Discussion in 'Electronic Basics' started by aman, Apr 23, 2005.

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  1. aman

    aman Guest

    I have seen in some analog circuits that if we need a 2.5 V supply
    there is way to do it that generate a 5V square pulse with duty cycle
    1/2 and pass it through a low pass filter to get a DC 2.5V.

    I was thinking of another way of generating 2.5V. Use a voltage divider
    and then use an opamp buffer.

    Which of the above 2 ways is a recommended way of generating 2.5V DC?
     
  2. This method has problems with additional voltage drop through the low
    pass filter when the load on the 2.5 volt output uses current.
    As long as the load doesn't upset the opamp (less current than its
    output can deliver without current limiting, and capacitance to ground
    that does not make the buffer unstable) this can work fine. You can
    also add a capacitor from the divider output to ground to clean up any
    high frequency noise that was present on the 5 volt supply. The 2.5
    volt output will be less accurate than the 5 volt supply (must include
    divider tolerance and drift and opamp offset voltage).
    I prefer the second for most low current needs. There are also fixed
    and adjustable regulators that are made for this sort of thing.

    For example:
    http://www.national.com/ds/LM/LM117.pdf
    http://www.national.com/ds.cgi/LM/LM4140.pdf
    http://www.national.com/pf/LM/LM4120.html#Datasheet

    These may improve on the voltage accuracy of the 5 volt supply.

    If the load current is more than a few tens of milliamps, you may want
    to look into a switching buck regulator, because of its higher efficiency.
     
  3. aman

    aman Guest

    I just need 2.5V to be at one of the inputs of the opamp. I will be
    using the virtual ground property of opamp getting 2.5V at the other
    input of the opamp also. I dont need practically any current to be
    drawn by the load because after opamp theren will be only a schmitt
    trigger(comparator). So that you know this is part of an water overflow
    detector. Still you think the regulator or voltage divider are good
    ideas for this.
     
  4. A simple two resistor divider may be all you need.
    If only an opamp input is connected to it (no current load except the
    opamp bias current) a pair of 100k resistors may be stiff enough.
     
  5. Jamie

    Jamie Guest

    normally a circuit like that is used to generate - voltages from a
    + source like in battery operated equipment etc. .
    if you already have a + supply and that is all you need, then using
    a simple Voltage regulator from the main source should work.
    something like a resistor and zener combo or a fixed reg. etc..
     
  6. Jamie

    Jamie Guest

    thats what she said!
     
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