Voltage Stepdown Converter

Discussion in 'The Projects Forum' started by KrisUK, Feb 7, 2011.

  1. KrisUK

    Thread Starter New Member

    Feb 27, 2009
    9
    0
    I am working on an automotive project and need to convert a voltage range of 22V to 28V down to a voltage range of 500mV to 5V to interface with the vehicle's controller.

    I've come up with a transistor design but only supplies a variation of around 1V over that range from 3V to 4V, which I believe to be too low a resolution.

    I am also trying to ensure that if a fault occurs, the input of the vehicle's controller will never see anything more than 5V as the maximum input voltage is 5V.

    Any suggestions or guidance in the right direction?
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,155
    3,061
    Automotive stuff may run up against the guidelines here, but what you need is an op-amp. Use resistors to divide down the high range to a peak of, say, 4v. Then compare that input voltage to a reference voltage of ~1v, which you can establish with a regulator, a zener diode, or perhaps an already existing 5v reference in your electronics. Feed the input and the reference to an op-amp with a gain of ~4 (go ahead and do the math). The op-amp can be powered off of your 5v reference regulator. The op-amp will output the voltage you can feed your controller (through a resistor). Choose an op-amp that can sense to the rails.

    This strategy will reconstruct a 0-4V signal, with the "base" voltage removed so that 22v = 0 and 28v = 4.
     
  3. John P

    AAC Fanatic!

    Oct 14, 2008
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    What's the input impedance of the controller? If it's high, all you need is a resistive voltage divider.
     
  4. KrisUK

    Thread Starter New Member

    Feb 27, 2009
    9
    0
    Thank you. I shall give that a go.

    The input impedance of the controller isn't very high. It's only about 36kΩ.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,155
    3,061
    22v/500mv = 44X gain

    28v/5v = 5.6X gain

    Resistors aren't going to do it; you need to subtract an offset. He needs more ∆V than you get after dividing down to under 5V.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    There is a 3 terminal IC, available from many places including Radio Shack, called a 7805. Add 2 capacitors, a 0.1µF on the input and output, and you are good to go. If you need less voltage than 5V a pot may work, depending on whether your load is variable. If it is let me know.
     
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