Need Help With Reducing Voltage

Discussion in 'The Projects Forum' started by gndn, Jan 8, 2013.

  1. gndn

    Thread Starter New Member

    Nov 9, 2012
    6
    0
    I need some help from the experts out there, I've been working on trying to design a circuit for awhile now and with my limited knowledge of circuit design, I've been unsuccessful thus-far in designing it. Here's what I'm trying to accomplish.

    I need to build a circuit that takes a variable 0-10V DC signal (lets call this the control signal), and based on what the voltage is, I need it to regulate the output of a 12V DC power supply. Let me give you an example...

    When the control signal is running at 10V DC, I want the power source to supply 12V DC. If the control signal is running at 5V DC (half power), I want the power source to supply 6V DC. If the control signal is running at 1V DC, the power supply would supply 0.83V DC, etc...

    Control Signal: 0-10V DC @ 20mA
    Power Supply: 12V DC @ 2A

    The reason for this circuit is I want to be able to take a programmable controller signal and control the voltage but still supply up-to 2A.

    I've been researching a solution for this for over 3 months and haven't quite found what I'm looking for yet. I've looked at resistors, pots, Regulators, switching regulators, op amps, and transistors, but still can't quite get the right design. Please Help!

    Thanks.
     
    Last edited: Jan 9, 2013
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    do you have money to spend? or are you on a hobbyist's budget?
     
  3. gndn

    Thread Starter New Member

    Nov 9, 2012
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    0
    That's a rather interesting question. More than likely hobbyist budget, it's for personal use, but you've got me curious now...
     
  4. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
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    Well, if you were interested in paying $100 or more, there are industrial devices that do what you want. They are called "Signal Conditioners" (google keyword). I briefly searched for one that has a selectable range of 0-12V, but did not find one. I know they exist though.
     
  5. Evil Lurker

    Member

    Aug 25, 2011
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    23
    Actually your going about it backwards. What sort of PSU are you using, a switcher or linear?

    If you are using a linear PSU, the easiest way is with a three terminal device like an LM317. Of course you may want to go with something heavier like an LM350 or a low dropout series like the LD1117s. Anyhoos there are a bunch of them floating about. As best I can remember (I don't have the schematic in front of me) to control the output, instead of using a feedback resistor for R2, you slap a TL431 shunt regulator on the adjust pin in it's place along with an electrolytic capacitor (want to say 22uF) in parallel with R1. Then you add in another connection to your input supply voltage with an appropriate current limiting resistor (8.2K??) to ensure that the shunt regulator maintains a slight bit of quiescent current (can't remember exactly how much but its definitely less than 3ma) flowing into the shunt regulator at all times to ensure proper regulation. Then you simply hook up your control voltage to the reference pin of the shunt regulator
     
  6. KrisBlueNZ

    Member

    Oct 17, 2012
    111
    14
    The answer depends on the power supply.

    Have you got a power supply you want to use? Or a circuit you want to build?

    Have you decided whether it will be a linear or switching circuit?

    Generally any DC regulated power supply consists of a control voltage, a variable pass element, and a feedback loop. The control voltage often comes from a voltage reference. The feedback loop takes a certain fraction of the output voltage and feeds that back for comparison with the control voltage. An op-amp or functionally equivalent circuit compares the control voltage with the feedback voltage, and varies the variable pass element to keep those voltages equal. This may involve varying the bias to a linear pass transistor, or varying the duty cycle to a switching device.

    Adding current limiting to the power supply complicates this design somewhat but the basic principle still applies.

    To use a 0~10V voltage to control a 0~12V output, you need to use the 0~10V control voltage as the reference input to the op-amp in the regulator, and you need to arrange your feedback loop so it has a gain of 10/12. So that when the output voltage is 12V, the feedback voltage will be 10V. The active part of the regulator will then ensure that the feedback voltage equals the control voltage, and you will get the behaviour you want.

    In practice you may have issues with voltages being too close to the negative rail. Some op-amps won't work with voltages closer than a few volts to their negative supply pins; you need a "single supply" op-amp. There could be other issues too; I'm just describing the general principle here.

    Post more details on your power supply design and you will get more specific answers.
     
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