Equations for LM317AHV

Discussion in 'The Projects Forum' started by OldSkoolEffects, Jan 29, 2010.

  1. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    Well, I got tired of buying new wall warts, so I want to use a 48V supply with a Vreg and pot to fine tune the voltage going to a solenoid.

    http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/LM317AHVT/?qs=taDQNAOcNDF8DV4C1kHvzw== seems as though it should work for my application, but the datasheet is unclear to me. Looking at the diagram on page 4, I can't quite figure out the equation that relates Vin, R, Radj, and Vout. I know the reference voltage is 1.25V nominal, but I'm not sure where that is applied in the diagram, and how it fits into the mathematical scheme. Any help would be much appreciated.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What voltage do you want for the solenoid? What current does it take to operate the solenoid?
     
  3. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
    68
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    I want to range the solenoid from around 24V all the way to 48V. The solenoid is about a 40 Ohm coil, so, as SgtWookie keeps telling me, current in the solenoid is a factor of Voltage/40 ohms.
     
  4. eng1ne

    Member

    Dec 4, 2009
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    I can't see any reference to Radj?

    From the datasheet:

    Vo=Vref*(1+R2/R1)+Iadj*R2

    Vref fits into the mathematical scheme simply in that way... The LM317 has an internal voltage reference which ~1.25 V, temperature dependent.

    Am I right in my assumption that you want to know how to value the components?
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Regardless how you set up the Voltage Regulator, you need to consider how much power it has to dissipate to do what you want it to do. Here is a simulation (using my favorite circuit calculator, LTspice) showing the power dissipation that will happen in any linear regulator while supplying a 40Ω resistance from 0 to 40V starting with 48V.

    Are you up to heatsinking the regulator so that it can dissipate more than 14 WATTS?
     
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  6. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    I can heatsink the regulator; that's no issue. This is mostly an exercise to figure out the best voltage to use with this solenoid.

    Eng1ne, you're correct. I'm just trying to find a starting point as far as R1 and a pot (R2). Should I just use the maximum Iadj and go from there?
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    R1 is usually 120 ohms for an LM317 and 240 ohms for a more expensive LM117.
    Then R2 is calculated using simple Ohm's Law arithmatic.
    Calculate the power dissipated in the pot when the output voltage is high and select a pot that can survive the heating.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You could also do it like this:

    [​IMG]

    Normally, 120 Ohms is used for R1 to satisfy the minimum 10mA load required to get the guaranteed regulation. But, that would mean a good bit of power dissipation in the pot, like AudioGuru mentioned.

    This way, the minimum 10mA load is satisfied by the LED - and you'll also have an indication that the output has a voltage on it.
     
  9. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
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    That makes sense to me Sarge. Are the capacitors required, or are they there to help smooth things out?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    If the regulator is more than 6" from the filter capacitor, it needs the 0.1uF input cap; otherwise it may oscillate at high frequency.

    The 1,000uF cap represents the one you already have. Using it will be a good thing.
     
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