Is it ok to use larger than necessary value for R2 with LM350 ?

Discussion in 'General Electronics Chat' started by iinself, Feb 23, 2015.

  1. iinself

    Thread Starter Member

    Jan 18, 2013
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    Hi,
    I am trying to regulate 18V from rectifier, I would like the output to be as close to 18V as possible, so can I use a larger than necessary R2. In this case if I use 150 ohms for R1 and 2.2K for R2 the formula yields an output of 19.5V, I know I will get somewhere around 16V but will LM350 regulate as expected due to the larger R2.

    Thanks
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    It would be REALLY helpful if you posted a schematic so that we had some hint as to what R1 and R2 are.
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The lm350 can handle input vs output voltages of up to 35 volts. Make sure you have a minimum load of 10 mA to insure regulation.

    A 2200 ohm resistor in the current path will not allow the required (worst case) 10mA load but the datasheet also says a typical load to maintain regulation is 3.5 mA. So, in typical cases you will be ok. Unfortunately, like audioguru says, you can't specify a typical lm350 when you buy one.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The R1 ,sets the regulator constant current through the variable R2.

    I would use 120 ohms for R1, then the current will be 10mA, easy to calculate the output voltage.
     
  5. iinself

    Thread Starter Member

    Jan 18, 2013
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    Sorry, here is the circuit from the datasheet. Thanks
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    For an 18V output, the input from the rectifier/capacitor should be at least 20V at the lowest peaks of the ripple. You will get better adjustability if you use two resistors in series for R2, one fixed and one variable. For example, with R1 = 150, a 250 ohm pot in series with (you figure it out) gives you an adjustment range of +/-1V.

    ak
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    R1 150 ohms, R2 1K with a 2k2 pot in series , gives 18V in the centre.
     
  8. iinself

    Thread Starter Member

    Jan 18, 2013
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    Thanks.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    To get the output voltage closer to the input voltage, you could use a low-dropout type regulator.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The reason that R1 is either 120 or 240Ω is to establish a minimum current through the regulator. Imagine this scenario. A load is connected to the output of the regulator through a switch which can be cycled on/off. With the switch open, the load is totally disconnected from the regulator. The only current that is drawn from the regulator is the current that flows through R1.

    The regulator requires a minimum load current to make it regulate properly. If the min. load current spec is not met, the output voltage shoots up several volts above the nominal regulated voltage. Now if you look closely, you will see that if the regulator is operating properly, the voltage across R1 is exactly 1.25V (derived from the internal band-gap reference). So the minimum load current is set by I=E/R = 1.25/120 ~=10mA or 1.25/240 ~= 5mA, which are the minimum specified currents for proper regulation for the two types of regulators.

    If the load on the regulator is static, and never drops below 5mA or 10mA (depending on which regulator), then you can make R1 somewhat higher in value. However, another factor comes into play, which is the change in current that flows out of the Ref pin Iref as a function of input differential across the regulator (Vin-Vout), or the change in Iref as a function of load current IL. Those are specified on the data sheet, and if you make R1 too big, you start compromising the regulation.
     
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