variable current

Discussion in 'General Electronics Chat' started by imraneesa, Apr 12, 2015.

  1. imraneesa

    Thread Starter Member

    Dec 18, 2014
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    Lm317.
    The formula for constant current is 1.25/R. If I want 300mA current. 1.25/R=0.3. therefore R=4.1ohm. But I want to be able to adjust current from 150mA to 300mA. How to attach trimmer? What size trimmer and how to calculate?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    a 10 ohms 1 Watt preset will give you the range you need,with the 4 ohm resistor in series rated at 1 watt .
     
  3. imraneesa

    Thread Starter Member

    Dec 18, 2014
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    But do I get small Smd 10 ohm trimmer. My board is very small.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is the starting voltage Vin? What is the min/max output voltage Vout (compliance)?

    At 300mA, you will likely be dissipating so much power that you will need substantial heat-sinking, which is not compatible with a small size, anyway...

    P = (Vin-Vout)*0.3

    Suppose Vin = 12V and Voutmin = 1V, then P= (12-1)*0.3 = 3.3W is huge...
     
  5. imraneesa

    Thread Starter Member

    Dec 18, 2014
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    Vin is 6v. Vout is 2.5v.
     
  6. imraneesa

    Thread Starter Member

    Dec 18, 2014
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    But only reference voltage passes through resistors which is 1.25v. Power on resistors = ixixR = .3*.3*4.1=0.369w.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Simple sim. Top plot pane is load current vs R1 for 0V<Vout<3.5V Note that the regulation stops at ~Vout=3V (Vdo+1.25V).

    Middle pane is power dissipation in the LM317. Note you will need lot of copper on the pcb or TO220 LM317 with tab.

    Bottom pane is power dissipation in R1, which is a fixed resistor plus a trim pot.

    50.gif
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You could use a 5Ω trimmer in series with a 4Ω resistor.
    But the trimmer should be at least a 1/2W device (to carry the maximum of 300mA current) which may be difficult to find.
    An alternate would be to use a 10Ω resistor in parallel with a 10Ω 1/4W pot (along with the 4Ω resistor in series with both).
     
  9. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    I thank everybody for the good information and help.
     
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