LM317 Power Supply

Discussion in 'General Electronics Chat' started by RodneyB, Jul 20, 2015.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I have built this power supply and its only going to 11.6 volts. I have a 24 Volt transformer. I wanted to set it to 16 Volts. I cant see where I have gone wrong
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What will be the load of the powersupply?
    C2 is rather small and R1 should be 120 Ohms for a LM317.
    For a LM117 the 240 Ohms would be OK.

    Bertus
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Wait until the battery is fully charged, and then measure the final voltage. If the battery is initially discharged (or is bad), then the design is current-limited, and the voltage will not come up until the battery stops drawing current. An SLA or flooded-cell 12V battery will never reach 16V.

    C1 is in the wrong place. If used at all (not really necessary here), it should be between pin 2 and pin 1.
     
    Last edited: Jul 20, 2015
  4. MikeML

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    Bertus, that is an issue only if the supply is unloaded. This one always has a minimum load (LED and battery) in parallel with the feedback sensing resistor.... R1 can be anything up to ~1K
     
  5. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    The load is 70mA. I have not got C2 connected at the moment. I am not sure if connecting the 2 windings is correct. The model transformer I have is 042-5052.0
     
  6. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I don't have the battery connected at the moment. I was trying to set the output voltage first but it will not go over 11 volts.
     
  7. MikeML

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    Oct 2, 2009
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    If the transformer is wired right, the voltage at J1 should be about 35Vdc with some ripple. Are you sure the LM317 is wired right? Does it get hot?
     
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  8. Jony130

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    Feb 17, 2009
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    Without C2 this circuit won't work properly.
     
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  9. MikeML

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    C1 might be needed for stability only if the charger is operated without the battery. It needs to be moved so that it is directly across the 317's pins 2 and 1.
     
    Last edited: Jul 20, 2015
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  10. Jony130

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    12.PNG
     
  11. gerty

    AAC Fanatic!

    Aug 30, 2007
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    What is the AC voltage into the bridge?
    What is the dc voltage out of the bridge?
    Is the voltage adjustable adjustable ?
     
  12. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I have fitted a 1000uf capacitor for C1. The circuit is now working properly.

    I am using transformer 042-5213.0 between pins 6 and 10 I have 21 Volts and I have bridged pins 7 and 9.

    What should the correct size of capacitor C1 be and how do you calculate it.
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Without C1 were you measuring the output voltage with a DMM? If so, it lied as all DMM's do. Without C1, there is full-wave rectified AC going through everything, not DC. This is not a problem if that's what you want and you know how to use it.

    With C1 installed, what is the voltage across C1 and what is the voltage on LM317 pin 2?

    ak

    EDIT - I meant C2, the input bulk filter capacitor.
     
    Last edited: Jul 20, 2015
  14. MikeML

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    To properly calculate C2, here is what you need to know:

    The rms voltage of the secondary winding.
    The effective winding resistance.
    The forward voltage drop of the rectifiers under load
    The maximum output voltage out of the 317.
    The maximum load current out of the 317 at the highest output voltage (above).
    The drop-out voltage of the 317.

    Give me all of these, and I will show you how...
     
    Last edited: Jul 20, 2015
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  15. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    The Voltage across C1 is 32.8Volts and the voltage on pin 2 is 16.1 Volts
     
  16. Jony130

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    But C2 is a input bulk capacitor not C1, you have a typo.
     
  17. MikeML

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    Me and everybody else screwed this up. As posted in the original schematic, C2 is the (input) filter capacitor, and C1 is the LM317 (output) anti-oscillation capacitor.

    Everything I said about the output capacitor should have referred to C1. I went and edited my previous posts... now my references to C1 and C2 agree with the schematic.
     
  18. Silvio

    New Member

    Jul 2, 2015
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    Is it possible that you got the multi turn pot wired the wrong way? If its a bourns pot the wiper is the pin on the back.
     
  19. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    The RMS voltage rating of the secondary winding is 30.8 Volts
    The effective winding resistance is 28.9 ohms
    The forward voltage drop of the rectifier under load is 1Amp
    The maximum load current out of the LM317 is 70mA
    The maximum load Voltage 16.00 Volts

    I don't know what drop out voltage is?
     
  20. paofanello

    New Member

    Jul 16, 2015
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