Variable DC Power Supply Using LM317T With LED Indication Of Voltage Levels

Discussion in 'The Projects Forum' started by InnocentOfTheWorld, Apr 15, 2010.

  1. InnocentOfTheWorld

    Thread Starter Member

    Apr 15, 2010
    47
    1
    Hello Guys ... !

    I'm assigned to complete an electronics project named 'Variable DC Power Supply Using LM317T With LED Indication Of Voltage Levels' ...

    Max. Output: 15 VDC
    Max. Current: 1 Ampere
    IC Allowed: LM317T Only
    Output Range: 0V to 15V (In steps of 3V ... i.e. 0V, 3V, 6V, ... , 15V)


    Using a transformer of 220/24 VAC (1 Ampere), I stepped down the input ac voltages ...

    Using 4 diodes (1N4007), I successfully rectified ac input signal ...

    What should be the value of filter capacitor, if the maximum voltage required at output is 15 VDC and maximum current allowed is 1 Ampere ... ?

    I've used a capacitor of 1000 uF/50 V, it works fine, if I'm using 15 VDC at output and using a maximum of 100 mA current ... Would it work fine if current at output is 1 A ... ?

    My LM317T is working fine ... I've regulated voltages required at output ... (Remember my output is: 0V, 3V, 6V, 9V, 12V, 15V ...

    Now, my second and last question ...

    I've to display the output voltage level using LEDs ... That is, if output from LM317T is 0V, there shouldn't be any glowing LED (indicating 0V) ... If output is 3V, there must be 1 LED glowing so as to represent 3V visually ... Similarly, if output is 6V, 2 LEDs must be glowing ... If output is 9V, 3 LEDs must be glowing ... If it's 12V, 4 LEDs ... And if it's 15V, 5 LEDs must be glowing ... !

    In short:

    OUTPUT / LEDs Glowing
    0V / 0 LEDs
    3V / 1 LED
    6V / 2 LEDs
    9V / 3 LEDs
    12V / 4 LEDs
    15V / 5 LEDs


    I'm unable to make a circuit with 5 LEDs, sensing voltage levels and displaying on LEDs ... Please, tell me how can I make a circuit, that would visually be displaying output voltage ... ?

    P.S. Ripple factor allowed is 3% ... I can only use ZENER diodes ... I cannot use any ICs for sensing voltage levels ... (I'm restricted to use simple electronics for indication from University) ... Help me out ... Please ...
     
    Last edited: Apr 15, 2010
  2. Bychon

    Member

    Mar 12, 2010
    469
    41
    √2 C Er F = I
    1.414 capacitance (p-p ripple voltage) Frequency = Current
    1.414 C (3% x 15V) 120 = 1 amp

    I was going to suggest zeners for the level indicator, but the assignment told you that. Make the output voltage struggle through progressively larger zeners before it can get to the LEDs.

    Enough clues?
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Build a zener ladder that when the output is at or above the zener voltage, the LED lights.

    [ed]
    Bychon, Hours later and I didnt see your post.. sorry
    But..great minds... ;)
    [/ed]
     
    Last edited: Apr 15, 2010
  4. Bychon

    Member

    Mar 12, 2010
    469
    41
    No problem.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    [ed]
    I had to edit this because I realized I was giving the answer without helping the OP find his way.
    [/ed]

    Do you have a specific zener to use? Did your prof. or University give you a kit to choose from, or do you have to choose the zeners on your own?
     
    Last edited: Apr 15, 2010
  6. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    How are you controlling it? If for example you were using a 6 position rotary switch, you could use a dual one and put the LEDs on the input side.
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
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    Can you really get the output of an LM317 down to 0 volts? I thought it would go down to 1.25 but no lower. You can always drop the output by putting 2 (maybe 3) diodes in series with it, though.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Yes, but you have to tie the ground reference to -1.25 volts.
     
  9. InnocentOfTheWorld

    Thread Starter Member

    Apr 15, 2010
    47
    1
    To Bychon ...

    Thanks for the formula, it helped me in evaluation of capacitance ... :) :)
    Actually, I'm doin' Mechanical Engineering ... So, 'struggle through progressively larger zeners' ... I didn't get it ... :) :) ... !

    To retched ...

    Can you, suggest me a circuit ... ?
    I'm NOT at all given any specified zener diodes to be used in my power supply ... I've to choose 'em myself ... :) :) ... !

    To Markd77 ...

    I'm actually using a '6-way sliding switch' for controlling of the voltages at output ... !

    To John P ...

    Hehehe ... Yeah, Your right ... Output is 1.25 V from LM317T ... I've considered it 0 V, I'm allowed to consider 1.25 V as 0 V, in this supply ... (Since no LED will be glowing at this voltage level, if 5 LEDs are used) ... !
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You can get 0v output easily.

    Unplug it from the mains supply. :)
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    That is a proven way to guarantee 0v.. From a LM317 or pretty much anything else on the mains..

    I can suggest you take a look at VU meter circuits.. These are used to progressively light leds with voltage increases.

    [ed]

    Take a look at, and read this page. It tells you how to pick and use zeners to light an led based on voltage. You can use this as a base and add zeners to ladder up each level
    http://jhau.maliwi.de/mot/voltmeter.html
    [/ed]
     
    Last edited: Apr 15, 2010
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Darn it Sgt...:D don't do that..
    I nearly sprayed on my monitor,
     
    Last edited: Apr 21, 2010
  13. bean1028

    New Member

    Apr 20, 2010
    2
    0
    You could get an led that displays different colors w/ different resistors like you find as smd's on a router or something, and when you select a different voltage a different R is selected, or do the same but with multiple LED's in a row all different color or labeled, but you would not want all 5 lit at the same time. Think about it, when have you ever seen such a simple circuit with a graph style display unless the voltages to the display, line out, LCD, etc. are all drawing off a fixed supply or the line out is regulated/powered separately. I actually don't know too much about electronics per se' but you can't supply 1A with 1A and still have 1A. You need to ditch the 1A limit or limit the 1a after the LED's use their share, if you really want all 5. If that is even possible. Not sure how much power LED's use off the top of my head, but that is what comes to mind now. I just built a small power supply with one this morning and think I might go with a digital readout/selector, and maybe selectable between voltage/current regulating...
     
  14. rajbex

    New Member

    Nov 2, 2009
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