LM317 Issue??

Discussion in 'General Electronics Chat' started by Dr.killjoy, Jun 28, 2015.

  1. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    I was bread boarding a simply lm317 circuit to drop 24vdc to 18vdc but I have some problems ..I am trying to charge a JumpBox or Jump Pack( It's what you use to jump your car when the batteries dead )... The original charger was a wall wart that was 120vac to 18vdc 500ma and it took a crap ..But I found a 120vac to 24vdc wall wart and I hook up my lm317 circuit in order to the 24vdc to 18vdc .. The problem came when I was measuring the output voltage ..When I breadboard it ,I used my power supply set to 24vdc -400ma and the output set to 18vdc with a small load ..I hook up the output to charging port of the jump box and the voltage suddenly jumped down and the power supply output voltage dropped too but I tried to increase the voltage but nothing changed but I could increased the current but not would change ..I am using ebay bought lm317 and I am not sure if they where the problem ..

    Specs
    R1-240ohm
    R2-3.3Kohm
    Power supply
    Mastech D3003
    Max 30vdc\3amps

    Thanks
    Jay Sr
     
  2. #12

    Expert

    Nov 30, 2010
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    1) The 317 needs 10 ma of load. The first resistor is usually 120 ohms, not 240. If you have a permanent load, that will not be necessary.
    2) Which 317? The TO-92 package only passes 100 ma.
    3) If your jump battery is really awfully bad, even a TO-220 will go into safety lock-out.
    30V - 12V is 18 watts per amp. Overheat threat will force the 317 into safety lock-out.
    4) Did you add the little capacitors in the datasheet? They really are important to keep it from oscillating.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    The data sheet suggest using the 240 and I am using a through hole T0-220 and I follow the datasheet to the T with caps before and after the lm317..
    I am not sure what I did wrong here but my though is the jump box is sink to much current and causing the voltage drop ?? The jump box has a charging circuit inside
    and I am not sure if the circuit is broken or if I am doing something wrong ?
    http://www.ti.com/lit/ds/symlink/lm317.pdf
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    The circuit I published works, and 120Ω is usually the required value.

    It guarantees the required 10ma though the LM317 to power it up.
     
    Dr.killjoy likes this.
  6. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    Listen here Wendy stop raining on my Parade with your smart responses to make me look stupid ..



    LOL
    JK
    Thanks and I will check it out later when I have some time and to better understand what going on ..
     
  7. #12

    Expert

    Nov 30, 2010
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    Read closer. The LM117 only needs 5 ma of idle current. The 317 needs 10 ma of idle current.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Doesn't this Jump Pack contain a 12V lead acid battery?
    What voltage did your new charger go to when you connected it?
    A 12V lead-acid battery needs about 14V to charge it. I suspect the original charger had internal resistance that dropped the voltage when it was charging.
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    Your power supply is current limiting. I doubt the 317 has anything to do with that. What do you expect the charge current to be? Are you sure you're current limit is high enough? BTW, you won't see any change while adjusting the current limit until you set it high enough to turn current limiting off. This, the voltage will suddenly jump up to it's setting.
     
  10. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    The box contains a sealed lead acid battery and I thing the voltage dropped to about 16vdc and I will have to change out the R1 resistor and see if that changes anything .. I also realized this is my last .1uf ceramic cap what the crap and I bought a couple packs of assorted ceramic packs from RS before they closed and between both packs I didn't get a single one ..I am about to head out and will post up my results later with pics and see if you guys see anything wrong ..
     
  11. #12

    Expert

    Nov 30, 2010
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    It isn't possible to make a lead acid battery come up to 16 volts just because you set your regulator there. It will always load down to the battery voltage. You have to hit a battery that size with about a dozen amps to get the voltage that high, and it's still temporary. The battery will revert to its normal voltage as soon as you quit driving large amounts of current into it.
     
  12. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    I set the power supply to 400ma of current and when the voltage dropped I increased the current to 3amps and still has the voltage dropped and won't come out of current mode at the 3 amps..
     
  13. Brownout

    Well-Known Member

    Jan 10, 2012
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    Then your load requires more than 3 amps.
     
  14. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    Then why does the original charger work at 18vdc 500ma ??
     
  15. Brownout

    Well-Known Member

    Jan 10, 2012
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    Good question. I wonder if the unit took a dump and blew the original power supply?
     
  16. Brownout

    Well-Known Member

    Jan 10, 2012
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    Have you tried just setting your power supply to 18v and connecting it directly?
     
  17. Dr.killjoy

    Thread Starter Well-Known Member

    Apr 28, 2013
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    Yeah but it sink all 3 amps at lower voltage .. I have a feeling I blew the charging circuit cause I opened the jump box case and it has a LM317 battery charger circuit and I think I wound up blowing up the circuit..I could be wrong but will post up pics up later of the circuit later ..
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    Is your jump pack lithium? - If so, you might need to be a bit careful how you charge it!

    If the precise voltage control for lithium is in the jump pack itself, you're probably OK - but if it relied on the wall wart for that, you could have a problem.
     
  19. #12

    Expert

    Nov 30, 2010
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  20. Brownout

    Well-Known Member

    Jan 10, 2012
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    That tells you your circuit is not the problem.
     
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