Dropping 12v to 1.5v with a LM317T, help!

Discussion in 'General Electronics Chat' started by DocOctavius, Oct 22, 2013.

  1. DocOctavius

    Thread Starter Member

    Jul 19, 2005
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    Hi guys, I've been reading like crazy trying to figure this out, my goal is to add this little battery powered timer into a 12 volt lamp that typically is plugged into a car lighter socket or a 12v lead acid battery.
    I bought a LM317T and was trying to use a 100k resistor as R1 and a 22k resistor as R2, as per online calculators all recommend. The problem is when
    I put the stuff together and test it I can't get it below 5v, so I don't know where to go from here. I also am not sure how to figure out what the timer draws in amps, so I was sort of guessing on the resistor values. It said I would get 1.5v but like I said it won't get anywhere near that low, I already
    blew out one of my timers getting frustrated and just hooking it up to see
    what happens.
    I don't think I need capacitors, it's a small thing and all wires are less than 3 inches, so what am I missing? The only thing I'm sure about is that this is very confusing and I thought it would be simple.

    Thanks in advance for the help guys
     
  2. BobTPH

    Active Member

    Jun 5, 2013
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    Those resistor values are way too high. Where did you get that calculator?
    Try 330 and 68. The resistors have nothing to do with the load current.

    And you do need capacitors.

    Bob
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    second that..
     
  4. DocOctavius

    Thread Starter Member

    Jul 19, 2005
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    EDIT: thanks for the quick response, just saw that my resistors are too high, I'll get the ones you recommended :)


    And I want to say I have tried all I can to search for details, there's not much clear info anywhere, here or other sites. Lots of discussion, not too many actual answers. The only people who have done something like this never came back to say what they used, so frustrating.

    BTW if it makes a difference, the 12 volt lamp that I'm integrating the timer into is a 9watt fluorescent curing lamp, and the timer is just a little digital kitchen timer so I can cure for 3 minutes and a buzzer goes off- just don't want to have to change the battery for the timer when there's already power to the unit, I'm sure it's simple I just can't make this voltage regulator do what it's supposed to! Maybe it's malfunctioning? any help would be greatly appreciated, I'll paypal you a 6 pack if you can figure this out for me I swear!!
     
  5. DocOctavius

    Thread Starter Member

    Jul 19, 2005
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  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  7. mikeinkaty

    New Member

    Oct 21, 2013
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    Why do you need capacitors when the input is constant DC from a lead acid battery?? I would say yes, if the device was to be used in an automobile with all the transient voltages running around. But I have several VR's that I use in the field with 6 & 12V batteries and I have never used capacitors for those. Would really like to know if I'm missing something.

    Mike
     
  8. DocOctavius

    Thread Starter Member

    Jul 19, 2005
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    I did use those, I clearly said I did. Obviously I did something wrong, but if you put 100 in the R1 space and 1.5v in the output, it says to use a 22 for R2. Since my local RS didn't have those numbers, I tried what I had
    and if you multiply the resistor values it still gets 1.5v so I figured it was just
    trivial. Obviously I don't have the understanding you do, or I would have used the table and not needed to post here.

    Glad you had a laugh at my expense, I was trying to be nice and everything :( It works with the 330 and 68 combo, thanks
     
  9. #12

    Expert

    Nov 30, 2010
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    6,852
    The capacitors are because the regulator chip has so much gain that it tries to switch on and off several thousand times a second. The capacitors slow it down.

    The resistors have to be in the low range because the chip needs at least 5 milliamps of load to keep it from overshooting. 1.25V/.005A = 250 ohms MAX. Some chips need 10 milliamps of load (125 ohms MAX) so people just memorize 120 ohms and use that for every 3 pin regulator. 120 ohms at 1.25V = .0104167 amps. You want 1.5V so you need that current to cause the extra .25V in some resistor...24 ohms.

    See the range we're working in? Little tiny ohms for little tiny voltages.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Little tiny ohms... so cute :D
     
    #12 likes this.
  11. DocOctavius

    Thread Starter Member

    Jul 19, 2005
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    Does it matter what type of caps I use? Like for a 0.1μF I have a polyester film one and then an electrolytic 1μF? Last question I promise
     
  12. #12

    Expert

    Nov 30, 2010
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    I think the poly type is right.
    anybody know for sure?
    Probably both will work because the capacitance is so low that the aluminum cap won't have much ESR.
    Short wires are important, so...try whichever one fits best.

    Ceramic is good, for sure. Used to be those little flat brown ones. Now, they come in lots of colors and some of them are square!:eek:

    @mcgyvr, Sometimes we get a customer that speaks my language.:rolleyes:
     
  13. BobTPH

    Active Member

    Jun 5, 2013
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    So I win the sixpack!

    Bob
     
  14. #12

    Expert

    Nov 30, 2010
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    Put your pitcher under the USB spout.:D
     
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  15. mikeinkaty

    New Member

    Oct 21, 2013
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    So, what are the adverse consequences of not using capacitors? My use without has always been for intermittent duty like with game cameras which may turn on for 1 or 2 seconds maybe 20 times a day.
    Mike
     
  16. #12

    Expert

    Nov 30, 2010
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    Fairly often, the regulator chip oscillates and the output is nowhere near where you want it.
     
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