Need regulated 5 volts

Discussion in 'The Projects Forum' started by LKS, Apr 17, 2009.

  1. LKS

    Thread Starter New Member

    Nov 7, 2008
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    Please help, anyone. Urgently need a circuit 12volts DC step down to variable 5volts 5milliamp regulators or can op-amps do that? Please help. Thanks alot
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    When you place a post into an existing thread, it can be confusing - especially when the topic is completely unrelated to the original. Please do not hijack threads. We make it quite simple to open a new topic.

    For a load of only 5 ma, a 78L05 is probably a good choice, although the "variable" part is confusing. Is this to be a fixed voltage regulator, or does the output have to vary? A 78L05 will handle current up to 100 ma.

    If it is to be variable, then an LM317 should be adequate.
     
  3. LKS

    Thread Starter New Member

    Nov 7, 2008
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    Hi I urgently need a 12vdc step down to 5volts dc with only a max 5ma current. Please help to give me a circuit. I was thinking LM317 step down to 5volts then series a LM317 current limiting to 5ma. is this workable. Thanks alot
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    Please stop hijacking other threads. Thank you.
     
  5. zimbarak

    Active Member

    Feb 8, 2009
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    i agree with beenthere the best way to dropdown voltage from 12 to 5 is to use Lm7805 votlage regulator since u wanted a fixed voltage !
     
  6. LKS

    Thread Starter New Member

    Nov 7, 2008
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    Thanks Zimbarak for your help. I need variable volts 0 volts to 5volts and maximum is current is 5ma. Does my initial idea of using the LM317 variable and series the LM 317 current limiting to 5ma circuit workable. Thanks alot
     
  7. LKS

    Thread Starter New Member

    Nov 7, 2008
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    Hi beethere and thingmaker3,

    Please pardon me for the intrusion into the wrong thread. I was'nt sure the right way to post it.

    I need a variable 0 to 5 volts, and variable current to max 5ma with a soft start ( delay on). Please help to provide a circuit or direct me to a datasheet where I can read. Thanks alot.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You'll need an LM317L wired as a current regulator to limit the current to 5mA. This will need to be after the voltage regulator. (placing the current regulator before the voltage regulator causes problems due to the current sunk from the Vreg's R1/R2/ADJ portion of the circuit.)

    Note that neither an LM317T nor LM317M will regulate current properly below 10mA; it must be an LM317L. These are available in TO-92 plastic cases; they look just like transistors.

    A resistor (R1) is wired from the OUT to the ADJ terminal, and current is taken from the ADJ terminal.

    Formula for LM317L:
    R1 = Vref / DesiredCurrent, where 5mA <= DesiredCurrent <= 100mA
    Vref may be from 1.2v to 1.3v, but is nominally 1.25v. [eta: typo; corrected from 1.5v to 1.25v]
    You can make it somewhat adjustable by using a 270 Ohm resistor and a 5k pot in parallel. It will not regulate below 5mA.

    That's just the current regulation portion. Note that the current regulator will have a minimum 3v drop between the IN and ADJ terminals.

    For the voltage regulator, use another LM317L. Use a 120 Ohm resistor from the OUT terminal to the ADJ terminal, and a 1k pot from the ADJ terminal to GND.

    Connect the OUT terminal to the IN terminal of the current regulator that was previously discussed. You will need to adjust the 1k pot until the voltage regulator output reads around 8v.

    You will need at least .1uF and 10uF caps on the input and output of the voltage regulator portion. Don't use caps on the output of the current regulator.
     
    Last edited: Apr 25, 2009
  9. Wendy

    Moderator

    Mar 24, 2008
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    With that low a current a zener is also a good option, though the tolerance isn't very good (they regulate well though).
     
  10. zimbarak

    Active Member

    Feb 8, 2009
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    yea bro it depends in wich lm317 regulator ur choosing ! u have 4 series L,M,T,K
    the LM 317 T can handle upt 1.5 ampere ,or if u want u can use the M one it can handle arround 1 ...
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    For our OP's application, out of the regulators you mentioned only the LM317L will get close to what they need to do; regulate current at 5mA. The rest won't regulate at lower than 10mA.
     
  12. LKS

    Thread Starter New Member

    Nov 7, 2008
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    Thanks Sgt Wookie, Bill Marsden and Zimbarak for your help, truly appreciate.
    It great to come to all about circuit forum.
    Nice job Bros.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Here's a schematic on how it should be wired up:

    [​IMG]

    Note that the model in the SPICE library simulation had a Vref close to the extreme limit; I had to increase the 270 Ohm resistor to 280 Ohms in order to achieve the desired limitation.

    Note that the two LM317's will have at least 1.7v+3v = 4.7v dropout across the both of them; so you should use a 10v supply.

    "Rload" simply indicates where you should connect your load.
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Sgt, I don't see how that's gonna work. Have you tried simulating it with a variable load? The constant-current will work, of course, but when the load is greater than 1k, it does not look like a good voltage source. A good current-limited voltage source should cross over from constant voltage to constant current fairly crisply.
    I think he needs a 5mA current source such as LM317L, driving a 5V shunt regulator such as TL431 (Fig. 12 in the Fairchild datasheet).
    If he doesn't want to waste a few mA, then the solution might get more complex.
     
    Last edited: Apr 26, 2009
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