How to only allow 3v - 6v in circuit

Discussion in 'General Electronics Chat' started by palerider2029, Mar 16, 2015.

  1. palerider2029

    Thread Starter New Member

    Mar 16, 2015
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    I've been using fixed 3volt LDO regulators, but I want to be able to allow any voltage from 3v - 6v. Are there similar components that will allow this?
     
  2. blocco a spirale

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    Jun 18, 2008
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    You need to provide more information...
     
  3. palerider2029

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    Mar 16, 2015
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    I've got multiple battery configurations: 3v to 7.4v
    I want to run an LED light between 3v - 6v depending on the output of the batteries not less not more.

    If 3v output is 3v (1 x cr123)
    If 3.7v output is 3.7v (1x 18650)
    If 6v output is 6v (2 x cr123)
    If 7.4 volt output is 6v. (2 x 18650)

    Best if the component allows >500ma of current.

    I've been limiting the voltage to 3v using this component, but this does not allow the LED to perform at the brighter 6v settings when I install the other battery configurations.
    http://www.digikey.com/product-detail/en/MCP1702T-3002E/CB/MCP1702T-3002E/CBCT-ND/1979827
     
  4. blocco a spirale

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    Do you have specs for the LED? Sounds like you need a constant-current source.
     
  5. palerider2029

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    Mar 16, 2015
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    The LED's already have drivers.
    One is 3v - 9v
    The other is 6v only

    Thank you for trying to help me. I really appreciate it.
     
  6. blocco a spirale

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  7. Hypatia's Protege

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    Mar 1, 2015
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    Well... If, as you say, the devices operate properly at 6v, your course would seem obvious? Am I missing something? :confused:

    Regards
    HP
     
  8. palerider2029

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    Mar 16, 2015
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    I was hoping for a simple surface mount component, some type of IC. I'm not very familiar with what is out there.
     
  9. palerider2029

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    Mar 16, 2015
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    Since I use 3v batteries I really need to limit the output to 6v, but still allow the 3v bulb to function normally if the input is only 3v.

    If I used a 6v regulator would it still function at 3v?
     
  10. Hypatia's Protege

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    It would not reliably pass the 3v 'through' if that's what you mean -- I strongly recommend you take the former poster's advice (Re: the linked devices) :)

    Best regards
    HP
     
  11. Hypatia's Protege

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    Mar 1, 2015
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    A diode "OR" arrangement may be useful -- though you'd need to select diodes exhibiting a low forward (bias) 'drop'...

    Really though, no need to be 'afraid' of the boost converters - they're 'self documenting' as it were :D

    Best regards
    HP
     
  12. Alec_t

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    Sep 17, 2013
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    Here's how you could do it :-
    LDO_3-6V_reg.gif
    Tempco is ~ -7mV per degree C.
     
  13. palerider2029

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    Mar 16, 2015
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    Thank you for taking the time to show me this.
     
  14. Dodgydave

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    use a J113 fet in constant current mode,

    uses just 2 components one transistor and one resistor .
     
  15. palerider2029

    Thread Starter New Member

    Mar 16, 2015
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    Would you mind being a little more specific?
    I like that the J113 comes in an sot-23 package.
     
  16. Dodgydave

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    Last edited: Mar 17, 2015
  17. palerider2029

    Thread Starter New Member

    Mar 16, 2015
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  18. palerider2029

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    Mar 16, 2015
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    In simplest terms I basically need gate that will allow any voltage through up to 6v, and then switch off if higher voltages are present.
     
  19. Dodgydave

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    Jun 22, 2012
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    If you want to shut off above a certain voltage you need a Comparator circuit, not a constant current,
    the constant current keeps the same level irispective of the voltage variation.

    What are you trying to make?
     
  20. palerider2029

    Thread Starter New Member

    Mar 16, 2015
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    It is a head mounted flashlight system. It can use 6v surefire incandescent or LED lamps, or 3-9v Solarforce lamps with a variety of battery types.
    I'm trying to regulate the voltage to protect the surefire lamp from 7.4v battery configurations while still allowing the solarforce lamp to run at 3v if the user decides to use that style of lamp.
     
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