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Thread: How to wire 3V LED into 12V battery

  1. #1
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    How to wire 3V LED into 12V battery

    So I want to install a red blinking LED (to simulate an alarm system) in my rear view mirror. I found this product at radio shack, however I want to wire it into my car's power supply instead of having to run it off batteries (http://www.radioshack.com/product/in...ductId=2062571).

    How would I go about doing this? Would I just a 9v resistor or is there a better way?

  2. #2
    FLAC SNOtwistR's Avatar
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    This little gem is perfect for this maybe overkill but perfect in my opinion SNO http://canada.newark.com/murata-powe...p-4/dp/72R3615
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    It's real small the same size as a regular lm780x but no heat to worry about
    Last edited by SNOtwistR; 08-19-2013 at 11:10 PM.

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    Awesome. How are the wires usually attached to the prongs? Are they soldered on?

    Also, is there any risk of this draining my car battery over time?

  4. #4
    FLAC SNOtwistR's Avatar
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    It's very low draw and yes you just solder on to the leads or use good crimps. Vin Ground Vout and the good thing is it is regulated for the variations of the vehicle voltage SNO
    Last edited by SNOtwistR; 08-19-2013 at 11:55 PM.

  5. #5
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    I thought Sno's recommendation was overkill until I saw it was a (super bright) LED flashing module. If it were the typical 20mA flashing LEDs (no attached module) then a dropping resistor would be fine.

    FYI - I think that little gem regulator's pins are common 0.1" "header" pins, hence you could solder to its female counterpart. PCs typically use such connectors for (3 pin) fans, hence saving you any soldering by cutting the connector with wires from discarded PCs or fans etc.
    Normally connectors are not recommended for automotive applications due to vibration etc (ie, PCB ICs should be soldered and not socketed), but IMO a connector in this case should be fine. Just don't reverse the polarity of the connector!


    And yes, any load will drain a battery, but it's an issue of how big the drain is.
    A normal car battery should handle a 20mA LED for weeks. (As opposed to a 3W bulb (250mA) which may flatten a battery in days.)
    Keep in mind that even if your supabright LED is 50mA - and hence with module maybe 60mA - by using Sno's "switchmode" regulator (instead of an "analog" or linear 3V regulator), you reduce the module/LED's current.
    EG - 60mA @ 3V (= 0.06 x 3 = 0.18W) means 15mA & 12V (.015 x 12 = .18W) and hence less than a plain 20mA LED through a resistor. (The regulator should be ~80% efficient or better, but even if it's a total of 0.25W, that's 12x less than a 3W bulb. Then factor in the duty cycle of the flash - ie, if it's on 20% of the time, then .25W becomes ~0.05W. IOW, the battery should handle that module for weeks... )


    PS - NOW I look at the specs... It's up to 25mA @ 3V, hence even better than my example 60mA above.
    Last edited by OldSpark; 08-20-2013 at 12:12 AM. Reason: PS...

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    Quote Originally Posted by OldSpark View Post
    I thought Sno's recommendation was overkill until I saw it was a (super bright) LED flashing module. If it were the typical 20mA flashing LEDs (no attached module) then a dropping resistor would be fine.

    FYI - I think that little gem regulator's pins are common 0.1" "header" pins, hence you could solder to its female counterpart. PCs typically use such connectors for (3 pin) fans, hence saving you any soldering by cutting the connector with wires from discarded PCs or fans etc.
    Normally connectors are not recommended for automotive applications due to vibration etc (ie, PCB ICs should be soldered and not socketed), but IMO a connector in this case should be fine. Just don't reverse the polarity of the connector!


    And yes, any load will drain a battery, but it's an issue of how big the drain is.
    A normal car battery should handle a 20mA LED for weeks. (As opposed to a 3W bulb (250mA) which may flatten a battery in days.)
    Keep in mind that even if your supabright LED is 50mA - and hence with module maybe 60mA - by using Sno's "switchmode" regulator (instead of an "analog" or linear 3V regulator), you reduce the module/LED's current.
    EG - 60mA @ 3V (= 0.06 x 3 = 0.18W) means 15mA & 12V (.015 x 12 = .18W) and hence less than a plain 20mA LED through a resistor. (The regulator should be ~80% efficient or better, but even if it's a total of 0.25W, that's 12x less than a 3W bulb. Then factor in the duty cycle of the flash - ie, if it's on 20% of the time, then .25W becomes ~0.05W. IOW, the battery should handle that module for weeks... )


    PS - NOW I look at the specs... It's up to 25mA @ 3V, hence even better than my example 60mA above.
    Thank you for taking the time to type of all of that...really appreciate it.

    Dumb question, but won't the alternator "recharge" the battery every time I start the car and negate whatever loss the led causes (albeit as minimal it may be)? When you say it will last for "weeks" surely this refers to the drain on the battery without ever turning the car on, correct?

  7. #7
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    Quote Originally Posted by addnon View Post
    ... won't the alternator "recharge" the battery every time I start the car and negate whatever loss the led causes...
    No - it's not a dumb question. (To quote another hereon - there is no such thing as a dumb question, only dumb answers.)
    But YES - you are quite correct. The alternator will recharge the battery.

    But one major consideration, the more discharged a battery is, the more (faster) sulfates form on the battery plates thereby reducing capacity.
    Sulfate formation is reversed by charging provided it is above 13.8V, hence the "new" charging standard of no more than 14.4V and usually 14.2V...
    BUT after about 1 week, the initial "soft" sulfates become "hard" and they are more difficult to remove. They require physical removal, typically done as part of "battery maintenance" using high recharge currents (>20A) or equalisation etc voltages (above the normal max of 14.4V) etc.

    When do sulfates start forming? Technically as soon as a battery is removed from charging and its "surface charge" dissipates. However the rate of sulfation at near full charge is negligible.
    But hence you'll usually see recommendations that a good and FULLY charged unused battery is recharged every 3 months at most (or 12 months for AGMs).
    And that any reasonably discharged battery (say below 80% State Of Charge for normal cranking batteries) be recharged ASAP - especially for AGMs.


    And yes, "lasting for weeks" refers to the battery powering a low power circuit without alternator recharging.
    But bear in mind other drains like alarms and central locking systems. Your LED will add to those drains.

    As a comparison, common Bosch "S&L" alternators from the 1980s & 1990s had a full-time "S" drain of ~12mA (= 150mW = 0.15W). IMO that is/was way too high for an alternator (or rather, its regulator), but I'm not aware of the industry nor owners making a big deal of it.
    Car alarms etc should only drain a few mA though I have heard of systems that drain tens of mA.
    And I know that my new Alpine HU was draining 120mA which caused starting problems after a few days (until I removed either its const +12V or IGN +12V from my traditional const +12V supply).

  8. #8
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    There are plenty of flashing red 12V LEDs around, why not use one of those instead?

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