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Thread: Need to drop 12v to 6v. What resistor to use...

  1. #1
    Flashaholic* AlexGT's Avatar
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    Default Need to drop 12v to 6v. What resistor to use...

    I want to adapt a coleman fluorescent microlight to the base of my Thor spotlight and want to power it from the 12V SLA. the microlight needs 6 volts to work (Uses 4AA batteries)

    What resistor value do I need to use to drop the SLA voltage to 6V-7V?

    Thanks in advance for your help.

    AlexGT

  2. #2

    Default Re: Need to drop 12v to 6v. What resistor to use..

    SLA probably starts about 12.6v and the coleman micro flourescent light is about ~300ma so you need to drop about 5.6-6.6v. Using a version of Ohms law R=E/I we get 6v/.3A= about 20ohms. Since it is dropping 6v at .3A you use the formule P=IE to get wattage dissapation or P=.3*6, 1.8watts. so a 2watt 20ohm resistor should do it.

    My advice though is to use something like an LM317 voltage regulator to do it instead of a resistor. It should be a little more efficient and generate less heat and maintain the voltage when your SLA keeps dropping. Beware though you could use the 6v light and drop your SLA below what is recommended.

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    Default Re: Need to drop 12v to 6v. What resistor to use...

    That will be hard to work out unless you know how much current the light uses, and it uses that much current all the time.

    A better solution is to look at a regulator chip, like a 7806. That will provide regulated 6V from a higher input voltage. You just need that part, two capacitors, and a heat sink. It will get hot, as it dissipates the excess voltage as heat. But any resistor that you choose will need to dissipate the same amount of heat anyways, so dissipating the heat is something you will need to deal with regardless.

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    Flashaholic* MrAl's Avatar
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    Default Re: Need to drop 12v to 6v. What resistor to use...

    Hello there,

    My advice would be to use a down converter such as one of
    National's line of Simple Switchers. Takes a few more
    parts but it's more efficient.
    Here's a brief rundown of what you need:
    input cap
    output cap
    switcher ic
    Schottky diode
    small inductor
    two 1/4 watt resistors

    Take care,
    Al

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    Default Re: Need to drop 12v to 6v. What resistor to use...

    Is a switching regulator a good idea for a flourecent light? Don't they use a small ballast to boost voltage and do its own wizzardry? I don't know switching would cause any problems, but to play it safe I would stick to a linear regulator like the LM317.

    The LM317 linear regulator is really easy to work with and easy to find. Radio Shack even has them. If you are too cheap to pay the dollar for it (probably close to what a high wattage resistor will cost), you can often find them in power supplies such as computers and wall warts.

    Efficiency will be the same for a linear regulator or resistor, just a resistor will not provide a constant voltage. You are asking for smoked parts using a resistor. As I said before, it will cost about thew same.

    Do an internet search for LM317 and have fun!

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    *Flashaholic* andrewwynn's Avatar
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    Default Re: Need to drop 12v to 6v. What resistor to use...

    as long as the output of the switching reg is filtered (capacitor).. it should be ok.. you could try to put two of the 6V lights in series that is certainly the most efficient method.. but they'd have to be matched or if one has a lower impedance that one will rob the other of power.

    -awr

  7. #7

    Default Re: Need to drop 12v to 6v. What resistor to use..

    One other option would be to figure out the ballast circuit and you might possibly be able to replace the transformer with one that is appropriate for a 12v input.

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    Flashaholic* AlexGT's Avatar
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    Default Re: Need to drop 12v to 6v. What resistor to use..

    Thanks for the replies guys, I will check them out.

    AlexGT

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    Flashaholic* fivemega's Avatar
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    Default Re: Need to drop 12v to 6v. What resistor to use..

    You need this

  10. #10

    Default Re: Need to drop 12v to 6v. What resistor to use..

    [ QUOTE ]
    fivemega said:
    You need this

    [/ QUOTE ]

    no workie, guess their generated links time out too quick to link to.

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    Flashaholic* fivemega's Avatar
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    Default Re: Need to drop 12v to 6v. What resistor to use..

    Digikey part number AN7706SP-ND

  12. #12
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    Default Re: Need to drop 12v to 6v. What resistor to use..

    That part will be a lot harder to heat sink and work with than a TO220 7806.

  13. #13
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    Default Re: Need to drop 12v to 6v. What resistor to use..

    I have some 25 ohm , 50 watt resistors - in 1206 format. (so approx 3mm long x 1.6mm wide). Before you get too excited, two minor challenges remain. You still need to heat sink the resistors to remove the heat - and - so far, the people I have sampled them to are not experienced with soldering surface mount parts - and don't realize just how small these little buggers are until they see them in person.

    If you are still interested - pm me.

  14. #14

    Default Re: Need to drop 12v to 6v. What resistor to use..

    The 'use a regulator instead' advice is right. FWIW, I too recommend the 7806, being fixed, it's probably easiest to use. The part and a cap or two and you're good to go.

    It is not at all safe to plan to use a simple resistor. The light is not a simple 'six volt, 300 mA' load. As the gas is cold, the tube (the load for the inverter in the light) changes dramatically as it goes through it's start up cycle. Current drawn changes a lot, typically. A high impedance 12 Volt source (what you have with a series resistor) is going to deliver wildly changing voltages, and may indeed dip far under six (stalling the cycle) as well as being high as the tube lights. In this (lighting) time, the very high impedance of the lamp tube is 'reflected back' into the driver (where the peaks can be many times higher, destroying it by overvoltage).

    A regulator fixes that.

    Doug Owen

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