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resistor help....

Discussion in 'General Electronics Discussion' started by t44florida, Jun 11, 2012.

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  1. t44florida

    t44florida

    7
    0
    Jun 11, 2012
    Just a single mom helping son building diorama...
    We purchased 1:160 scale LED street lamps.
    Each led lamp specs are 3v 20ma.
    Manufacturer supplied resistors for 12v power supply.
    Resistors from manufacturer are 1000ohm, .25 watt.

    When I plug in circuit data (12v, 3v drop, 20ma) to resistor calculator:
    First calculator says:
    450ohm resistor, .18 watt, recommend .3 watt

    Second calculator says:
    650ohm resistor, .26 watt recommend .433 watt

    Question: which resistor values are correct?

    Second question: what about the amps from the power supply.... if I have a 500ma supply current vs a 5A supply.... don't you need larger wattage resistor to protect it from 500ma than a 5A?

    Your help is appreciated!
     
  2. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    First thing don't worry about the amp rating of the power supply, not relevant here unless you are doing a lot of lights that exceeds the limits... The reason for the resistor is to limit the current from the supply...

    Next before you commit to any resistors, how many of these are you hooking up? There are more efficient ways of hooking them up then putting a resistor on each one... We can help in that regard :) Also do you by chance have alternate power supplies? If you do the one resistor one LED hookup you can use a lot less voltage...

    As for the correct value resistor, the 1000Ω ones that came with are a safe bet... You won't get full brightness but it's a generic fit all value that the manufacture can use for several different types of LEDs...

    450Ω would be the minimum you would want to go, lots of people will bump up the minimum value a little bit just to be safe, the difference in brightness between the 450Ω and 650Ω will hardly be noticed but 650Ω gives you a little more room for error... My guess is that the second calculator might have had a fool proof margin of error built in for automotive enthusiast since automotive really don't operate at 12 volts ;)
     
    Last edited: Jun 11, 2012
  3. t44florida

    t44florida

    7
    0
    Jun 11, 2012
    Hi Cocacola,
    All total there are 3 meters of LED strip lights (specs state: 12v, 0.4A/meter),10 lamps at 3v 20ma and another 10 incandescent 6v 70ma.
    thanks
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,819
    Jan 21, 2010
    As well as all of that, it may just look better if the LEDs aren't at maximum brightness.

    Connect one up with the supplied 1000 ohm resistors. Note that the LEDs are polarised and you should have been told which way around to connect them.

    Just to be sure, connect one end of the resistor to one lead of the LED, and power connects to the free ends of the resistor and LED.

    with a 500mA supply, you can connect about 50 of these up, but I'd limit it to 30 or so just to keep everything extra happy. A 5A supply could drive 500ish.

    With other wiring arrangements you could drive many more, but let's keep it simple if you only have to drive a few :)
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,473
    2,819
    Jan 21, 2010
    The LED strip can be connected directly to the 12V power supply. This will require 1.2A.

    The LEDs if connected as recommended will require 200mA (ish)

    The 6V lamps should be connected in strings with 2 in series (or they will blow). You could actually experiment with 3 in series if you want a warmer yellow clow from them). In pairs, you would expect a total current of 350mA.

    The grand total is around 1.8A If your power supply is rated at 2A or greater, you're fine.
     
  6. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Can I assume this is for a short run temporary thing? The reason I asked is if so we are going to cheat on the incandescent lamps and run two in series, this will work fine as long as both work, if one blows the other goes out as well... Same with the LEDs...

    Look at the picture attached for bulb and LED wiring, the two LEDs should use a single 330Ω resistor... We could technically run 4 LEDs with no resistor but I don't recommend that... We could also run a group of three with a 150Ω resistor but doing them in 2's works out easier for a ten count ;) Not as efficient but straight forward...

    The strip LEDS just attach to the 12volt supply... You stated a 5A supply and that will be plenty...

    BTW, you can take 3 of your 1000Ω resistors and do them like picture 2 this will get a 333Ω for the LED arrangement I suggested... Or you could only use two (instead of three) that will slightly dim the LEDs but still work fine...
     

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    Last edited: Jun 12, 2012
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