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Need help reducing voltage

Discussion in 'General Electronics Discussion' started by Marshseav, Nov 24, 2017.

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  1. Marshseav


    Nov 24, 2017
    I need to reduce the 12v 5a that comes from a computer power source to about 3v. I put a 620 ohm resistor in the pos wire and it doesnt seem to have reduced voltage. It's 12.09v before and after the resistor. The resistor has 614 ohms resistance. I'm a noob and would appreciate if you could tell me what Im doing wrong.Thank you
  2. 73's de Edd

    73's de Edd

    Aug 21, 2015
    If you are trying to . . . and expecting for a 3VDC being at the other lead of that dropping resistor, that is not going to happen until you connect on that 3 Volt at 3 amp load which would actually compute as needing the use of a 1.8 ohm @ 27 watt power resistor for that level of voltage drop.

    With your present miscalculation in the use of such a high value of . . . 620 ohm . . .resistor you should
    experience that 9 volts of expected drop, down to 3 VDC, but with it ONLY having the capability of supplying a feeble . . .and thats heavy on the feeble . . . . . .14 or so milliamperes of current at that 3V level.
    As you try to pull more current, that voltage plummets.

    Sooooooo what current consumption level of are you needing at the 3V end of the resistor ?
  3. AnalogKid


    Jun 10, 2015
    The voltage "drop" across the resistor will vary with the current going through it, according to Ohm's Law.
    E = I x R
    The voltage across a resistance equals the value of the resistance times the current through it. So if you have a 620 ohm resistor, and the current being drawn by whatever is connected to the "output" side of the resistor is 10 mA (0.01 A), then the voltage drop across the resistor will be 6.2 V. If the source is 12.0 V, then the load will see a voltage of 5.8 V. But if the load current increases to 15 mA, then the voltage across the load will decrease to 2.7 V.

    This is why the only way to get a stable voltage reduction that does not vary with the load current is to use a regulating device or circuit.

    AND, the reason you do not see any effect from the 620 ohm resistor is because you probably are measuring it with a meter that has a very high input impedance. Using the same equation, calculate the voltage drop across the resistor when the load current through it is 1 uA (1 microamp, or 1 one-millionth of an amp).

  4. Marshseav


    Nov 24, 2017
    Wow, I am a noob. I guess I should have given more info. I am trying to use this power source to power 20 5mm leds and 10 3mm leds. I have already ''popped''several leds and a bluetooth wireless speaker with the12v supply. What is the accepted volt level for leds? Or is this too general of a question?I am lost , what should I do? Also ,on a separate circuit, I need to supply a wireless bluetooth speaker with the same 12v supply. It requires 5v, I don't know the amps but it has a 3.7v battery.
    Last edited: Nov 24, 2017
  5. davenn

    davenn Moderator

    Sep 5, 2009

    have a look here in our resource section about driving LEDs

    come back with any Q's on anything you don't understand
    it's all pretty straight forward

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