Question about lowering voltage

Discussion in 'General Electronics Chat' started by BrandonNC, Jun 10, 2012.

  1. BrandonNC

    Thread Starter New Member

    Jun 10, 2012
    Hello all, I am new to the site and forums so hopefully this isn't considered a stupid question.

    I am just curious if the calculations for determining what resistor to use for an LED can be used with anything? For example, I know that for a 2.6V 28mA LED I would use a 100 ohm resistor of at least 1/8 watts.

    My question is, if I want to connect a Garmin GPS receiver that requires 5V 80mA to a circuit which has an input of 12V, would I use the same math?:

    (12V - 5V) = 7V
    7V / .080 = 87.5 ohms

    So would a 100 ohm resistor work for this as well, like it would for an LED?

    Also, can anyone explain the math in determine what wattage resistor I should use? (1/8, 1/4, 1/2, 1, 2, etc)

    Thanks a ton!
  2. absf

    Senior Member

    Dec 29, 2010
    I won't do that. A LED is just a component, and its characteristic is very much spelled out on the datasheets. As long as you don't exceed its Max current, it would operate happily with a resistor that limits the current to a safe value.

    A Garmin GPS is not a single component. It's a collection of components built into a working circuit. Though it says 5V 80mA, but the current would vary during operation. Say if the current is increased by 20mA to 100mA, your 100Ω resistor would drop 10V across it leaving 2V for your GPS. I am sure the GPS wouldn't be able to work on 2V.;)

    A proper solution would be to use a voltage regulator like 7805 to supply 5V to your GPS. Try google with 7805 and learn more about it.

    Last edited: Jun 10, 2012
  3. dataman19


    Dec 26, 2009
    Try one of these voltage regulators...
    The Regulators are at the bottom of this entry page.
    I would use the LM7805, and if I needed more current use one of the other voltage regulator circuits that uses a bypass current transistor.
    Incidentally, the bypass current transistor will work with any 3-pin regulator (assuming you are using a PNP transistor for a positive voltage, and an NPN transistor for a negative voltage).
    In reality, I don't think the Garmin device uses over an amp of current, but I would still use a bypass transistor anyway, it takes the load off the regulator, yet you still get all the benefits of the 3-pin regulator (such as short circuit/over cirrent protection).
    Hope the link helps
    Phoenix, AZ
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
    Yes Ohms law works for other than a LED circuit. It is rather universal because current flowing through a resistor causes a voltage drop determined by the resistance and current flowing. V(drop)=I x R as you used in your calc.

    In your example of the Garmin requiring 5V @ 80mA using a 100 Ohm resistor would drop 8 volts instead of the desired 7. You calculated the resistor correctly as 87.5 Ohms but then substituted a 100 Ohm in its place. You would need to use a value closer to the calculated value. The 100 Ohm may work but it depends on how the Garmin device works with only 4 volts supply instead of 5.

    And as suggested by others the resistor method of reducing voltage isn't always the preferred method as it does have some disadvantages which other methods can overcome.