I'm currently working on some 9v batteries with led, so what does a resistor reduce? Both current and voltage? Or just current?


closed as unclear what you're asking by Wesley Lee, R Drast, Dmitry Grigoryev, Leon Heller, Andrew Mar 17 '17 at 12:43

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    \$\begingroup\$ Use a meter. That works. \$\endgroup\$ – R Drast Mar 17 '17 at 11:10
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    \$\begingroup\$ With V=IR, if you know the resistance value and the current going through it, you get the voltage. Anything other than that will need more info than you are providing. \$\endgroup\$ – Wesley Lee Mar 17 '17 at 11:10
  • \$\begingroup\$ @WesleyLee does a resistor reduce both current and voltage? \$\endgroup\$ – user142208 Mar 17 '17 at 11:17
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    \$\begingroup\$ The voltage drop varies with the current. There's no way to get a 'this resistor will drop my 12v battery to 5v to work this logic' statement, if that's what you're looking for. \$\endgroup\$ – Neil_UK Mar 17 '17 at 11:17
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    \$\begingroup\$ Why do we even entertain questions like these where OP has clearly done 0 research on topics that have been covered 10^6 times? \$\endgroup\$ – Bort Mar 17 '17 at 11:50

Resistors don't "reduce" anything. They can be used in a circuit together with other components to reduce current or voltage to something else. For example, consider the basic resistive voltage divider:

With no current being drawn at V2 and V3, those voltages are:

  V2 = V1 * R1 / (R1 + R2)

  V3 = V1 * R2 / (R1 + R2)

One way to think about this is that the current thru the two resistors is equal since they are in series. By Ohm's law, the voltage across a resistor is proportional to the current thru it and its resistance. Expressed in common units:

  V = A Ω

where V is the voltage across the resistor, A the current thru it in Amps, and Ω the resistance in Ohms.

This is the basic equation describing what a resistor does. Think about it until you understand it, then see how the voltages at V2 and V3 follow from it.


The voltage drop across any resistor that adheres to Ohm's Law is always governed by this equation:

V(Across Resistor) = I(through Resistor) * R(Nominal Resistance Value)

The condition for the resistor to obey Ohm's Law is that it has a linear I-V(Current to Voltage) curve, which means that if you plot current vs voltage where current is the y-axis and voltage is the x-axis, you will have a straight line. In other words, increasing the voltage across the resistor by some fixed amount will increase the current through the resistor proportionally.

Again, the voltage drop across the resistor is the energy required to move charge across the resistor, and in any simple circuit where you have a resistor connected to a voltage source, the voltage drop across that resistor is just equal to the voltage of the voltage source. And if you are given the current through the resistor, you just multiply the total current by the resistance value.

Hope that helps!


From Ohm's Law Voltage = Current x Resistance

If you know the current & resistance, then you can easily find the voltage.

To solve circuits the following techniques can be used:

  1. KVL or KCL(derived from Ohm's Law)
  2. Node Voltage Method,
  3. Loop Current Analysis
  4. Superposition Theorem
  5. Thevenin Theorem( or Norton)

Other ways to get the voltage is to use Pspice like circuit simulators.


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