Voltage, Current, and Resistance

Modified: Mar. 14, 2026

Published: Mar. 2, 2026

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Every electronic circuit, from a simple LED blinker to a complex microcontroller system, operates on three fundamental quantities: voltage, current, and resistance. Understanding how these three relate to each other is the single most important concept in all of electronics. Once you internalize Ohm’s law and Kirchhoff’s laws, you can analyze any circuit you encounter. #AnalogElectronics #OhmsLaw #CircuitFundamentals

The Water Analogy

Before diving into equations, let’s build an intuition using water flow.

This analogy breaks down at advanced levels, but for building circuits on a breadboard, it works remarkably well.

Ohm’s Law

The relationship between voltage, current, and resistance is expressed by Ohm’s law:

Where:

• = voltage across the component (volts)
• = current through the component (amperes)
• = resistance of the component (ohms)

You can rearrange this equation three ways depending on what you need to find:

Worked Example: LED Current

Suppose you have a 5V supply and want to drive a red LED. A typical red LED has a forward voltage drop of about 2V and a safe operating current of 20 mA (0.02 A). What resistor do you need?

The resistor must drop the remaining voltage:

Using Ohm’s law to find the resistance:

A standard 150 ohm or 220 ohm resistor will work. Using 220 ohm reduces the current slightly to about 13.6 mA, which is still bright and extends the LED lifetime.

Kirchhoff’s Voltage Law (KVL)

Kirchhoff’s Voltage Law states that the sum of all voltage drops around any closed loop in a circuit equals zero. In practical terms, this means the supply voltage is distributed among all the components in a series circuit.

For a simple series circuit with a battery, resistor, and LED:

This is exactly what we used in the LED example above. The battery provides 5V, the LED drops 2V, and the resistor must drop the remaining 3V. The voltages balance.

Why KVL Matters

KVL lets you find unknown voltages in any loop. If you measure two out of three voltages in a series circuit, KVL tells you the third. When debugging a circuit with a multimeter, KVL is your primary tool: if the voltages around a loop don’t add up, something is wrong (a bad connection, a burned component, or a measurement error).

Kirchhoff’s Current Law (KCL)

Kirchhoff’s Current Law states that the total current entering a node (junction) equals the total current leaving it. No charge is created or destroyed at a junction.

If a wire splits into two branches carrying 10 mA and 15 mA, then 25 mA must be flowing in the wire before the split. This principle is essential when analyzing parallel circuits.

Series and Parallel Resistors

Series Resistors

    VCC
     |
   [R1]  1K
     |
   [R2]  2K
     |
   [R3]  3K
     |
    GND

  R_total = R1 + R2 + R3 = 6K
  Same current through all resistors

Resistors in series carry the same current, and their resistances add:

The voltage divides across them proportionally to their resistance values. A larger resistor drops a larger share of the voltage.

Parallel Resistors

         VCC
          |
    ┌─────┼─────┐
    |     |     |
  [R1]  [R2]  [R3]
  1K    2K    3K
    |     |     |
    └─────┼─────┘
          |
         GND

  Same voltage across all resistors

Resistors in parallel have the same voltage across them, and the total resistance is calculated by:

For two resistors in parallel, there is a convenient shortcut:

Key insight: the parallel combination is always less than the smallest individual resistor. Adding resistors in parallel provides more paths for current, reducing overall resistance.

Quick Reference

Power Dissipation

Every resistor converts electrical energy into heat. The power dissipated is:

Power is measured in watts (W). Standard through-hole resistors are rated for 0.25 W (1/4 watt). If your calculation shows a resistor needs to dissipate more than its rating, it will overheat and potentially burn.

Worked Example: Power Check

Using our 150 ohm LED resistor with 20 mA flowing through it:

This is well within the 250 mW rating of a standard 1/4 watt resistor. No problem.

But if you used a 10 ohm resistor to drop 5V directly:

A 1/4 watt resistor would smoke immediately. Always check power dissipation.

The Voltage Divider

A voltage divider is two resistors in series that produce an output voltage that is a fraction of the input voltage. This is one of the most common circuits in all of electronics.

    Vin (5V)
      |
    [R1]  10K
      |
      +-----> Vout
      |
    [R2]  10K
      |
     GND

  Vout = Vin x R2 / (R1 + R2)
       = 5V x 10K / 20K = 2.5V

Where is connected between and , and is connected between and ground.

Worked Example: 3.3V from 5V

To get 3.3V from a 5V source:

Using and :

Close enough. In practice, you might use and for a higher impedance divider that draws less current from the source.

Voltage Divider Limitations

A voltage divider is not a power supply. When you connect a load to the output, the load acts as a resistor in parallel with , changing the output voltage. Voltage dividers work well for:

• Reducing a signal level (e.g., 5V logic to 3.3V logic input)
• Reading a potentiometer position
• Creating a reference voltage for a high-impedance input (like an ADC or op-amp)

They do not work well for powering circuits that draw significant current. Use a voltage regulator for that (see Lesson 6).

Practical Build: Voltage Divider and LED Circuit

Components Needed

Circuit 1: LED with Current Limiting Resistor

1. Connect the power rails Connect your 5V source to the positive rail and ground to the negative rail on the breadboard.

2. Place the resistor Insert a 220 ohm resistor between the positive rail and a breadboard row.

3. Place the LED Insert the LED with the anode (longer leg) in the same row as the resistor, and the cathode (shorter leg, flat side) connected to the ground rail.

4. Power on and observe The LED should light up. If it does not, check that the LED is not reversed.

5. Measure the voltage across the resistor Set your multimeter to DC voltage mode. Place the red probe on the resistor lead closest to the positive rail, and the black probe on the other lead. You should read approximately 3V (since the LED drops about 2V from the 5V supply).

6. Measure the voltage across the LED Move the probes across the LED. You should read approximately 2V.

7. Verify KVL Add both measurements. They should total approximately 5V (the supply voltage). This is Kirchhoff’s Voltage Law in action.

8. Measure current Break the circuit (remove one jumper) and insert the multimeter in series, set to mA mode. You should read approximately 13 to 14 mA with a 220 ohm resistor.

Circuit 2: Voltage Divider

1. Build the divider Connect a 1k ohm resistor from the positive rail to a middle row. Connect a 2k ohm resistor from that middle row to ground.

2. Measure the output Place your multimeter probes across the 2k resistor (the middle row to ground). You should read approximately 3.33V.

3. Try different ratios Swap in different resistor values and predict the output voltage before measuring. Try 10k + 10k (should give 2.5V), or 1k + 10k (should give about 4.55V).

4. Use a potentiometer Replace both resistors with a 10k potentiometer. The wiper (center pin) gives you a continuously variable voltage from 0V to 5V as you turn the knob. This is exactly how analog volume controls work.

Verification Checklist

Common Mistakes

Resistor Color Code Quick Reference

Most through-hole resistors use colored bands to indicate their value. For a 4-band resistor:

Common colors: Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9.

Example: Red, Red, Brown, Gold = 22 x 10 = 220 ohm, 5% tolerance.

When in doubt, use your multimeter to measure the actual resistance. It takes five seconds and eliminates guesswork.

How This Connects to Embedded Systems

When you see a 10k pull-up resistor on an I2C bus in a later course, you will understand why 10k was chosen (low enough for reliable communication, high enough to limit wasted current) because you understand Ohm’s law.

Summary

In the next lesson, we will add capacitors and inductors to the mix, introducing the concept of time-varying behavior that is essential for understanding decoupling, filtering, and timing circuits.