Why start here
Almost every question in electronics, from “why did my LED burn out” to “why does my project reset”, comes back to three quantities. Understanding them in plain terms makes the rest of basic electronics much easier. As always, learn with low-voltage sources such as small batteries, and avoid mains electricity, which can injure or kill.
The water analogy
Imagine a closed loop of pipe with a pump and a narrow section.
- Voltage is like the pressure the pump creates. It is the push that makes charge move.
- Current is like the flow rate, how much water passes a point each second.
- Resistance is like a narrowing of the pipe. The narrower it is, the less flows for a given push.
Voltage is measured in volts, current in amperes (amps), and resistance in ohms. Small circuits often use milliamps, which are thousandths of an amp.
A few things this picture gets right:
- Pressure exists even when nothing flows. A battery has a voltage across its terminals with nothing connected, just as a closed tap holds pressure.
- Flow needs a complete loop. Break the circuit anywhere, and the current stops everywhere.
- Current is the same at every point along a single loop, just as the same amount of water must pass every point in a closed pipe.
Ohm’s law
For many simple parts, the three quantities are connected by one relationship:
voltage = current x resistance, or V = I x R.
Rearranged, current is voltage divided by resistance, and resistance is voltage divided by current.
Some hypothetical examples, with the arithmetic checked:
- A 9 volt source across a 1,000 ohm resistor gives 9 / 1,000 = 0.009 amps, or 9 milliamps.
- Double the resistance to 2,000 ohms and the current halves, to 4.5 milliamps.
- A 5 volt source with a current of 0.01 amps through a resistor implies a resistance of 5 / 0.01 = 500 ohms.
So with the same voltage, a larger resistance means less current, and with the same resistance, a larger voltage means more current. That is the idea behind most resistor choices.
Applying it to an LED
An LED is not a simple resistor, so Ohm’s law applies to the series resistor next to it rather than to the LED itself. Suppose a 5 volt supply, an LED that drops about 2 volts when lit, and a wanted current of 10 milliamps. The resistor must take up the other 3 volts. R = 3 / 0.01 = 300 ohms. A standard nearby value, 330 ohms, gives 3 / 330, about 9 milliamps. The LED’s real voltage drop varies, so confirm with its datasheet. This is a typical use of the passive components that every beginner meets.
Power: the cost of current
Power is the rate at which energy is used, measured in watts, and equals voltage times current. In the example above, the resistor dissipates 3 volts x 0.009 amps, about 0.027 watts. A small resistor can handle that. Larger voltages and currents mean more heat, which is why a supply, a wire or a resistor can get hot when overloaded, and why matching a supply to its load matters.
Series and parallel
When resistors are placed in series, one after another, their values add. Two 1,000 ohm resistors in series total 2,000 ohms. In parallel, the current has more paths, so the total resistance is lower than either: two equal resistors in parallel give half the value of one, so two 1,000 ohm resistors give 500 ohms.
Voltage behaves in complementary ways. In a series chain, the supply voltage is shared among the parts in proportion to their resistance. In parallel, each branch sees the full supply voltage.
Where the analogy runs out
Water is helpful, but electricity differs.
- Not every part obeys Ohm’s law. Diodes, LEDs and transistors have more complex behavior, and their current does not rise in a straight line with voltage.
- Resistance of real materials changes with temperature. A lamp filament, for example, has a much lower resistance when cold than when glowing.
- A supply does not force a fixed current. The circuit decides how much to draw, within what the supply can provide.
- AC and fast-changing signals introduce more ideas, such as how capacitors and inductors respond to changes.
Measuring them
A multimeter measures all three, though in different ways. Voltage is measured across a part, with the meter in parallel. Current is measured through the circuit, so the meter must be placed in series, which needs more care. Resistance is measured with the part out of the circuit and unpowered. The article on taking multimeter readings explains the technique safely.
A useful mental habit
When a circuit does something odd, ask three questions. What voltage is each part getting? What current is flowing? Is something in the path offering far more or far less resistance than intended? A short or a missing connection shows up as exactly such a mismatch, which makes these quantities a good basis for circuit troubleshooting too.