Electronics Basics: Current

Current is the flow of electric charge through a conductor. In a wire, the charge is carried by electrons moving between two points at different voltage.

It is measured in ampere (A). One ampere means one coulomb of charge passing through a point every second. A coulomb is a huge amount of electrons, around 6.24 × 10¹⁸ of them.

In electronics we usually deal with much smaller values. A typical LED wants around 20 milliamperes (mA), which is 0.02 A.

One convention to keep in mind: in circuit diagrams, current flows from the positive terminal to the negative one. Electrons actually move in the opposite direction. The convention was set before electrons were discovered, and we all stuck with it.

How current relates to voltage and resistance

Ohm’s law ties the three fundamental quantities together:

I = V / R

Current equals voltage divided by resistance. Take a 9V battery connected to a 450Ω resistor:

I = 9V / 450Ω = 0.02A = 20mA

Lower the resistance, and more current flows. This is why you never connect an LED directly to a battery: with almost no resistance in the circuit, the current spikes far above what the LED can handle, and it burns out in an instant. The fix is a resistor in series, sized using Ohm’s law to keep the current around the value the LED is rated for.

In a series circuit, the current is the same at every point. Whatever flows through the resistor also flows through the LED.

AC and DC

We have 2 types of current: alternating current (AC) and direct current (DC)

Anyone listens to AC/DC?

In direct current (DC) the current flows in one direction, and it’s the type of current generated by batteries.

In alternating current (AC) the current periodically changes direction, and it’s the current provided by the grid that we connect to in our houses, where we add sockets into our walls.

Those two different types of current have very different characteristics, and they allow very different usages and applications.

For example, DC is not good for long distance transmission of current. We use AC instead, because transformers can raise its voltage cheaply. Higher voltage means lower current for the same power, and lower current wastes less energy as heat in the wires.

Most electronic devices run with DC.

That’s why we use rectifiers that allow us to transform AC into DC. The power brick of your laptop is one: it takes AC from the wall and outputs DC.

We can also do the opposite, using devices called power inverters, used often in recreational vehicles for example.