Basic Electrical Principals

Definitions (from Wikipedia)

Voltage:

Voltage or electric potential tension (denoted V or U and measured in units of electric potential: volts, or joules per coulomb) is the electric energy charge difference of electric potential energy transported between two points.[1] Voltage is equal to the work done per unit of charge against a static electric field to move the charge between two points. A voltage may represent either a source of energy (electromotive force), or lost, used, or stored energy (potential drop)

In the Hydraulic analogy this is equivalent to water pressure in a pipe.

Current:

An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma.[1]

The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second.

In the hydraulic analogy this is equivalent to the flow of water in a pipe (gallons per second)

Resistance:

The electrical resistance of an electrical conductor is the opposition to the passage of an electric current through that conductor. Electrical resistance shares some conceptual parallels with the notion of mechanicalf riction. The SI unit of electrical resistance is the ohm (Ω).

In the hydraulic analogy, resistance is like restrictions in the piple.

# Ohms Law

Ohms Law is the relationship between voltage, current and resistance.: V = I * R

also:

I = V/R

R = V/I

Power is I*V (Amps times Volts) and is measured in Watts

## LED Current Calculation

A typical Red LED has a forward Voltage of 1.7 V and a Max current of 20 mA (.02 A)

If our power supply is 5V:

FIrst subtract the Voltage drop from the Power supply voltage:

V = 5-1.7 = 3.3

3.3 = .02 *R

R = 3.3/.02 = 165    (ohms)

If you put two LEDs in series, the voltage drop would be 3.4V:

R = 1.6/.02 = 80    (ohms)

Here’s a good tutorial on this (and Kirchov’s Voltage law)