## Parallel Circuits

A parallel circuit allows for more than one path for current to flow. We have the same two loads from the series circuit below, but we have two paths now for electrons to flow. Each pathway for current to flow is called a branch.

In a parallel circuit, if the light (load) in branch 1 were to be faulty, the light (load) in branch two would still function as there’s another pathway around the faulty load to take for the electrons to flow.

## Ohm’s Law

So let’s quickly recap the representations for voltage, resistance, and amperage in the Ohm’s Law triangle:

E = Voltage (electromagnetic force)

I = Current (amps)

R = Resistance

For a given resistance, the current is directly proportional to voltage. That said, if you increase the voltage in a circuit with a fixed resistance, the current will increase. If you decrease the voltage in a circuit with a fixed resistance, the current will decrease.

Circuit breakers and fuses will protect against both overloaded circuits as well as overcurrent in a circuit. It’s important to know the difference between both.

An overload in a circuit is when there’s an increase of current (amperage) above the rating for the circuit. If the circuit is allowed to draw the excess current, the wire insulation could become damaged or even catch fire from the added heat. An overload in a circuit usually happens over time and not instantly.

## Circuit breakers and fuses

#### Fuses

A fuse is designed to break current flow in the circuit when there’s excessive current flowing through a circuit.

A fuse contains a strip of metal, which when exposed to higher current than designed for will melt/break before any damage can be done to the wires in the circuit.

Example: A circuit with #14 wire is rated for 15A with a suggested 15A fuse. If you have 18A flowing through this circuit, the wire insulation could become damaged and possibly start a fire. The fuse is designed so the metal link inside the fuse will heat up and break the flow of current.

## Ground fault circuit interrupter (GFCI)

A GFCI is not the same as a breaker or fuse. It exists to protect people whereas a fuse or breaker exists to protect the conductors in your house. A GFCI works on the principle that the current moving through a circuit should be the same at all points of the circuit. The GFIC monitors current going from hot to neutral. If the current going from hot differs from neutral, the GFIC trips and stops the flow of current.

## Solenoid

Solenoids operate on the principle of electromagnetism. With current moving through a conductor, a magnetic field forms around the conductor. A single straight conductor by itself will not generate a strong magnetic field, but if the conductor is wound into a tight coil, it will produce a strong magnetic field. Each loop of the coil will create a small magnetic field, and every loop combines to create a larger magnetic field. The more wound loops, the larger the magnetic field produced.

## Back EMF and impedance

When passing an alternating current through a coil of wire, like a transformer or a solenoid valve, the expanding and collapsing magnetic field created by the alternating current induces a back-voltage (back EMF) in the coil which goes against the supply voltage. All coils of wire will have this.

If you have two sets of windings near one another, you can “induce” a voltage from one to the other. A transformer has two sets of windings. The primary and secondary windings. One coil (primary winding) is connected to alternating current, and the expanding and collapsing magnetic field from that coil will “induce” a voltage in the nearby coil (secondary winding). The voltage produced is in the opposite direction, opposing the original voltage.

## Flame rectification

The flame sensor exists to prove a flame across the burners. Without this flame sensor, there’s a risk of releasing gas without flame and risking an explosion upon ignition. The flame sensor is positioned on the opposite end of the burners from the igniter to ensure all burners ignite correctly.

Upon a call for heat, before the burners ignite, the furnace control board will send AC voltage to the flame sensor. This amount of voltage varies from manufacturer to manufacturer, but it’s usually between 80v – 100v. This voltage goes through a wire from the control board, to the wire that connects to the flame rod. You can think of it like the electrons are building up at the tip of the flame rod just waiting for a path so they can keep moving.