Page 166

NORA Oilheat Technicians Manual

Chapter 8 Basic Electricity Fuse Switch 6 Amps Figure 8-7: Volts, amps and ohms in a circuit Power 120 Volts Source 6 Amps 2 Amps 20 Ohms 40 Volts 20 Ohms at Each 2 Amps 20 Ohms Power Source 8-8 Basic Electricity 120 Volts Load 20 Ohms Figure 8-8: Multiple loads in series Series Circuit A series circuit is one where there is only one path for the electric current through the loads. Remember the old Christmas tree lights (each light is a load) where, if one burned out, they all went out? That was a classic series circuit. Aside from the problem of losing one load and shutting off all the rest, the other drawback to a series circuit is that each load steals electricity from the other loads. In a series circuit, the power is delivered in the greatest quantity to the point of largest resistance. This is why poor connections in a circuit, where the resistance is high, tend to overheat. Since voltage applied to a circuit is voltage used by that circuit, more than one load in the same pathway will share the supply voltage. Loads in the same pathway reduce the voltage to each load as well as the current through the entire circuit. The volts drop as they go from load to load. The current flow (amps) is affected by both the volts and the total ohms of the circuit, and is the same in all parts of the circuit. To review, in a series circuit the total resistance (ohms) is equal to the ohms of all the individual conductors, switches, and loads present in the circuit. The current flowing (amps) in all parts of the circuit is the same. By adding all the voltage drops of all the loads together it equals the applied voltage (volts from the source). Figures 8-7 and 8-8. Parallel circuit A parallel circuit has separate branches for each load. This way, if one load burns out, it will not affect the other loads. Also, all the loads receive the same voltage. The loads are parallel to each other. In a parallel circuit, some of the loads and not others can be turned on. For example, the burner motor and the ignition can run separately, so the spark can be shut off once flame is established. In a house circuit, all the lamps and appliances are wired in parallel. This way each can be turned on and off separately. See Figures 8-9B and 8-10B. The resistance of the load in each branch circuit determines the current (amps) delivered to that branch. The current in the common lines is equal to the current flowing in the branch circuits served by


NORA Oilheat Technicians Manual
To see the actual publication please follow the link above