In a series circuit, all voltage sources and loads are connected, end to end, to form a single path.
If any component of a series circuit becomes open or burnt out, current will be unable to flow.
This circuit shows 2 lamps in series connected to a battery. Because there is only one path, the current is the same through each lamp. But if Lamp 1 burns out, there will be no more current flow and therefore Lamp 2 will go out as well.
You may have experienced this type of connection with a string of lights.
If one of the bulbs burns out, all of the bulbs will go out.
Here's a typical series circuit operating a lamp. If the fuse blows or the switch is opened, the lamp will go out.
In a series circuit, because each load is connected one after the other, we determine the total resistance by adding all the loads together.
100Ω + 25Ω + 120Ω + 80Ω = 325Ω
By using Ohm's Law we can take the total resistance and the source voltage to calculate what the circuit current is.
12V / 325Ω = 0.037A = 37mA
A consequence of this is that if the load is increased the current in the circuit will decrease.
A very powerful troubleshooting tool is Kirchhoff's Voltage Law (KVL). It states:
Start at point A and go around the loop adding up the voltages.
6V + 6V + 12V - 12V - 12V = 0
Now if you add up all of the voltage drops that you measured across the obvious loads (lamps, motors etc.) and it did not equal the voltage sources, then you know that you missed a "load".
That missed load may be some unintended resistance such as corrosion or oxidation on some terminal connection.
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