Engineering Expert Witness Blog

    What happened during the short circuit is that the bulk of the current began to follow through the path of least resistance, that of the derailed wheels, rather than the higher resistance of the electric motor.  Electric current, always opportunistic, will flow along its easiest course, in this case the short, thick metal of the wheels, rather than work its way along the many feet of thin metal wire of the motor’s electromagnetic coils.  With its wheels sparking at the site of derailment the train had become an electric toaster within seconds, and the carpet beneath the track began to burn.  Needless to say, mom wasn’t very happy.

     In this instance Ohm’s Law was at work, with a decidedly negative outcome.  The Law’s basic formula concerning the toy train would be written as:

I = V ÷ R

where, I is the current flowing through the metal track, V is the track voltage, and R is the internal resistance of the metal track and locomotive motor, or in the case of a derailment, the metal track and the derailed wheel.  So, according to the formula, for a given voltage V, when the R got really small due to the derailment, I got really big.

     But enough about toy trains.  Let’s see how Ohm’s Law applies to an unregulated power supply circuit.  We’ll start with a schematic of the power supply in isolation.

Figure 1

     The unregulated power supply shown in Figure 1 has two basic aspects to its operation, contained within a blue dashed line.  The dashed line is for the sake of clarity when we connect the power supply up to an external circuit which powers peripheral devices later on.  The first aspect of the power supply is a direct current (DC) voltage source, which we’ll call V_DC.  It’s represented by a series of parallel lines of alternating lengths, a common representation within electrical engineering.

     And like all electrical components, the power supply has an internal resistance, such as discussed previously.  This resistance, notated R_Internal, is the second aspect of the power supply, represented   by another standard symbol, that of a zigzag line.

     Finally, the unregulated power supply has two output terminals.  These will connect to an external supply circuit through which power will be provided to peripheral devices.  Next time we’ll connect to this external circuit, which for our purposes will consist of an electric relay, buzzer, and light to see how it all works in accordance with Ohm’s Law.

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