Engineering Expert Witness Blog

     Last time we looked at a circuit schematic containing an unregulated power supply.  This power supply was connected to an external supply circuit containing a number of components such as electric relays, buzzers, and lights.  Each of these components has a resistance factor, and combined they have a total resistance of R_Total.  We saw that when R_Total increases, the electrical current, I, decreases, and when R_Total decreases, I increases. 

     In contrast to this increasing/decreasing activity of the total resistance R_Total,  the fixed internal resistance of the unregulated power supply, R_Internal, doesn’t fluctuate.  Let’s explore Ohm’s Law further to see how the static effect of R_Internal  combines with the changing resistance present in R_Total to adversely affect the unregulated power supply output voltage, V_Output, causing it to fluctuate.

Figure 1

     In Figure 1 R_Total and R_Internal are operating in series, meaning they are connected together like sausage links.  In this configuration their two resistances add together as if they were one larger resistor.  

     Generally speaking, Ohm’s Law sets out that the current, I, flowing through a resistor in an electrical circuit equals the voltage, V, applied to the resistor divided by the resistance R, or:

I = V ÷ R

     In the case of the circuit represented in Figure 1, the resistors R_Internal and R_Total are connected in series within the circuit, so their resistances must be added together to arrive at a total power demand.  Voltage is applied to these two resistors by the same voltage source, V_DC.  So, for the circuit as a whole Ohm’s Law would be written as:

I = V_DC ÷ (R_Internal + R_Total)

     But, Ohm’s Law can also be applied to individual parts within the circuit, just as it can be applied to a single kitchen appliance being operated on a circuit shared with other appliances.  Let’s see how this applies to our example circuit’s R_Total next week.

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