Engineering Expert Witness Blog

      Last time we began our discussion on unregulated power supplies and how they’re affected by power demands within their circuits.  Our schematic shows there are two basic aspects to the circuit, namely, its direct current source, or V_DC,  and its internal resistance, R_Internal.  Now let’s connect the power supply output terminals to an external supply circuit through which electrical current will be provided to peripheral devices, much like all the kitchen gadgets mentioned above.

Figure 1

     The external supply circuit shown in Figure 1 contains various electronic components, including electric relays, lights, and buzzers, and each of these has its own internal resistance.  Combined, their total resistance is R_Total, as shown in our schematic.

     Current, notated as I, circulates through the power supply, through the external supply circuit, and then returns back to the power supply.  The current circulates because the voltage, V_DC, pushes it through the circuit like pressure from a pump causes water to flow through a pipe.

     R_Total and I can change, that is, increase or decrease, depending on how many components the microprocessor has turned on or off within the external supply circuit at any given time.  When R_Total increases, electrical current, I, decreases.  When R_Total decreases, electrical current I increases. 

     Next time we’ll continue our discussion on Ohm’s Law, introduced last week, to show how the static effect of R_Internal  interacts with the changing resistance present in R_Total to adversely affect an unregulated power supply’s output voltage.

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