Column: Circuit drill


Does a short-circuit detection suit all applications?


By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions Company, Jordan


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lectronics design egineers commonly apply a detection part in their circuits to alert to a short. This is typically a simple circuit (Figure 1) that detects the resistance of a load – in Figure 1 represented by R6. When the connected load resistance drops severely, the red LED turns on;


otherwise, the green LED stays on, indicating that the circuit is healthy and there is no short. As we all know, a short circuit is just a form of low


resistance, which imposes some very important questions: What is the value of the resistance in the circuit to be considered a short? Does this resistance cause any limitations to circuit designers when deciding to use it in various applications? To answer these questions, I’ve created the following simple setup:


Methodology Since the connected circuit resistance is the parameter used to determine a short circuit, we will use different values for R6


to measure its performance. This will help determine the threshold resistance at which the detection circuit triggers.


Assumptions Throughout the experiment I kept the values of all circuit components fixed: a load resistance of 1kΩ and lower, and a 5VDC voltage source.


Results The load resistance values were plotted against the voltage across the red LED to identify the point at which the detection circuit triggers. Regardless of the varying values for R6, the green LED


stays on and the red off, except when the R6 drops to 42Ω or below, which clearly indicates that this is the detection circuit’s trigger point as a short circuit. If our short-circuit detection setup is used with lower


resistance loads, the circuit will not function and will continuously keep flagging up shorts.


Figure 1: Short-circuit detection setup


Figure 2: Load resistance (R6) vs voltage across the red LED www.electronicsworld.co.uk October 2021 11


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