It is very common to apply a short-circuit detection circuit to a design to alert to a short; see Figure-1. This circuit is simple and widely used by electronics designers since it detects the resistance of a connected circuit – represented by R6. When the connected circuit resistance drops severely, the LED-Red turns on. Otherwise, the LED-Green stays on, indicating that the circuit is healthy and has no short-circuit.

As we all know, short-circuit is just a form of low resistance occurring, which puts a very important question. The question is, what is the resistance that the circuit starts considering it as a short-circuit? Does this resistance cause any limitation to electronics circuits designers when deciding to use this circuit in various applications? This article will explore the answers to these questions.

Experiment Methodology:

Since the connected circuit resistance is the parameter used to determine the existence of short-circuit, we intended in this experiment to use various values of this resistance (represented as R6) to measure the performance of our short-circuit detection circuit. Using different resistance values, we will identify the threshold resistance where our detection circuit will be activated if we go lower. 

Experiment Assumptions:

Throughout this experiment, we fixed the values of all detection circuit components.

• The values of the load resistance were used from 1K Ohm and lower.
• The voltage source was fixed at 5VDC

Monitored Impacts:

In the following section, the performance of the short-circuit detection circuit is observed when different values of R6 (presumed load resistance) are used. The load resistance (R6) values are plotted against the voltage exists across the (LED-Red) to identify the detection circuit reacted.

Regardless of the different values of R6, the LED-Green stays on, and LED-Red stays off except when the R6 values are at 42 Ohms. Any resistance values that are used below 42 Ohms will activate our detection circuit. This means that if we use any load with 42 Ohms or lower, then our detection circuit will flag it as short.

Figure-2: Load Resistance (R6) vs. Voltage Across LED-Red Diode.

Conclusion:

From the above, we identified that threshold load resistance that flags it as short when connected to our subject short-circuit detection circuit. This load resistance was found as 42 Ohms and lower.

This is an important finding and helps the electronic circuit designers avoid using loads with 42 Ohms or lower input resistance. If our short-circuit detection circuit is used with lower resistance loads, it will not function and continuously keep flagging shorts.