Isn't a short circuit just a parallel circuit with one path having very low resistance? Shouldn't both paths still have the same voltage across them? So why does all the current go through the short while the rest of the circuit goes dark (at least this is my understanding of shorts). You might say that it's because the short has super low resistance and real power sources have physical limitations, but for example: in a NOT gate there are two resistors that prevent the transistor (junction transistor) from getting overwhelmed, and the current remains at a reasonable value. In my mind this clearly creates a parallel circuit, yet the current only flows through one path depending on the input. Why?


closed as unclear what you're asking by CuriousOne, ACuriousMind, Norbert Schuch, user36790, Kyle Kanos Feb 27 '16 at 23:39

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  • $\begingroup$ The design of digital logic is an engineering question, but much of your question indicates that you are not very familiar with electric circuits. "A short" is usually an expression for a conducting path in the wrong place (just like "dirt" is matter in the wrong place). The transistor that is pulling the output of a gate to ground is in the right place, it is not creating "a short". $\endgroup$ – CuriousOne Feb 27 '16 at 2:28
  • $\begingroup$ @CuriousOne While your explanation is correct, I'm going to disagree with your assessment of the term "short." Laymen and non-science trade-level electricians may use short in the sense you say, there are many electrical engineers that will use the term short to describe an appropriate low resistance path. Just a small disagreement. Still appreciate your contributions. $\endgroup$ – Bill N Feb 27 '16 at 4:24
  • $\begingroup$ @BillN: Quite the contrary, I am using it in the way an electrical engineer uses "short". A short doesn't belong in your circuit. If it's there, things are usually going South fast. If I want a conducting path in my circuit, I make a "connection" or a "net", but I don't make a "short". $\endgroup$ – CuriousOne Feb 27 '16 at 4:30
  • $\begingroup$ It would help if you include a schematic of the circuit you're asking about. For example, because there is more than one current path in an inverter circuit, and so when you talk about "the current" doing something it's not clear which current you mean. $\endgroup$ – The Photon Feb 27 '16 at 5:38

A not gate (inverter) is not designed as you describe. The input does not short circuit to ground, it only acts as the Gate to the transistors. The not gate is attached to input, output, Vcc and GND. There are 2 opposite transistors between Vcc and GND. At any time one transistor is high resistance and the other is low resistance. So there is always resistance between Vcc and GND. It's just a matter of whether the transistor "above" the output is most of the resistance or the transistor "below" the output is most of the resistance. Low and High output respectively.

  • $\begingroup$ You seem to be describing a field effect transistor NOT gate. I definitely should have specified but I'm talking about a junction transistor NOT gate. In this design there is only one transistor and two paths to ground. One that bypasses the transistor and another that goes through it. The input is the base of the transistor. When the input is off the current is forced to go through the path that bypasses the transistor. But when the transistor is open the current goes through the transistor directly to ground. But why does the current only go through the transistor when it is at low resistanc $\endgroup$ – Derek Farkas Feb 27 '16 at 4:49
  • $\begingroup$ Whether they're FETs or junction transistors or whether the design uses 2 transistor or 1, the strategy is the same. There are 2 resistors between Vcc and ground. One or both is a transistor or FET. Either there's more resistance "above" the output or there's more resistance below the output. The input is the base or gate of the transistor(s) and so makes the difference in which resistor is stronger. $\endgroup$ – Joe C Feb 27 '16 at 9:31

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