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Just starting to learn basic concepts of electronics, and I'm a bit confused looking at this diagram. Previously, we've always had a wire connected to the negative end of the terminal, and that made sense as electrons flowing from negative to positive would re-enter the battery. In this case, how can current flow? Maybe I'm misunderstanding, but this doesn't seem like a closed circuit?

I feel like I'm also pretty shaky on ground, but if the only path for electrons is to ground, how can there be a voltage difference? Also, where does current flowing through the diode go? It can't go through the positive terminal of the battery?

Sorry for the dumb questions!

Circuit Diagram

Analysis

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Just to follow on from the answer given by @JohnRennie.

Those unlabelled triangle in your diagram indicate that those nodes are at the same constant potential which is often chosen to be zero volt as in the case of your circuit diagram.

As far as your circuit diagram is concerned it is exactly the same as if you had connected the two triangles together with a line but there is extra information in that looking at the CRO traces those triangles are assumed to be at a potential of $0\,\rm volt$ and other potentials are measured relative to those unlabelled triangle nodes.


The use of such a symbol reduces the complexity of a circuit diagram as the one below for a Precision Stereo Headphone Amplifier.

enter image description here

Just think what the diagram would look like if all the unlabelled triangles were connected together with lines as were the triangles labelled $+12\,\rm V$ and the triangles labelled $-12\,\rm V$.

I have chosen this diagram to show that the triangle can be labelled to show its potential relative in this case to the unlabelled triangles which are to be taken as being at zero volts.
So in this diagram some nodes are all at a potential of $+12\,\rm V$ and other nodes are all at a potential of $-12\,\rm V$ relative to the unlabelled triangles.

The symbol (inverted T) is sometimes used to designate zero volts but with a label also a stated fixed potential as shown in this diagram with an upright T.

enter image description here

In this diagram there is the familiar earth/ground (GND) symbol which I cannot reproduce as a text symbol and that is taken as being at a potential of zero volts.

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The ground is effectively a current sink or source of infinite capacity so anything at a greater or lower potential than ground will flow current to or from the ground.

If the bottom connectors on V22 and V18 were connected together with a wire then they would have the same potential. However it also means that the current flowing out of V22 has to be the same as the current flowing into V18, and vice versa, because there is no way for current to enter or leave. Connecting both to ground also means they have the same potential, but it removes the requirement that the current flowing out of V22 has to be the same as the current flowing into V18, because ground can act as a current source or sink.

In this particular case the current flowing through V22 and V18 has to be the same because they're connected together by the top wire, so just treat the problem as if it was a closed circuit.

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  • $\begingroup$ @Farcher: Hmm, I thought I'd explained it reasonably well but maybe I'm too familiar with the idea. I'll have a second look and see if I can make it clearer, or you're welcome to edit my answer if you would like to. $\endgroup$ – John Rennie Mar 4 '18 at 8:23
  • $\begingroup$ Ah, I see. Yes, the last paragraph is rubbish. $\endgroup$ – John Rennie Mar 4 '18 at 12:12

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