Hello.
I am planning a circuit similar in principle to the following:
Falstad Circuit
A couple of points:
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The arduino will be powered by the USB connector from a PC, and so meaning I may upload a new sketch to it without worrying about disconnecting the power supply of the second part of the circuit.
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The second part of the circuit is powered by a separate power source that is around 12V. This is the higher power part of the circuit.
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Would I have issues of ground loops etc if I connect the two grounds from the emitter of the transistor?
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Would a transistor be a suitable way of controlling this sort of circuit, obviously assuming it can handle the voltage and current of the high power side of the circuit.
My main issue is ensuring I can power and reflash the arduino without worrying about disconnecting the power on the second half of the circuit.
Thank you!
Hello there,
Thank you, yes I had already read that topic a couple of times over before posting. The thing is, it does not say why I would want to isolate the grounds from each other. Is my circuit where everything is contained in a small project box something which I would have to worry about ground loops enough to warrant isolating the grounds?
Thanks.
You can't isolate the grounds in the circuit you have, it won't work. The emitter is common to both halves of the circuit. If you have read the tutorial then you know why they have to share a ground.
As for ground loops, if you connect as drawn, with the emitter as the common point physically exactly as shown then there no loops. Each power supply has it's own wire for -ve back to the supply, the only place they meet is where they have to meet; at the transistor emitter. Well, that's assuming there isn't something missing from the schematic.
Hello Perry,
Lovely tutorial thank you very much for the write-up, it has been a help.
You can't isolate the grounds in the circuit you have, it won't work. The emitter is common to both halves of the circuit. If you have read the tutorial then you know why they have to share a ground.
I don't think I was very clear, yes, I am aware that using a transistor that they must share a common ground. However, I was implying replacing the transistor with something like an optoisolator etc. to ensure the grounds are isolated.
As for ground loops, if you connect as drawn, with the emitter as the common point physically exactly as shown then there no loops.
Lovely thank you very much.
I know it's a lot of work and rather time consuming but have you thought about expanding your tutorial about the actual reasons why you would ever want to/need to isolate grounds.
Thanks again.
Lovely tutorial thank you very much for the write-up, it has been a help.
Thanks, it's good to know it helps. It takes quite a lot of effort to write something like that, most of the effort goes on deciding what NOT to include. Robin2 and Ballscrewbob were a great help in that regard.
Just one thing I want to expand on that I hinted at in my previous response when I said:
that's assuming there isn't something missing from the schematic
What might be missing is that either or both of the USB power supplies might be grounded. The one to your Arduino from your PC will be, or at least it should be, this is not shown on your schematic. The other one I do not know. If it's a typical wall wart type power supply with no earth connection then it won't be. If both are grounded then there is a loop through the power supply grounds. In my experience, for digital circuits, this is not normally a problem. For analogue circuits you can expect hum and noise from ground loops.
I know it's a lot of work and rather time consuming but have you thought about expanding your tutorial about the actual reasons why you would ever want to/need to isolate grounds?
Yes, it's too big a subject! One of the aims of that tutorial was to keep it simple. I'd been trying to write it for months and had a draft that was too complicated and not at all clear. After reading and answering many questions on common grounds it became clear that the thing many people new to electronics don't get is that there must always be a complete circuit for the electrons to circulate around; that's why they are called electrical circuits. That was the key point I wanted to address. The rest follows in answers to questions as they arise.
I am considering adding something about 0V, ground and earth. They are used as if they are the same thing, probably because often they are. It's made more difficult by different use of the words 'ground' and 'earth' in different parts of the world.
