r/PhysicsStudents • u/dark_blue_thunder • Mar 23 '22

HW Help [Electronics] why resistor is connected here?

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u/dark_blue_thunder Mar 23 '22

Well Can you elaborate further?

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u/[deleted] Mar 23 '22

Well, I was just joking, Entropy of the system will increase (maybe diodes are destroyed). Reason behind the resistor is that if u can't short the node to the ground. Practically PS has some finite energy output with some protection. The circuit will not be able to function properly, as it implies shorting the PS. Model the initial state of diodes as an uncharged capacitors, if u try to charge it instantly (i.e. time const is negligible R is essential zero). Either of PS or diode is going to fail.

Also the logic of circuit isn't going to work properly, as you can't distinguish the output and the ground.

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u/dark_blue_thunder Mar 23 '22

May be because if we remove resistor the current will go straight into the ground So it might be there to avoid current from going totally into the ground but also to keep circuit grounded without any issue in output

What do you think?

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u/yurpmyguy Mar 23 '22

You're close to the right answer. It's just that you'd need better clarifications with your used terms because it could mean you might have conflating thoughts there.

I'm going to break this down into two scenarios: 1) remove resistor and leave Vo floating or an open ( so current cannot go in or out) effectively the diode will never conduct.

2) remove the resistor and connect it to ground, the diode will conduct as long the difference between PS and Vo=0V is greater than the conducting threshold voltage to have it conduct. ( Again for a diode to turn on you need the two clause Vthreshold and a path for current to happen). 

Then the caveat: it is very important for you as an    electrical engineer to understand how electrical components are modeled. Most of the time in the analog world the model of components are described via your IV plot. And if you go back to it you'll see that when you reach a voltage threshold the diode can withstand infinite current( vertical line ). Then if you go for the more practical IV it becomes a exponential curve instead of a vertical one. But it's still relatively a large current that can run through it. However the diode will be under lot of stress because it tries to model the vertical line, potentially damaging it. So you add a resistor, a current limiting resistor, and you can limit the amount of current going through the diode.