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Femme_physics
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The way I see it, the more the plates charge, the harder it is to charge them. Therefor capacitors always charge exponentially and linearly. Is that right?
Femme_physics said:So since we have a transistor mediating between the voltage source and capacitor (like in my last exercise), that means it charges from a constant current source, yes?
the more the plates charge, the harder it is to charge them.
I like Serena said:It depends on how the capacitor is charged.
If the capacitor is charged from a constant voltage source, it will be charged exponentially as you say.
If the capacitor is charged from a constant current source, it will be charged linearly (as in your recent thread).
The current source will have to work harder and harder to keep its current constant though. ;)
If the capacitor is charged in another way, it will yet be different.
sophiecentaur said:Woa there A constant voltage source has zero source resistance. The capacitor will charge 'instantly' because the voltage across it will suddenly become equal to the supply volts. You must make it clear that a series R is involved if you want to avoid misunderstanding.
I like Serena said:It's still exponential. :)
In the extreme theoretical case where the resistance is zero, the exponential function turns into a step response.
Only if the transistor is arranged to operate as a constant current source. The transistor circuit could be designed to provide some other characteristic, perhaps something non-linear, so you mustn't assume it to be a constant current source without analysing it.Femme_physics said:So since we have a transistor mediating between the voltage source and capacitor (like in my last exercise), that means it charges from a constant current source, yes?
You would need to look at the circuit or add one that you had designed. What is the exact context of all this? Do you not believe the theory?Femme_physics said:Hmm. How would I know if the transistor acts as a constant current source or not?
You derive equations for its voltages and currents, to establish which variables influence the current charging the capacitor. Just like you did in that other memorable thread.Femme_physics said:Hmm. How would I know if the transistor acts as a constant current source or not?
Femme_physics said:Hmm... ok then, I think I understand. But as far as the constant voltage issue from before... I am in basic electronics and we're only working with ideal voltage sources. I'm not sure how can a capacitor charge "instantly" if everything in nature takes time. Even if it's 0.000000000000000000000000000000001 nanosecs
I'm not sure how can a capacitor charge "instantly" if everything in nature takes time. Even if it's 0.000000000000000000000000000000001 nanosecs
It depends on how the capacitor is charged.
If the capacitor is charged from a constant voltage source, it will be charged exponentially as you say.
If the capacitor is charged from a constant current source, it will be charged linearly (as in your recent thread).
The current source will have to work harder and harder to keep its current constant though. ;)
If the capacitor is charged in another way, it will yet be different.
If you have a symbol in your circuit with the text "current source" next to it, it is also a current source. ;)
Note that your mediating transistor also needs constant voltage between the base and the emitter, before it will behave like a current source.
In your previous threads you have seen a couple of examples of that (for instance a zener diode).
What you need is that the current is the same whether the capacitor is full or empty.
If you can calculate the current in both those cases and it is the same, you have a current source.