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I'm currently working on some simple circuits to learn about circuitry, and being a Physics major I have a pretty good grasp of the mathematics and concepts, but I'm a little confused about capacitors. I understand how they work, but am curious about their placement in a circuit.

As a simple example lets say I wanted to charge a capacitor in a circuit quickly and then discharge it slowly. Obviously I would slow the discharge rate by connecting the capacitor across a resistor, but the charging process is where I have a question. Would I still need a resistor? I feel like it depends on what the input to the capacitor would be, and resistors could be used as needed to control the rate of charging. Even still I feel like having a resistor would be a safer approach. Not really sure.

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Well, you know that to charge a capacitor instantly will require infinite current, right? And we know that all real-world power sources have some finite resistance associated with them, as do capacitors (ESR). However, as your intuition suggests, if you just pop your capacitor across a supply you are depending on the parasitic resistance to limit your current. If it doesn't limit your current enough your supply may not be able to handle it and you can get droop or brownout.

So it's better to control the charging impedance to a known value that doesn't stress the rest of the system.

Sometimes, especially in AC/DC supplies a positive tempco thermistor is used to limit the inrush that charges the main cap. As it heats up the resistance decreases and the circuit begins operation.

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  • \$\begingroup\$ So I should compare the voltage being provided by the circuit with the voltage and current capabilities of the capacitor I want to use. Then I can implement resistors as needed to maintain a sustainable setup. If the input voltage and current is at a level that provides an acceptable charging rate though, then I don't need it. Am I understanding right? \$\endgroup\$ – user144153 Aug 4 '14 at 23:39
  • \$\begingroup\$ You have to consider how much current your power supply can handle, and certainly the ripple current and voltage rating of the capacitor. If your supply along with it's output impedance, layout impedance and the ESR of the capacitor gives you a charging current that's acceptable then you don't need a resistor in series. It might be preferable to have one anyway, as those quantities typically don't have minimum specified values. \$\endgroup\$ – John D Aug 5 '14 at 3:29
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Charging up a capacitor quickly and discharging it a bit more slowly is something that you probably do every day without noticing - charge your phone much? That battery acts a lot like a cap. It all has to do with the time constant, which is a product of resistance and capacitance, simply put. So, to charge fast, what kind of resistor would you have to use to shrink your time constant T=RC, C being fixed? And to then discharge it slowly, you might want to choose a resistor that is bigger than the first.

There are tons of analogies between circuit elements and physical components described using DE's. Might help the physicist in you get a better grasp on this. As noted above, it might not always be safe to just hook power supplies up to caps, depending on the cap, power supply, etc...

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    \$\begingroup\$ A battery is not like a a cap. \$\endgroup\$ – Matt Young Aug 5 '14 at 16:32
  • \$\begingroup\$ I agree that they are not the same circuit element, but they are both charge storage devices which charge and discharge according to the impedance of the circuit they are attached to. The key difference is the amount of energy or power they can store/deliver. Batteries are good at storing energy, caps are good at delivering power (energy over a short time). \$\endgroup\$ – RYS Aug 5 '14 at 17:53
  • \$\begingroup\$ What about my truck battery that can deliver 200A for thousands of milliseconds? \$\endgroup\$ – ACD Sep 4 '14 at 14:06
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    \$\begingroup\$ Thousands of milliseconds is...seconds. Anyway, if your truck is at 24V delivering 200A like you say, we're looking at 4.8kW of power (J/s). Some capacitors that they use for lasers can deliver something like 70MW. This is quite a difference in power, but like I said, the cap can't do that for long, but your battery might be able to do 200A at 24V for some minutes. \$\endgroup\$ – RYS Sep 5 '14 at 6:05
  • \$\begingroup\$ Still, there is a distinction to be made between chemical energy storage (battery), vs. electric field storage (capacitor.) The physical differences are what make batteries better at bulk storage but slower delivery, and capacitors the opposite. They operate on two very different principles. \$\endgroup\$ – rdtsc Jan 15 '16 at 21:38

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