Calculating Capacitor Charge Time: 1000 uF, R Value

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Discussion Overview

The discussion revolves around determining the charge time for a 1000uF capacitor, particularly focusing on the role of resistance (R value) in the charging process. Participants explore the implications of resistance on charge time and the factors that influence it, including circuit parameters and capacitor specifications.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the R value is arbitrary or should be sourced from a datasheet.
  • Another participant asserts that the resistance should come from the circuit, noting that very low resistance would lead to a rapid charge of the capacitor.
  • A participant inquires about typical resistance values and their dependence on desired current levels.
  • It is suggested that the charge time depends on the resistance and inductance of the entire circuit, emphasizing that capacitors do not charge instantaneously in practical scenarios.
  • One participant highlights critical parameters from manufacturer datasheets, such as effective series resistance (ESR) and ripple current, which affect charge time and safety during charging.
  • A link to an overview of large capacitors is provided for additional context.

Areas of Agreement / Disagreement

Participants express varying views on the role of resistance in determining charge time, with no consensus reached on typical resistance values or the implications of different circuit configurations.

Contextual Notes

Participants mention that the charge time is influenced by various factors including the internal structure of the capacitor, the presence of resistive and inductive components, and the specifications outlined in manufacturer datasheets. There is an acknowledgment of the limitations of charging capacitors too quickly, particularly with electrolytic types.

OZwurld
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Hello all,

I was reading this post on calculating the capacitor charge time:
Code:
https://www.physicsforums.com/showthread.php?t=145203
.

My problem is i have a capacitor (1000uF) and i want to determine its charge time but don't know the R value, is it something i chose arbitrarily or should it be in a datasheet.

Thanks.
 
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It should come from the circuit. If your resistance is at or near 0, then your current is very large and your capacitor would fill up instantaneously.
 
is there a typical value that i should use, or does that all depend on how much current i want?
 
OZwurld said:
is there a typical value that i should use, or does that all depend on how much current i want?

In the limit, it absolutely depends upon the resistance (and the Inductance) of the rest of the circuit. In real life, a Capacitor is never charged "instantaneously". The leads and internal structure of the C will also have resistive and inductive components; when using Capacitors at RF, the self Inductance can be very relevant; any Capacitor can resonate and behave as a short circuit, open circuit or even an Inductor, above self-resonance.
 
The critical parameters relating to minimum charge time will be specified in the manufacturers data sheet.
1000uF will almost certainly be electrolytic, so the parameters will be;
1. ESR, the effective series resistance. 2. Ripple current, I_ripple. 3. Temperature rating.

If you try to charge an electrolytic capacitor too quickly, the high current flow may fuse the internal foil. That can also happen if you short circuit a capacitor. If you repeat the charge-discharge cycle often, the I2R power dissipated in the capacitor ESR may overheat the electrolyte. Keep the current below the ripple current specified.

To charge a capacitor, C, from a fixed DC voltage, V, through a resistor, R, will require the minimum resistor value to be R = V / I_ripple.
 
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here's a pretty good overview of large capacitors

http://www.vishay.com/docs/28356/alucapsintroduction.pdf
 

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