RC discharging

Thread Starter

Drmario5237

Joined Oct 14, 2018
65
I want to build something that uses a capsitor to store charge and discharge the capsitor though a resistor. I looked up the formula for this and a RC discharging calculator that has time of discharge, supply voltage , starting voltage, ending voltage, resistance and capacitance but what if like to know is how much current/amps is discharging from the capsitor at any time though the resistor in the process and how to figure out what ohms resistance I use to get a certain current/amps from the capsitor because I have a load that needs a certain current/ amps to run it. What do I need to do to do this and how do I figure out how to get that certain current/amparage from the farad supercapsitors I'm using. Thanks.
 

Papabravo

Joined Feb 24, 2006
21,157
You have an equation for the voltage on the capacitor for any condition charging or discharging. The current through the resistor at any point in time is simple V(t)/R. Since R is a constant the current must be an exponential just like the voltage. The only condition where this is not true is the condition where there is no voltage difference across the resistor, so the current flowing in the resistor is 0.

If you have a device that requires a constant current you'll have a very difficult time using a capacitor to supply that current. What I can't figure out is why you would want to do that. Do you think capacitors are like cheap batteries? Do you think they operate by magic if you don't let the smoke out? Come to think of it what do you think?
 

ebeowulf17

Joined Aug 12, 2014
3,307
I want to build something that uses a capsitor to store charge and discharge the capsitor though a resistor. I looked up the formula for this and a RC discharging calculator that has time of discharge, supply voltage , starting voltage, ending voltage, resistance and capacitance but what if like to know is how much current/amps is discharging from the capsitor at any time though the resistor in the process and how to figure out what ohms resistance I use to get a certain current/amps from the capsitor because I have a load that needs a certain current/ amps to run it. What do I need to do to do this and how do I figure out how to get that certain current/amparage from the farad supercapsitors I'm using. Thanks.
Generating a constant current will involve wasting a significant portion of your stored energy as heat dissipated at the regulator.

How much current are you hoping to get out of a super capacitor, and how long are you hoping to get it for? It may not be feasible to use a capacitor at all - you may need a battery instead.

If the power demands actually are low enough to get away with using a cap, then a simple linear regulator can be configured as a current source.
 

oz93666

Joined Sep 7, 2010
739
Instead of a standard resistor try using these halogen bulbs , the intensity of the light will show how much current

Twenty of these are only $2 including delivery (eBay) .... 12V ...available in many different Wattages (and so resistances) , up to 100W , by wiring in series or parallel you can get the resistor you require
 

MrChips

Joined Oct 2, 2009
30,706
If you are having difficulty with the math, try simple calculations with what you know.

V = voltage (volt)
I = current (amp)
R = resistance (ohm)
C = capacitance (farad)
Q = charge (coulomb)
t = time (second)

The charge on a capacitor is Q = C x V
Charge is also Q = I x t

First-order time estimate is t = Q / I = C x V / I

For example, if you have
C = 100 F
V = 10 V
I = 1 A

By calculation
R = 10 Ω
t = 1000 s

Time constant = R x C = 10 x 100 = 1000 s. In this time your current and voltage would have fallen to 37% of the initial values.
However your voltage and current are both falling exponentially.

So realistically speaking, you can supply 1A @ 10V for 100 seconds with a 100 F capacitor.

Edit:
A simpler way of doing this is:
Q = C x V = 100 F x 10 V = 1000 coulombs
If we take one-tenth of this, i.e. 100 coulombs = 1 A x 100 seconds
 
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