
Homework Help Stuck on a textbook question or coursework? Cramming for a test and need help understanding something? Post your questions and attempts here and let others help. 

Thread Tools  Display Modes 
#1




Equation for capacitor discharge
I've got the following circuit here:
I want to find an equation to model the capacitor discharging. If I'm correct, this is the needed equation: V = V0 * e^(t/RC) In my case, I am using a nine volt battery, 1K resistor and 5μF Capacitor. When I calculate the RC I get 0.064, which surely can't be right here? I'm using Circuit Wizard, and this is the output of graph when it runs on the three probe points (as above): Basically, when the equation for the capacitor discharges meets the equation of the voltage divider, the LED will turn off. I'm trying to find this equation so that I can include it in my write up. Thanks for your help  pasta 
#2




Visit http://www.allaboutcircuits.com/vol_1/chpt_16/4.html for more information.
Your image doesn't show a 9V discharge curve. The tau is only 5 milliseconds and the 9 V should have decreased to zero in about 25 milliseconds. You might want to ensure you have a good discharge path.
__________________
Joe Navy Electricity and Electronics Training Series (NEETS)  pdf files  http://www.fcctests.com/neets/neets.htm 
#3




Yes indeed  the discharge path into the LM741 negative input will be of fairly high resistance. I doubt the 10k plays any significant part in the discharge time.

#4




I mean the 1k  R10.

#5




In the interests of circuit predictability, the 1k should probably be in series with the switch to limit the current pulse magnitude drawn from the 9V supply when the switch is pressed  otherwise you have an effective momentary "short circuit" when C is even partially discharged.

#6




Quote:
Quote:
When I graph 9 * e^(1/0.064) over the domain [0, 1], I get the following image: Which certainly fits what JoeJester was saying 
#7




To proper "discharge" you need to add resistor parallel to C1 to get good discharge path.
Now capacitor is discharge by base current of LM741 and this current is very small typ. 20nA, max 200nA. So to discharge 5uF capacitor to 0V you have to wait C=Q/U=I*t/U t=(C*U)/I=(5uF*9V)/200nA=225s For example, if we have a capacitor charge to Vcc=9V and we wont to discharge capacitor to Vc=4.5V at time t=5s We need parallel resistor R R = t / ( C*In(Vcc/Vc) ) = 5s / ( 5uF * In(9/4.5) )= 5/(5uF*0.693)=5/3.465u=1.44MΩ=1.5MΩ Last edited by Jony130; 11022009 at 08:49 AM. 
#8




If you want the LED to energize when you press the switch and then deenergize after a delay when you release the switch, you might want to use the circuit illustrated in the 091101_B picture. 091101_A illustrates the waveforms.
If you want delay energizing the led, (less than one tau), then place the resistor in series with the switch. The resistor placement in the OPs diagram is in series with the HighZ OPAMP input, making the discharge path the Input Z and the resitor.
__________________
Joe Navy Electricity and Electronics Training Series (NEETS)  pdf files  http://www.fcctests.com/neets/neets.htm 
Tags 
capacitor, discharge, equation 
Related Site Pages  
Section  Title  
Video Lecture  RC Time Constants (Part 1)  Pulse Circuit Response  Inductors,Capacitors,Transformers  
Worksheet  Timer circuits  
Worksheet  Time constant calculations  
Worksheet  Time constant circuits 
Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Simple question on characteristic equation.  MichealY  General Electronics Chat  8  06262009 03:18 AM 
Need quick answer: Does POS simplify into equivalent SOP simplification equation?  andrewle8  Homework Help  1  02232009 08:26 PM 
Hie im new . POTENTIAL DIVIDER EQUATION ..!  Giggabyte  General Electronics Chat  2  09192008 01:17 PM 
Have to make equation with given equation  justinneedshelp  Homework Help  5  11082007 12:46 AM 
Simultaneous Equation techniques  Gears  Math  7  03182007 12:25 PM 
Thread Tools  
Display Modes  

