Calculating Resistor Size for Time Delay in Alarm Circuit

Click For Summary
SUMMARY

The discussion focuses on calculating the resistor size required for a time delay in an alarm circuit using a 2200 µF capacitor and a 5V supply. The goal is to achieve a voltage of 4.3V across the capacitor after a delay of 60 seconds. The equation used is V=V(initial)e^(-t/RC), leading to a calculated RC value of approximately 719.58. The student expresses confusion regarding the initial voltage of the capacitor, indicating a need for clarification on the formula's application.

PREREQUISITES
  • Understanding of capacitor charging equations
  • Familiarity with the concept of time constants in RC circuits
  • Basic knowledge of logarithmic functions
  • Ability to perform calculations involving microfarads (µF) and voltage
NEXT STEPS
  • Review the principles of RC time constants in circuits
  • Learn about capacitor discharge and charging curves
  • Study the application of natural logarithms in electronic formulas
  • Explore practical examples of alarm circuit designs
USEFUL FOR

Electronics students, hobbyists designing alarm systems, and engineers working with timing circuits will benefit from this discussion.

ehabmozart
Messages
212
Reaction score
0

Homework Statement


A student is designing an alarm circuit. She wants a time interval between closing a switch and the circuit becoming active. She decides to charge a 2200 mu (x10 -6) F capacitor through a resistor from a 5V supply. The potential difference across the cap. must rise to 4.3 V for the alarm to become active. Calculate the size of the resistor that she should connect in the circuit to achieve a 1 minute delay


Homework Equations


V=V(initial)e^-t/RC


The Attempt at a Solution


t should be 60 seconds
ln(4.3/5)=-60/Rx2200x10-6
RC=719.5831403
R should be .. Oh .. That's too big.. I NEED HELP!
 
Physics news on Phys.org
Notice that the problem says the voltage across the capacitor should rise to 4.3 volts, yet when we plug in t=0 into your formula, you get 5 volts. I would consult notes and make sure that this is the correct formula for the capacitor's voltage.
 

Similar threads

Capacitor circuit time constant problem
  • · Replies 2 ·
Replies
2
Views
2K
Resistor Calculation for Alarm Clock Circuit
  • · Replies 3 ·
Replies
3
Views
2K
RLC Circuit Analysis -- Two sources and two switches
  • · Replies 3 ·
Replies
3
Views
2K
Time constant for parallel RC circuit
  • · Replies 2 ·
Replies
2
Views
2K
Calculation of current in driven series RLC circuit
  • · Replies 8 ·
Replies
8
Views
3K
How Long Does It Take for a Capacitor to Charge to 8V in an RC Circuit?
  • · Replies 2 ·
Replies
2
Views
2K
Opamp RC-circuit -- Capacitor time constant calculation
  • · Replies 6 ·
Replies
6
Views
4K
Time Constant, Maximum Charge, and Current in an RC Circuit
  • · Replies 11 ·
Replies
11
Views
3K
Maximum current through a capacitor
  • · Replies 1 ·
Replies
1
Views
3K
Circuit with resistor, switch and capacitor
  • · Replies 62 ·
3
Replies
62
Views
6K