You are Here: Home >< Physics

# Signal Generator Watch

1. http://www.electronics-tutorials.ws/...scillator.html

Using the square wave signal generator.
For the 555 timer, how have they got the times for charging to be t=ln(2)RC
The time for charging through 2 resistors for a capacitor should be Vin/3 / (R1 + R2) = I
Because we are only charging 1/3 of Vin, from 1/3 of Vin to 2/3 of Vin.
The voltage of the capacitor should be 2Vin/3 and put into the equation C=Q/V we get C = 3Q/Vin.
The charge transfered during that time are equal so we sub for Q.
Getting t = 2C(R1+R2), where have they gotten ln(2)(R1+R2) = t for charging time?
2. Anyone
3. (Original post by JokesOnYoo)
http://www.electronics-tutorials.ws/...scillator.html

Using the square wave signal generator.
For the 555 timer, how have they got the times for charging to be t=ln(2)RC
The time for charging through 2 resistors for a capacitor should be Vin/3 / (R1 + R2) = I
Because we are only charging 1/3 of Vin, from 1/3 of Vin to 2/3 of Vin.
The voltage of the capacitor should be 2Vin/3 and put into the equation C=Q/V we get C = 3Q/Vin.
The charge transfered during that time are equal so we sub for Q.
Getting t = 2C(R1+R2), where have they gotten ln(2)(R1+R2) = t for charging time?
during charge the pd across the capacitor goes from 1/3 Vcc to 2/3 Vcc... i.e double it's initial pd

it's being charged by a RC circuit with a pd of Vcc.
2/3 Vcc is halfway between 1/3 Vcc and Vcc

ln(2)RC gives the time for the pd across a capacitor to double - in this case R is R1+R2

during discharge the pd across the capacitor goes from 2/3 Vcc to 1/3 Vcc... i.e. half it's initial pd.

ln(2)RC gives the time for the pd across a capacitor to half - in this case R is only R2

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 28, 2016
Today on TSR

### Should I ask for his number?

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE