Put t on the left side? Vc = V * (1- e^(-t / R*C)) - Page 2 - Electronic Circuits Projects Diagrams Free

Roff · 1st September 2007, 04:34 PM

Quote:

Originally Posted by eng1

You've tested for equality, but my point was if 1-(V/Vc) is positive (negative) at any instant, also Vc/(Vc-V) is positive (negative) at the same instant because it's the reciprocal.
Len's equation seems correct to me. I got it from Roff's equation applying the formula ln(1/x) = -1*ln(x)

As I pointed out above (we were posting at the same time), my original equation was wrong. Sorry, Len.

eng1 · 1st September 2007, 04:34 PM

Quote:

Originally Posted by Roff

t=-R*C*(ln(1-(Vc/V)))

Yes, now the argument of the log function is positive

After your correction, an equivalent equation is:
t=RC ln[V/(V-Vc)]

ljcox · 1st September 2007, 11:06 PM

I'm glad we sorted that out in the end!

Roff · 2nd September 2007, 12:36 AM

Quote:

Originally Posted by ljcox

I'm glad we sorted that out in the end!

With apologies to Sir Walter Scott...

Oh, what a tangled web we weave,
When first we practice to... help someone.

blueroomelectronics · 2nd September 2007, 01:32 AM

Well I put the formula into Just BASIC

Code:

VDD = 5
R = 649
C = .00018
Vc = 3.125
print R*C*(exp(1)*(1-(Vc/VDD)))

Turns out I've got to use the TLC555 version as it will be eaiser to implement.

Shall we start discussing ideal values for R1 & R2

ljcox · 2nd September 2007, 01:48 AM

Bill,
I don't understand why you have exp(1)* rather than ln

And it is RC ln{1/(1 - Vc/V)}

blueroomelectronics · 2nd September 2007, 02:46 AM

The "Just BASIC" compiler does not have an ln() statement.
exp(1) [2.71828183] seems to work fine, I'll remove the brackets.

ericgibbs · 2nd September 2007, 07:38 AM

Quote:

Originally Posted by blueroomelectronics

The "Just BASIC" compiler does not have an ln() statement.
exp(1) [2.71828183] seems to work fine, I'll remove the brackets.

Bill,
Some Basics consider the 'log' statement as meaning natural logs to base 'e'

Try 'print log(2.71828183)' you most likely get '1', this will confirm that your Basic considers 'log' as a natural log reference.== 'ln'

eng1 · 2nd September 2007, 09:17 AM

Quote:

Originally Posted by ericgibbs

Bill,
Some Basics consider the 'log' statement as meaning natural logs to base 'e'

Eric, you're right. The manual of that compiler says:

Quote:

LOG(n) Description:
This function returns the natural logarithm of n.

Usage:
print log( 7 ) produces: 1.9459101

BTW, it would be easy to get natural logarithm of x if you could use only base-10 logarithms:
ln (x) = 2.302585 * log10 (x)

Hero999 · 2nd September 2007, 02:27 PM

I never saw the point of the exp function, surly it's easier to do 2.71828183^x.

tkbits · 3rd September 2007, 05:53 AM

Not really.

x ^ y where y is non-integer is calculated as base ^ (y * log-base x).

If base is 10, then x ^ y is 10 ^ (y log10 x). We used to do this with paper, pencil, and a table of logarithms.

If base is e, then x ^ y is e ^ (y log-e x), or exp(y * ln(x)). There are approximating polynomials for doing this on a computer.

Papabravo · 7th September 2007, 01:14 AM

The "ln" is an abreviation for "natural" or "Naperian" logarithms. Named after the Scottish mathemetician John Napier.

http://en.wikipedia.org/wiki/John_Napier

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1st September 2007, 11:06 PM	(permalink)
ljcox	I'm glad we sorted that out in the end! __________________ Len

2nd September 2007, 01:32 AM	(permalink)
blueroomelectronics	Well I put the formula into Just BASIC Code: VDD = 5 R = 649 C = .00018 Vc = 3.125 print RC(exp(1)*(1-(Vc/VDD))) Turns out I've got to use the TLC555 version as it will be eaiser to implement. Shall we start discussing ideal values for R1 & R2 __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

2nd September 2007, 01:48 AM	(permalink)
ljcox	Bill, I don't understand why you have exp(1)* rather than ln And it is RC ln{1/(1 - Vc/V)} __________________ Len Last edited by ljcox; 2nd September 2007 at 01:53 AM.

2nd September 2007, 02:46 AM	(permalink)
blueroomelectronics	The "Just BASIC" compiler does not have an ln() statement. exp(1) [2.71828183] seems to work fine, I'll remove the brackets. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

2nd September 2007, 02:27 PM	(permalink)
Hero999	I never saw the point of the exp function, surly it's easier to do 2.71828183^x. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez

3rd September 2007, 05:53 AM	(permalink)
tkbits	Not really. x ^ y where y is non-integer is calculated as base ^ (y * log-base x). If base is 10, then x ^ y is 10 ^ (y log10 x). We used to do this with paper, pencil, and a table of logarithms. If base is e, then x ^ y is e ^ (y log-e x), or exp(y * ln(x)). There are approximating polynomials for doing this on a computer.

7th September 2007, 01:14 AM	(permalink)
Papabravo	The "ln" is an abreviation for "natural" or "Naperian" logarithms. Named after the Scottish mathemetician John Napier. http://en.wikipedia.org/wiki/John_Napier