# Time calculation problem

Discussion in 'Electronic Basics' started by Brian, Mar 5, 2006.

1. ### BrianGuest

In a pdf file from Rifa on "Electrolytic Capacitors Application Guide" it
states the following:

The ripple current from the main and from the load has to be known. First
calculate the capacitor voltage charge time.

tc = [arccos (Umin / UMax)] / [2 x 3.14 x fmain]

Then it give an example:

tc = [arccos (359 / 361)] / [2 x 3.14 x 50] = .000335

I have to be missing something, because I can't get the same answer as they
do in the example. I get tc = 0.0192, which is not right. Can someone tell
be what I am doing wrong?

Thanks,

Brian

Hi,

Cheers - Joe

3. ### BrianGuest

Thanks for your reply Joe. Isn't [2 * 3.14 * 50] the number of radians in 1
second at 50 Hz.?

Brian

4. ### Joe McElvenneyGuest

Hi again,
Yes, but the arccos of a number gives you radians and the
2*pi*f bit gives you radians per second. Dividing one by the
other, the radians cancel and you're left with seconds.

Cheers - Joe

5. ### BrianGuest

Isn't that what I want -- charging time in seconds? That's what this formula
is suppose to give me. Am I missing something here?

Brian

6. ### Jonathan KirwanGuest

Grab my equation tool at:
http://users.easystreet.com/jkirwan/new/zips+src/EQ_C.ZIP
and extract out the file EQ.EXE that's inside.

At a DOS prompt, type in:
EQ acos(359/361)/2/pi/50

It will print out the figure you are getting from the PDF file.

The others have pointed out that your ARCCOS() function is probably
returning something other than radians -- in particular, degrees. For
example, if you use my equation tool and enter this:
EQ deg(acos(359/361))/2/pi/50

you will then get your 0.0192 out.

This whole thing is called "operator error."

Jon

7. ### Joe McElvenneyGuest

Brian,

and you will get the answer they do.

So Tc = 0.105/314.159 ~= 0.335 sec (allowing for rounding)

Cheers - Joe

8. ### BrianGuest

Thanks Joe, that is what I was missing (switching the calculator to
radians). Sometimes, the simplest things, are the hardest to figure out
where you went wrong. Thank you for your patience.

Brian

9. ### BrianGuest

Thanks for your offer Jon. Since Joe told me what I was doing wrong, I won't