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### Network # Time calculation problem

B

#### Brian

Jan 1, 1970
0
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

J

#### Joe McElvenney

Jan 1, 1970
0
Hi,
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?

Yes! Use radians NOT degrees.

Cheers - Joe

B

#### Brian

Jan 1, 1970
0
Joe McElvenney said:
Hi,
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?

Yes! Use radians NOT degrees.

Cheers - Joe

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

Brian

J

#### Joe McElvenney

Jan 1, 1970
0
Hi again,
Joe McElvenney said:
Hi,
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?

Yes! Use radians NOT degrees.

Cheers - Joe

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

Brian

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

B

#### Brian

Jan 1, 1970
0
Joe McElvenney said:
Hi again,
Joe McElvenney said:
Hi,

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?

Yes! Use radians NOT degrees.

Cheers - Joe

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

Brian

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

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

J

#### Jonathan Kirwan

Jan 1, 1970
0
Joe McElvenney said:
Hi again,
Hi,

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?

Yes! Use radians NOT degrees.

Cheers - Joe

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

Brian

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

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?

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

J

#### Joe McElvenney

Jan 1, 1970
0
Brian,

As I said at the beginning, switch (your calculator) to radians
and you will get the answer they do.

ACOS (359/391) = 0.105 rads

2*pi*50 = 314.159 rads/sec

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

It is simply rads over rads per second.

Cheers - Joe

Joe McElvenney said:
Hi again,
Hi,

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?

Yes! Use radians NOT degrees.

Cheers - Joe

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

Brian

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

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

B

#### Brian

Jan 1, 1970
0
Joe McElvenney said:
Brian,

As I said at the beginning, switch (your calculator) to radians
and you will get the answer they do.

ACOS (359/391) = 0.105 rads

2*pi*50 = 314.159 rads/sec

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

It is simply rads over rads per second.

Cheers - Joe

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

B

#### Brian

Jan 1, 1970
0
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

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

Thanks,

Brian

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