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Old 03-19-2013, 09:55 AM   #1
UKscooby
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Default Formula help in Excel



I'm working on the above equation and trying to get it into Excel. Currently this is what I have:

=0.418/(L21+(0.75*L15)*(H3-B3)/(M23*J3))

Where:

L21 = 8.7989
L15 = 0.5688
H3 = 30.5
B3 = 24
M23 = 0.00002451
J3 = 147294.59

Giving an answer of 0.04369


However, if I change the formula by adding 2 parenthesis' to:

=0.418/(L21+(0.75*L15))*((H3-B3)/(M23*J3))

I get an answer of 0.08157

Which is the correct way to write the formula?
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Old 03-19-2013, 10:04 AM   #2
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Whichever way you need the formula to be evaluated?

Order of operations, how do they work?
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Old 03-19-2013, 10:09 AM   #3
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the first one is righter.
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Old 03-19-2013, 10:18 AM   #4
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I would do option 2, but I would add another set of parenthesis around everything after 0.418/

edit: oh, that's option 1. I would do option 1.
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Old 03-19-2013, 10:23 AM   #5
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Is this homework?
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Old 03-19-2013, 10:27 AM   #6
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It looks like homework.
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Old 03-19-2013, 10:28 AM   #7
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One of them is very clearly not what you want, look at it more closely
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Old 03-19-2013, 10:32 AM   #8
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Nope, its work from home.
I'm going through calculations of a power supply design because it has a couple of issues and I cant figure out where I went wrong. The readings all match the design specs but some of the components are getting very hot even though they were spec'd with a +30% margin. Also, the circuit runs perfectly if I disconnect the slope compensation network but screws up if I reconnect it, even though calculations seem to be correct and values are within tolerance.
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Old 03-19-2013, 10:33 AM   #9
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Quote:
Originally Posted by Bud Fox View Post
One of them is very clearly not what you want, look at it more closely
=0.418/(L21+(0.75*L15))*((H3-B3)/(M23*J3))
=0.418/(L21+(0.75*L15))*((H3-B3)/(M23*J3))
=0.418/(L21+(0.75*L15))*((H3-B3)/(M23*J3))
=0.418/(L21+(0.75*L15))*((H3-B3)/(M23*J3))
=0.418/(L21+(0.75*L15))*((H3-B3)/(M23*J3))
=0.418/(L21+(0.75*L15))*((H3-B3)/(M23*J3))

yep, advertizing symbol for a maxi pad. do not want.
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Old 03-19-2013, 10:40 AM   #10
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Well thanks chaps. The first method is what I've had since day one. I'm going crazy second guessing everything.
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Old 03-19-2013, 10:55 AM   #11
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Try reversing the equation then seperating the division from the multiplication.
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Old 03-19-2013, 11:25 AM   #12
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adding the second parentheses like you did, brings that last term into the numerator.
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Old 03-19-2013, 11:37 AM   #13
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=sum() you put the stuff in the brackets thats all i got
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Old 03-19-2013, 12:09 PM   #14
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try going through this and see if you come up with same values.

Current or voltage mode regulator?
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Old 03-19-2013, 12:50 PM   #15
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Quote:
Originally Posted by tegan_ca View Post
try going through this and see if you come up with same values.

Current or voltage mode regulator?
Peak current mode buck-boost.
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Old 03-19-2013, 12:55 PM   #16
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Quote:
Originally Posted by UKscooby View Post
Nope, its work from home.
I'm going through calculations of a power supply design because it has a couple of issues and I cant figure out where I went wrong. The readings all match the design specs but some of the components are getting very hot even though they were spec'd with a +30% margin.
Put a fan on it.
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Old 03-19-2013, 12:57 PM   #17
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Excelsior
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Old 03-19-2013, 01:16 PM   #18
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Quote:
Originally Posted by Davemeister View Post
Put a fan on it.
passive cooling only.
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Old 03-19-2013, 01:19 PM   #19
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Originally Posted by UKscooby View Post
passive cooling only.
Get someone to stand next to it and fan it, but only if they feel like it?
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Old 03-19-2013, 02:02 PM   #20
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.04369 is what I got doing it on paper...I double checked my order of operations
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Old 03-19-2013, 02:46 PM   #21
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Quote:
Originally Posted by Davemeister View Post
Get someone to stand next to it and fan it, but only if they feel like it?


I'll give you $1 to stand there and fan them. All 10,000W worth.
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Old 03-19-2013, 02:58 PM   #22
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You can't even cool 1.21 gigawatts passively, let alone 10. Ask Doc Brown if you don't believe me!
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Old 03-19-2013, 07:34 PM   #23
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WTF. After not being able to figure this out I called up the apps guys and they told me the equations in the datasheet are wrong. The device has been on the market since 2010 and the data sheet has been revised 5 times. How can this still be wrong?!

I ask for the correct equations and this is the response I got:

"The compensation equations are not correct in the data sheet on pages 18/19. (An ECN edit is on the way) Below are the corrected equations:

blah blah"

Fantastic.

Oh wait, those are the equations for a different configuration.

I call back to get the corrected equations for buck-boost mode.

Response: Those are not ready for publication yet because they have not been tested and approved.

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Old 03-19-2013, 08:13 PM   #24
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I'd have written it this way in excel, if that's any help:

=0.418/(L21+((0.75*L15)*((H3-B3)/(M23*J3))))
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Old 03-19-2013, 08:36 PM   #25
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Quote:
Originally Posted by Omega_2nr View Post
I'd have written it this way in excel, if that's any help:

=0.418/(L21+((0.75*L15)*((H3-B3)/(M23*J3))))
This is correct. When in doubt, wolframalpha.
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