# confusion with getting Ct with a series circuit

Discussion in 'General Electronics Chat' started by james7701, Feb 18, 2016.

1. ### james7701 Thread Starter New Member

Jan 5, 2016
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ok so, the first problem i was able to apply the formula to get my answer but, the 2nd problem i don't understand why im not getting the answer. (which is 320 )
any suggestions as of different approach to this?

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2. ### #12 Expert

Nov 30, 2010
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The answer isn't 320 as far as I can tell. I see 470 uf in series with 1 uf, and the answer to 2 caps in series is always less than the smallest capacitor. I see .9978769 uf on my calculator.

Still, I prefer a method that can do more than 2 capacitors, so don't get too invested in this formula. It's right for 2 caps, but life serves up different problems eventually.

Jan 5, 2016
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4. ### james7701 Thread Starter New Member

Jan 5, 2016
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but, why are they showing an answer of 320?

5. ### GopherT AAC Fanatic!

Nov 23, 2012
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Unit issues

470 nF is equal to 0.47 uF

So, 1uF = 1000 nF in series with 470 nF

Try again

6. ### #12 Expert

Nov 30, 2010
16,703
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It's the units labels that went wrong. You have to convert 1 uf to 1000 nf to get the formula to work.

Nov 23, 2012
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8. ### james7701 Thread Starter New Member

Jan 5, 2016
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so, i do 470÷1000 and 471÷1000?

9. ### #12 Expert

Nov 30, 2010
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(470 x 1000) /(470+1000)

10. ### GopherT AAC Fanatic!

Nov 23, 2012
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Take reciprocal of c1 + reciprocal of C2

11. ### #12 Expert

Nov 30, 2010
16,703
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That's the formula I was trying to think of in post #2. You can add up the 1/x's all day long, then 1/x the result to get the answer.

12. ### james7701 Thread Starter New Member

Jan 5, 2016
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so, it's best to change the units? and than proceed?

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14. ### WBahn Moderator

Mar 31, 2012
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If you track your units, you would have seen where you went wrong.

$
\frac{470 \, nF \, \times \, 1 \, \mu F}{470 \, nF \, + \, 1 \, \mu F}
$

Now when you look at the denominator you don't see 470 + 1, you see two things that can't be added together that way because they don't have the same units.

$
\frac{470 \, nF \, \times \, 1 \, \mu F}{0.470 \, \mu F \, + \, 1 \, \mu F}
$

NOW you can add the denominator

$
\frac{470 \, (nF)(\mu F)}{1.470 \, (\mu F)}
$

Now the uF cancel, leaving you with 320 nF.

Always, always, ALWAYS track your units!!!