Solving Voltage Ripple Formula: Get the Right Answer!

[Rafeng404]

Hi lads,

Working through my HND course books and I have come across a formula that I cannot get to give me the same answer as the book. Nor can some of my engineering student friends.

Values are:

F=50
C=500^-6
Rl = 100
Vc=31.03

The formula is

=(1/(2FsCRl))*(Vc-(Vr/2))

Inputting the values to make it look clearer it becomes

= (1/5)*(41.03-(Vr/2))

Solve for Vr.

My attempt:

=0.2(41.03-(Vr/2))
=8.206 * 0.1Vr
Vr= 8.206/0.1
Vr = 82.06

Clearly we have missed something stupid here and transposed it wrong. I am a bit rusty but I have lent my maths bible to someone else.

The right answer according to the uni is 7.46.

Have I completely forgotten how to do maths?

Thanks in advance

 

[collinsmark]

Hi lads,

Working through my HND course books and I have come across a formula that I cannot get to give me the same answer as the book. Nor can some of my engineering student friends.

Values are:

F=50
C=500^-6
Rl = 100
Vc=31.03

The formula is

=(1/(2FsCRl))*(Vc-(Vr/2))

Inputting the values to make it look clearer it becomes

= (1/5)*(41.03-(Vr/2))

Solve for Vr.

My attempt:

=0.2(41.03-(Vr/2))
=8.206 * 0.1Vr
Vr= 8.206/0.1
Vr = 82.06

Clearly we have missed something stupid here and transposed it wrong. I am a bit rusty but I have lent my maths bible to someone else.

The right answer according to the uni is 7.46.

Have I completely forgotten how to do maths?

Thanks in advance

Hello @Rafeng404,

It would greatly help if you clarified a few things:

• Your values for F, C, Rl, and Vc don't have any units. I can assume that they're all in SI units, but it's good to specify.
• In your formula, "=(1/(2FsCRl))*(Vc-(Vr/2))", there's nothing on the left hand side of the equation. What is all that equal to? You can't solve for Vr with nothing on the left-hand-side. It doesn't make any sense unless it is equal to something.
  
• Where did this formula come from? Was it given to you like that or did you derive it in an intermediate step?
• In my experience with these types of problems, I would expect there to be a $\pi$ in your formula, usually between the 2 and the $F_s$. I find it curious that it's not present. Do you have any comments on this?
  
• When specifying your constants, you stated that Vc = 31.03. But when plugging it into your formula it became 41.03. I don't know what to make of that.
  
• In your attempted solution, I'm not sure how you solved for Vr since there was nothing originally on the left-hand-side of the equation. This goes back to an earlier bullet.

 

[Babadag]

It is difficult to understand the formula. Usually s is the operational parameter but then you don't need F. If s=pi then the parameters are in complex Xc=-j1/(2*pi()*F*C). If the formula represents the current in a [mesh?] circuit the circuit diagram could be useful in order to solve the problem.

 

[Rafeng404]

Lads,

Apologies, perhaps I should not have done this on my phone after a back shift.

1. F= frequency, C= capacitance, Rl = Load resistance, Vc = Peak voltage across capacitor, Vr = voltage ripple peak to peak.
2. The formula is for Vr ( see attached)
3. The formula is in our books, you need to transpose to get Vr (see attached)
4. I agree, normally I would expect to see Pi in a formula like this, perhaps this is where the uni has buggered up??
5. Vc is 41.03, the 31.01 is a typo

Hope this helps, Vr is what I am looking for.

I am sure there is something obvious that I am missing but I cannot seem to make it work for 7.46Volts.

Many thanks

 

[NascentOxygen]

My attempt:

V_r = 0.2(41.03 - (Vr/2))
.....⤴
=8.206 * 0.1Vr

Try simplifying that again, moving all V_r terms to the left side.

 

[Rafeng404]

"ALL Vr terms" head slap! Bloody hell! cheers mate i realize what I haven't been doing now. I have been negating the first Vr term. I am such a spanner sometimes.

Thank you all.