Electronics Forum  Help Search Members Calendar 
Welcome Guest ( Log In  Register )  Resend Validation Email 
treez 
Posted: August 18, 2012 04:17 pm

Unregistered 
Hello,
Do you know of mathematical expressions that give the RMS value of this Discontinuous current waveform? http://i49.tinypic.com/152gryo.jpg (i.e. supposing I know the peak value, the rise time, fall time and dead time) 

Abdullah M.A. 
Posted: August 19, 2012 01:58 pm

Forum Addict ++ Group: Spamminator Taskforce Posts: 4,288 Member No.: 10,604 Joined: August 16, 2007 
> 1st Tri.
i(t) = [ t / (4.71  4.7)] * 2 > 2nd Tri. i(t) = [(4.5  4.4 )/(4.74  4.71)] * 2 Now, Find the RMS. Abdullah  "A scientist can discover a new star, but he cannot make one. He would have to ask an engineer to do that."
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engineer is twice bigger than necessary." 
Sch3mat1c 
Posted: August 19, 2012 06:11 pm

Forum Addict ++ Group: Moderators Posts: 20,249 Member No.: 73 Joined: July 24, 2002 
Define duty cycle D as the ratio between the time when current is nonzero versus the full period (eyeballing, about 75%). RMS is 1/sqrt(3*D) times the peak current. Note that average is only Ipk/(2*D), so the crest factor gets huge for light loads.
Tim  Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.

treez 
Posted: August 22, 2012 07:21 pm

Unregistered 
D = 0.8
The RMS is actually 1.03A. The formula above......I.E...... 1/[sqrt(3D)] gives 0.645A 

Sch3mat1c 
Posted: August 22, 2012 08:45 pm

Forum Addict ++ Group: Moderators Posts: 20,249 Member No.: 73 Joined: July 24, 2002 
I specifically stated "times peak". You applied it wrong. I get about 1.3A RMS. Was 1.03A calculated in LTSpice? Then why ask for a formula you don't know how to use?
Tim  Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.

treez 
Posted: August 24, 2012 06:16 am

Unregistered 
1.03A was indeed from ltspice
But its also the sum of the sqares of the individual rms's of the waveform split into two trains of triangles (both right angled). so it seems correct.........i know i should be doing the integration to check it, though i find i need a single excel spread calc...as its quicker and clearer for those jobs that you need to do quickly 

:: support us ::