# Will cutting off an AC current affect its measurement?

Discussion in 'General Electronics Chat' started by pokenan, Dec 3, 2015.

1. ### pokenan Thread Starter New Member

Nov 19, 2015
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0
Hi all

Let's say we have a 200A(p-p) sinusoidal AC current. After reaching the peak for the first time, it is cut off at 80A. As the wave hasn't reach its half period, so I am now wondering whether cutting off can affect some properties of AC current. That is to say, firstly, does it still have an RMS value at 200/root2 as a normal consistent AC current? Can the 200A be measured by a current meter? Will it be considered as a changing DC current since it has always been positive? Will the magnetic field be totally different from that excited by a normal consistent AC current?

Thank you for your attention and help.

Kind regards
pokenan

2. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
what happens to the light when you turn it off? same thing, a few switching transients, then nothing left to measure. how can anyone measure 200 amps on an open circuit?

3. ### AnalogKid Distinguished Member

Aug 1, 2013
4,696
1,297
The RMS value of a waveform is a measure of a steady-state condition. Turn-on and turn-off transient conditions and discontinuities are fundamentally different from steady-state conditions. That is why an early course in EE is called transient analysis. Yes, you can calculate the RMS value of a partial half-cycle of a sinewave, but it is not simply Vpeak/root2.

And yes, an interrupted AC or DC magnetic field is totally different from a steady state field. That's where the flyback kick comes from.

ak

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4. ### pokenan Thread Starter New Member

Nov 19, 2015
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Thank you for reply. Another question, for transient DC current which rises from 0 to 200A then drops back to 80A, when analyzing, does it make sense if I consider the rising and dropping part as a part of an interrupted AC?

5. ### AnalogKid Distinguished Member

Aug 1, 2013
4,696
1,297
Not to me. In my mind, AC or DC - anything AC or DC - is a steady state condition. Rising and falling DC isn't very D, is it?

ak