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Old 21st Jan 2021, 9:09 pm   #8
ms660
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Join Date: Apr 2011
Location: Cornwall, UK.
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Default Re: Question on a bit of theory

Quote:
Originally Posted by Radio Wrangler View Post
It's actually very simple. The induced voltage is an instantaneous thing.

V = -L (di/dt)

di/dt is an expression stolen from calculus, it means the slope of the current versus time waveform in amps per second.

Draw the current/time waveform. Decide on what instant of time you're interested in the voltage. Place a ruler on that point, angled to be asymptotic to the curve. Calculate its slope, amps/second, and multiply by L. You now have the voltage at that instant. the minus sign just tells you which way round the voltage is.

If you're worried about risetimes, then notice that a rise (or fall) is usually an S-shaped curve with slower sections at each end. It goes fastest in the middle, so you get the highest peak voltage there.

If you want an average, place your ruler across the two places which define the beginning and end of the period you want to average, and use that slope.

Does the change of current make the voltage? or does the voltage make the change of current?

Both! they are inseparable, like voltage and current in ohm's law. You get both at once, inseparably and for free!

David

Edit: Crossposted..... not aware of any disappearing posts. I'll check... No, nothing's been done. Sometimes there are delays. Sometimes I forget to click 'post quick reply' and wonder where the occasional one has gone.
So where did he go wrong then?

Lawrence.
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