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This circuit measures the inductance (and displays it digitally) by tracking the number of microseconds it takes to charge an unknown inductor with a constant 5v drop up to a current threshold of 5A. The resultant inductance is simply equal to the time reading - (minus) the 5.3% EverCircuit error in micro-henries. If interested, you can refer to the equations below for more info. You can easily change the inductance on the bottom right of the schematic and press the button to start the count. Just reset the sim each time you wanna measure another value.
Uses a 1MHz clock to count the microseconds as soon as you press the push-button, it automatically stops counting once the inductor has reached the current limit. After that, the value (in micro-henries) should simply be equal to the microseconds reading. You can also prove this by referencing the simple formulas below by substituting the reading you got into the time variable within the equations!
As an example, there is a 1uH inductor in the test circuit, so plugging in all the values means that it should only take 1us to to charge it up to 5Amps at a constant voltage drop of 5v.
***Important Accuracy Note***
*The value will sometimes be off by 1 to several digits depending on how high the inductance your measuring is… I believe this is due to problems of synchronizing the transition between the clock and when you press the button, but could also be caused by all the propagation delay between when the the circuit actually generates and receives the stop/reset counting signal. Based on various measurements I’ve observed, the error seems to be consistently +5.3%*
*** Edit: Reduced the original error of 8.2% down to just 5.3%
Equation to solve for inductance:
L = Δt …or… L = 5Δt / 5
derived from: L = (vΔt) / Δi
Equation to solve for the expected time (microseconds reading) for a known inductance:
Δt = L …or… Δt = 5L / 5
derived from: Δt = (LΔi) / v
Both of the above equations were derived from this fundamental inductor equation shown below, I just rewrote it to solve for different variables and rearranged/simplified with the values of i and v from the circuit:
v = L(Δi / Δt)
Here’s a small table of different example inductor values, and what their real corresponding times (in microseconds) should be:
if L = 1uH, Δt = 1us
if L = 50uH, Δt = 50us
if L = 1mH, Δt = 1000us
if L = 999mH, Δt = 999,000us
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