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thebugger
modified 8 years ago

How to measure unknown inductors with a few basic parts

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07:54:10
Okay it's pretty simple, i just came by this method myself. There's a way to measure capacitors too, by the way, but it's hard to represent it here in EC. Let's start with inductors. What happens when a pulse is injected in a parallel LC circuit? It starts ringing, at its resonant frequency. The two things you need to know is the value of the capacitor in the circuit, and the resonant frequency (you'll need an oscilloscope to measure this). Most of the times this method will give you a result with a 10% margin of error. So let's go through it step by step: 1st, you need to inject the LC circuit with a fast rise/fall impulse with an appropriate frequency. Usually every frequency will do, but if the frequency is too low, the dampening will be too high and it'll be harder to measure. Let's use here in the example 100Khz. 2. You need to measure the decaying oscillations frequency. Here in EC, the oscilloscope will show it directly, in reality you must measure it (assuming you have an oscilloscope, you surely know how to measure frequency). With a lot of oversampling i measured 1.58MHz 3. Now we use the formula Fr= 1/ 2п.sqrt.L.C and substitute to find the inductor. The formula takes shape L= 1/(2пFr)^2.C. [uH] 4. Let's do the math. L=1/(2x3.14x1.58)^2.0.001= 10.14uH, with a margin of error 1.4% 5. You can use Electrodroid to calculate the value of the inductor. Once you have the two constants (frequency, capacitance), it can accurately find the 3rd (inductance).
published 8 years ago
hurz
8 years ago
Finally you found that method yourself and better late then never. However, C must be set to 1.1nF. Keep it up.
nikisalli
8 years ago
i did this automatic with arduino , so you plug the inductor and it says the inductance
hurz
8 years ago
@niki, and whats the range? 100uH to 10mH?
nikisalli
8 years ago
yes it's something like 1uh to 10mH
nikisalli
8 years ago
i took that from Internet
thebugger
8 years ago
This circuit is useful for a wide variety of ranges, if your pulse generator and oscilloscope can handle it. You can probably measure inductances down to a few nH and up to a few H. The error margin also depends on the oscilloscope. Old analog oscilloscopes will get you around 10% error, but a digital oscilloscope will get the error down to 1% or even less. Anyway any method has its perks and disadvantages, i just found this one interesting.
hurz
8 years ago
1%????? Down to a few nH????? Nononono, no way!
thebugger
8 years ago
Why not? If your oscilloscope is very accurate and has a high bandwidth, and your pulse generator is good i don't see why not. Digital oscilloscopes often have a vertical mark, with which you can accurately measure the period of the decaying oscillations.
hurz
8 years ago
Tolerance of all caps???????
thebugger
8 years ago
You can also measure caps, but that tutorial is hard to represent in EC.
nikisalli
8 years ago
if you have tobmeasure caps you need an rc network and how about the tolerance of a resistor?
jpoulin0901
8 years ago
Its often easier to just put your reactive component in series with a known resistance, apply a known sinusoidal voltage at a known frequency and then, 1. Find the resistor voltage and use it to find circuit current. 2. If measuring a good quality AC cap, measure Vc and assume Xc=Vc/I, and C=1/(2πfXc). 2.1. If measuring a coil, disconnect after step 1 and find the DC resistance of the coil (RL). 3. Let ZL be the impedance of the coil, and VL the voltage across it. ZL=VL/I and the reactance of the coil, XL=√(ZL²-RL²) 3. The inductance is then L=XL/(2πf). This method can be used at 60 Hz but only with particularily large value caps and coils. Also, at higher frequencies, skin and proximity effects will introduce larger differences between DC and AC resistance in coils which leads to progressively poorer accuracy.
jpoulin0901
8 years ago
My method is not too accurate but all you need is a sinusoidal signal source and a true rms meter. Your idea is not bad but many folks don't have a scope.
thebugger
8 years ago
Niki, capacitors and inductors are typically not variable (not always), so you cannot predict their tolerance, but resistors come in the form of potentiometers and trimmers, which you can trim to match the needed resistance very accurately. As I said, everything depends solely on your equipment's quality.
thebugger
8 years ago
Jpoulin0901 I often use the reactance method with larger inductors (such as mains chokes), often in the Henry range, but i guess given the right equipment, you can measure things down to a few uH. I prefer this method only for large inductances and capacitances because, even if the error is quite high, there's usually not a problem.
thebugger
8 years ago
Yes the resonance method uses an oscilloscope, and if it's a cheap oscilloscope, you won't get much accuracy, but on the other hand it's kinda simple and works with fairly low values, with somewhat good tolerance.
nikisalli
8 years ago
i use a 132€ pocket oscilloscope and i have to say that it's really accurate, also at 10Mhz
yra
8 years ago
Bugger do you have a simple circuit for testing circuit pulled mosfet & scr, i just need a good - bad indicated. Any ideas.
thebugger
8 years ago
Well you can try it the same way you test BJT's just put a bleeding resistor to ground of let's say 1Mohm. Other than that I'm not really sure, haven't had much experience with mosfets, just in a few switching applications.
thebugger
8 years ago
Those pocket oscilloscope are goos for preliminary testing. Most of the times you'll find your wave undersampled and not accurate. Just like my old analog oscilloscope
yra
8 years ago
I'll give it a try thanks.
thebugger
8 years ago
The bleeding resistor goes from gate to ground, i forgot to mention
gsiangyu
8 years ago
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