Determining key values for unknown inductor - Page 1
Forum section: electronics / beginners

author: retrev
author_rank: Contributor
timestamp: June 06, 2018, 06:27:11 pm
content: I've been stripping parts out of some unused DSL line filters, mostly for the decent film caps. There are some unknown inductors, 4 lead ones that are probably RF chokes. I'm wondering if there's a way to measure some of the key values like inductance, impedance, and resonance without using an expensive network analyzer? I've got a couple of multimeters, DS1054Z, and a DDS signal generator that'll probably push about 10Mhz (probably half that for a clean sine wave).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2018, 08:07:13 pm
content: Yes, but you need an AC voltmeter with more bandwidth than a DMM.The general procedure is to resonate the inductor with a capacitor, connect this to the generator through a resistor to make an impedance divider, and measure the resonant peak and width to determine frequency, bandwidth and resistance. Use a calculation like this to solve for the unknowns:https://www.seventransistorlabs.com/Calc/RLC.htmlWhat you can do right now, at least, is ohm out the multi-pin parts, find how many windings and taps they have, and make some guesses as to what is what. You may find most things have too low resistance to measure (they're all under an ohm or so?). If you find higher resistances, they may be more for signal filtering or coupling, than inductive action.When you get something that can measure AC voltages accurately (usually a scope; even a little Analog Discovery board would be a nice complement to that DDS), you can use the resonant method and start figuring things out.Regarding transformers vs. inductors, I use this distinction:- Inductors are intended to store energy as reactive power. If there are multiple windings, it's a coupled inductor.- Transformers aren't intended to store energy. The inductance is very high (ideally infinite), minimizing energy storage.Examples: DSL coupling transformer (a type of pulse transformer): actually a transformer. Common mode choke: actually a transformer. Flyback transformer: actually a coupled inductor.Transformers have much larger values of inductance, and much crappier inductance (higher losses, low saturation current). This can make it difficult to measure transformers with the resonant method. If you find one, just do the best you can in figuring out its value -- it's not like it's going to be very stable anyway. Actually, that's something you can test, also: measure it at low signal level, like 10s of mV; then measure again at higher level, 100mV or 1V. See how the measured value shifts. Or do it with different capacitors = different resonant frequencies, and see how the inductance and resistance (core loss) change.Tim

author: strawberry
author_rank: Super Contributor
timestamp: June 07, 2018, 07:31:46 pm
content: Quote from: T3sl4co1l on June 06, 2018, 08:07:13 pmAm I rightInfinite inductance mean zero current , therefore transformer wont startup or at least before end of infinite timeopposite happen when reduce turns to zero and get infinite currentwhen I deal with school math there is always paradoxes and stuck

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2018, 09:22:30 pm
content: Zero current just means zero magnetizing current.Transformer action depends on flux, not current.Tim

author: strawberry
author_rank: Super Contributor
timestamp: June 08, 2018, 03:16:11 pm
content: Quote from: T3sl4co1l on June 07, 2018, 09:22:30 pmInductance is a property of an electrical conductor which opposes a change in current. It does that by storing and releasing energy from a magnetic field surrounding the conductor when current flows, according to Faraday's law of induction. When current rises, energy (as magnetic flux) is stored in the fieldprimary generate electron field and fill it up but secondary pick up that field. similar to car alternator

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2018, 05:03:41 pm
content: Yes, in free space, where field intensity is proportional to flux density: B = mu_0 * H.When magnetic cores are used, the field intensity can be arbitrarily low, say by a factor of a million (see superpermalloy).Even though the windings in a transformer are not fully surrounded by core (they're in air, or dielectric), the voltages still match, very well. Indeed, they'd match quite poorly if surrounded -- as badly as in an air-core transformer.Faraday's law is defined by the enclosed flux, so a transformer having a core through the center works out perfectly.Tim