Electronics > Projects, Designs, and Technical Stuff
Impedance sweep on a coil
(1/1)
drummerdimitri:
I'd like to plot the impedance vs frequency graph of a coil using my function generator set to sweep from 20 Hz to 20 KHz sine waves and use my graphical multimeter to record the resistance over time.
That being said, I haven't been able to get even one resistance measurement at a random frequency (10 KHz, 1V RMS) since the resistance is in the mega ohms.
What am I doing wrong here? Can this not be done this way?
duak:
When you say resistance of the coil, do you mean the DC resistance or do you mean the AC reactance or impedance?
If you want the AC impedance you have to measure the current flowing through the coil. If the meter does not have an AC current input this can be done by measuring the AC voltage across a resistor in series with the coil. Even better is to use a transimpedance amplifier that has a low input impedance input and produces a voltage out proportional to the input current. Once you have the current, you can determine the impedance because Z = V/I where V is the voltage across the coil and I is the current through the coil.
What is the make and model of your graphing meter?
drummerdimitri:
--- Quote from: duak on August 24, 2019, 05:41:04 pm ---When you say resistance of the coil, do you mean the DC resistance or do you mean the AC reactance or impedance?
If you want the AC impedance you have to measure the current flowing through the coil. If the meter does not an AC current input this can be done by measuring the AC voltage across a resistor in series with the coil. Even better is to use a transimpedance amplifier that has a low input impedance input and produces a voltage out proportional to the input current. Once you have the current, you can determine the impedance because Z = V/I where V is the voltage across the coil and I is the current through the coil.
What is the make and model of your graphing meter?
--- End quote ---
Yes surely I meant the Impedance of the coil as resistance is the DC equivalent.
I have a Keithley DMM6500. I can digitize current so that shouldn't be an issue.
Thanks!
T3sl4co1l:
The most basic circuit, assumes a constant 50 ohm source resistance and constant amplitude from the generator.
Connect the generator directly to the coil. Tee this connection off to the voltmeter. Sweep frequency while measuring voltage.
You don't measure phase this way, and must reconstruct it from a curve fit, or the Kramers-Kronig relations.
This is a shunt method, and works best for impedances less than 25 ohms.
You can also do a series method, where the coil is in series between source and load; in that case, replace the tee with a coupler (tie the grounds with a much lower impedance, e.g., a metal box with two connectors bolted into it) and connect the coil between the center conductors. Terminate the voltmeter end so that the system is 50 ohms all the way through. This works best for impedances more than 100 ohms.
You can measure phase this way, but you need a reflectance bridge. A VNA (vector network analyzer) contains one of these each for both source and load, and measures phase (hence "vector"). This also lets you determine any line-to-ground impedance the coil may have (which can be substantial when high impedances are being measured).
Tim
Navigation
[0] Message Index
Go to full version