Products > Test Equipment
ET432 LCR Meter - Wild Results - Range Function?
killingtime:
--- Quote from: Tycho_Brahe on January 24, 2024, 10:41:12 pm ---This is depressing. I thought I had a decent brand/model picked out....
--- End quote ---
Not so fast ...
I remember reading some posts on a guitar amp forum about valve output audio transformers. They love to measure them because the output inductance on the amp forms a low pass filter in the audio chain, and that affects the way it sounds. Conclusion was the level a transformer is driven at alters its electrical properties.
I was taught that as the AC voltage on an unloaded transformer primary (or inductor) increases for a given frequency, the core becomes more saturated with magnetic flux, and this affects the permeability of the core (assuming it's not air). As the permeability goes down, so does the inductance as the two are directly related. This means less reactance (Ohms) and the transformer will pull more current as you increase the driving voltage on the primary, even if the secondary is unloaded.
Spent last night searching the web for some charts on transformer\inductor driving voltage and inductance but couldn't find anything. This topic isn't discussed much, but it's important for LCR meters as you'll see. So I ran my own tests today.
Results and data attached.
I'm driving the original transformer (unloaded) with a variac, all the way from 300mV to 250V AC RMS. Recording the voltage and current using two DMMs. Reactance (XL) calculated using Ohms law as we know the current through the Xfmr and the voltage across it. Inductance then calculated using L = XL/2(pi)F.
The inductance starts off low and increases, peaks then decreases as the core starts to saturate. Did the same test again for an MOT I had lying around, as I know these saturate easily.
I don't know what's causing the low inductance at low voltages, but the figures don't lie.
I also ran 600mV AC RMS from the variac as this is used by the ET432 LCR meter (see post above), and got 0.68H. That's not too far off the 0.528H I got in AUTO on the LCR meter. A 22% difference, which sounds poor but when you look at the data I collected the inductance hits 7H @ 100V, so about 10x that.
What does all this mean? You need to drive an inductor or transformer at the voltage it will be used at to determine its actual inductance.
This brings LCR meters back into focus. They only output about a volt, so they're not going to tell us a whole lot unless the target circuit runs at that voltage as well. I'd hold off on that 1K bench LCR meter for now.
Incidentally, capacitors have a similar issue with voltage as well, but depends on the dielectric used. As you increase the bias voltage, the capacitance goes down, so you need to test them at working voltage.
Randy222:
--- Quote from: mawyatt on January 25, 2024, 04:19:15 pm ---This is often done to reduce thermal EMF effects.
Best,
--- End quote ---
The tiny handheld, DC near 200mV, will run into a thermal EMF effect issue?
2nd, we know the meter cant do 200mv when measuring 1m-ohm, I just don't see how it can drum up 200A.
mawyatt:
--- Quote from: Randy222 on January 25, 2024, 04:36:10 pm ---
--- Quote from: mawyatt on January 25, 2024, 04:19:15 pm ---This is often done to reduce thermal EMF effects.
Best,
--- End quote ---
The tiny handheld, DC near 200mV, will run into a thermal EMF effect issue?
2nd, we know the meter cant do 200mv when measuring 1m-ohm, I just don't see how it can drum up 200A.
--- End quote ---
As said this is often done, whether this LCR meter does this we don't know, as we don't have one.
The lower the impedance measurement the more influence Thermal EMF has on the measurement. Thermal EMFs can easily produce many microvolts which for example measuring 1m-ohm with 10ma current (what our Hioki IM3536 can produce), that's only 10uV across the DUT, so a few microvolts due to Thermal EMF can introduce significant errors..
Obviously the LCR meter measuring 1m-ohm can't produce 200amps, what if you short the test leads!!!
Seems you need to study up on how these LCT LCR meters actually work, how measurements are made, and how these relate to the parameters displayed on these LCR meters.
Best,
csuhi17:
Has anyone figured out how to properly calibrate this device?
That is, I mean the correct use of the CAL button.
Phil1977:
--- Quote from: csuhi17 on March 02, 2024, 09:33:09 pm ---Has anyone figured out how to properly calibrate this device?
That is, I mean the correct use of the CAL button.
--- End quote ---
I get the best results if I use the calibration function twice. Once with the short circuit bridge and once without.
I also have no idea why the instrument uses this square wave for the DCR measurement. I´ve never noticed these problems before I´ve read about them in this forum. So far my ET432 did and does a good job for characterizing caps and inductors over the frequency range. But the more I read about it the more difficult it gets to interprete its results :-//
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