Just pinging for any interest in inductor tester - Page 1
Forum section: electronics / projects

author: Buriedcode
author_rank: Super Contributor
timestamp: January 28, 2016, 12:26:23 pm
content: I have seen a number of projects to test inductors' inductance and saturation current (for power electronics, the two biggest parameters) all of which are well designed, but require an oscilloscope to see the current ramp, and to look for the 'knee' where it enters saturation. Nothing wrong with that, because frankly if someone is designing SMPS's then they should probably own a fairly decent scope, and these test devices make for quick analysis.However a few years ago, I didn't have access to a decent scope, and I was playing around with flyback converters for nixie tube projects. I looked into a way of checking inductance and saturation current of hand-wound transformers (well, coupled inductors.. as they were used in flyback configuration)without requiring a fast scope, just a standalone 'box'.Does anyone think there would be much use/interest in such a tester? There are many, many 'inductor testers' about already, ranging from the basic resonant frequency LC meters that can be fairly accurate, to the 'pulse generators' used with scopes, all the way up to pretty complicated LC meters.Examples:http://www.electronics-lab.com/project/simple-lc-meter/http://www.dgkelectronics.com/quick-project-inductor-saturation-current-tester/https://www.elektor.com/500-ppm-lcr-meter-kit-with-main-board-and-lcd-boardhttp://elm-chan.org/works/lchk/report.htmlI had built an LC meter based off the PIC16F628A (link above, pretty common) but couldn't get a sensible reading for an inductor with more than one winding - with the second winding open - so it seemed fairly useless for coupled inductors/transformers. But a setup with a borrowed scope of course worked fine, seeing the current ramp and knee, so I could calculate its inductance and get a rough idea of saturation current.The idea was pretty much the same set up as the pulse generator types, but using comparators to measure the time between two set current points on the ramp, therefore giving inductance (across a fixed voltage), and then shifting this window up... as it enters saturation, the ramp gets steeper, so the time difference gets smaller, once the inductance falls to 10% of its value at lower currents.. that'll be the 'rough saturation point'. I also thought of reusing the energy stored in the inductor dumping it back to the supply so it could generate fairly high current pulses from a portable device that runs on a lipo cell.So, whilst I probably ranted a bit, does anyone think this would be worth pursuing? As I said, there's already lots of these things about, so its not a new idea, just asking if there is a 'need'.BC

author: macboy
author_rank: Super Contributor
timestamp: January 28, 2016, 02:37:16 pm
content: I am also interested in such a thing. I recently had thought about knocking up one of the oscilloscope/pulse based testers, but didn't get around to it. It seemed like such an clumsy and inelegant way to do it (even if clever).

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 28, 2016, 03:04:48 pm
content: Hi,Have a look at this thread:Particularly this message:https://www.eevblog.com/forum/projects/inductor-saturation-tester-alternative-route-to-dump-the-excess-energy/msg183197/#msg183197This inductor tester recirculates the energy. A few people have built this successfully.The current can rise fairly quickly, so it may be challenging, but not impossible, to do the measurements without a scope.You may want to start with a short pulse measure the peak current, increase the length of the pulse measure the peak again etc. This would require a fast peak detector.Regards,Jay_Diddy_B

author: Kleinstein
author_rank: Super Contributor
timestamp: January 28, 2016, 05:20:24 pm
content: Measuring with increasing pulse lenght is a rather good idea. Taking the reading of the currnt after a well defined delay is usually not that difficult as the µC knows the timing - so it is more like a sampling circuit, not a classical peak detector that needs to get the timing from the signal. So it is similar to having a digital sampling scope to measure the curve, one point per pulse. Still it is quite some task for the µC to find the right slope and find the break in the curve.The picture on the scope can give more than just inductance and saturation current, but also some idea on how hard saturation starts.The down side with these pule type measurements is that they may not be accurate with inductors that have a high remanance.

author: Buriedcode
author_rank: Super Contributor
timestamp: January 30, 2016, 12:40:29 pm
content: Thanks for the replies lads.Back when I was thinking about this, I was thinking rather too basic in terms of which microcontroller to use - was just going to prototype with a PIC16F (!) using its two comparators, and SR latch to gate a timer. The micro turns on MOSFET manually, and the comparators have thresholds set a fixed voltage apart, say 1V. Starting at 0.5V and 1.5V, as the current ramps (after a suitable amplifier for the current sense resistor) it'll measure the time taken to move through two current points. Because the comparators in a PIC aren't particularly fast, propagation delay will be significant compared to the ramps rise time, but the difference means any delay will be subtracted out - although the propagation delay will be different for different voltage thresholds.That was the simplest way I could come up with, however, to ensure a half decent resolution and minimum inductance the timer would have to run very fast, and to slow the current ramp down, the voltage across the inductor was set at 2V. At such a low voltage, the DCR of the inductor becomes significant, as does the sag of the supply (which would come from a cap bank), and even the MOSFET's on resistance. Instead of thinking more practically I went down the rabbit hole of trying to make a very fast transient response linear regulator, fed by the cap bank, that tries to keep the voltage across the inductor fixed at the time of the test. Also I would need to measure DCR, and factor this in when calculating inductance from the current ramp. LTspice helped a great deal here, first using ideal models, then replacing parts with real-world devices. Whilst it worked, it wasn't exactly accurate at <10uH with DCR's of over 0.2 ohms.Even back then Corex-M0's were pretty cheap, so upping the spec of the controller would increase speed as well as allow for faster processing using 'clever software' to calibrate out things like regulation error, MOSFET resistance etc.. but I'm now starting to think it would be better to place the inductor in an actual boost converter, with a variable on-time, dumping the inductors energy back into a cap bank.By letting it run and changing the on-time, we can take an average of multiple cycles for higher accuracy for higher frequency switching. With a higher supply voltage, the DCR and MOSFET on-resistance has less of an impact.So really its just a glorified SMPS designed to measure inductance by fixing other parameters. Letting the inductor reach a higher peak current should mean when the inductor starts to saturate, the measured inductance falls. As always, these things can balloon in complexity, reaching the pint of diminishing returns, so I'm still at the idea's stage - again.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: January 30, 2016, 03:14:41 pm
content: I think that anyone specialized enough to be using this has an oscilloscope and would be interested in actually seeing the waveform.The device you are describing sounds like a go/no go device suitable for automated testing in a factory. A well made oscilloscope accessory would be interesting, so long it is easy to trigger the scope for a measurement. Like those LCR meter accessories jigs that cost stupid amounts of money. This would allow you to focus on making a dope ass analog front end for the thing. Temperature dependence of saturation accessory can be added with the money/time saved avoiding an actual readout. + you can have nice engineering graphs if you stick to a oscilloscope.I have a little jig that plugs into my oscilloscope directly and has a function generator/multimeter connection. This way I can have a swept frequency vector volt meter, and I can make bode plots of inductance/capacitance over frequency, voltage dependance, etc.Nothing can really replace this unit if I plug in a 6.5 digit multimeter, high end tektronix function generator and oscilloscope into it (granted phase is the worst part of the measurement). I need to remake it in a nice box and figure out how to give the option of DC bias using a buffered voltage calibrator, etc. I think I need to figure out some isolators for this but you really can't beat it with a stand alone tool, I think, so long the impedances of your measuring instruments don't mess with it. So long you design it right I don't think there is any reason why you can't go to high frequencies too. I think when I make it I will make it over size and leave room in the box for additional stimulation options etc. That way you can conduct some really interesting experiments.Or just buy a network analyzer :p, granted they don't typically make these that work in the low end of the spectrum.

author: Buriedcode
author_rank: Super Contributor
timestamp: January 30, 2016, 05:27:22 pm
content: sarepairman2,yes you're right, thats why I was asking if it was actually worth the effort. It wasn't so much a 'go/no-go' device, but without seeing the waveform, one could really only provide a few specs, namely, DCR, inductance, and saturation current - which would only indicate the current at which inductance drops to 10% of measured value. That should be enough for basic boost, and buck converters, but not much more. That would make it rather niche, and only for hobbyists who regularly wind their own inductors (who again, would probably have access to a scope anyway).I think there are already plenty of jigs for speccing inductors, as most are glorified pulse generators with a MOSFET, current sense resistor and flyback diode - more than good enough for the job.As I have lots of projects on the go, I'll do a few tests with my scope. I still *could* be worth it, if made cheap enough, but then that would hinder accuracyAlso, a standard LCR meter could find the saturation current, if combined with a high current supply (which again, many will have to hand). It was the combination of these two (oscilloscope and high current supply) that made me think of a 'box with LCD' type affair.I think its pretty conclusive its a bit too much of a niche (lots of effort, for little gain) so shall remain a curiosity rather than something that will be used regularly