inductance meter - Page 1
Forum section: electronics / projects

author: thefatmoop
author_rank: Contributor
timestamp: June 04, 2013, 04:00:22 pm
content: I'm in the process of making an inductance meter that measures inductance using AC steady state analysis. If using a R-L voltage divider i've found that the only measurements required are: Vin peak, Vout peak, and phase shift of the Vout. Vin is the reference, so its phase shift is 0degFrequency is known and so is the resistance. For now i'm not paying attention to equivalent series resistance (ESR) of the inductor.In the future I want to measure ESR and take that into account when calculating L. Originally I thought of passing a DC current through the inductor to measure the ESR, but couldn't I test at two nearly same frequencies and calculate the ESR off of that? How do the professional meters calculate ESR?As far as circuitry i'm using a DDS sine wave module and using relays to switch between 3 known resistive loads. The input and output sine waves are converted to digital using an LM339 and then the two signals go through a nand network to find the phase shift which is measured by a micro. I'm then using peak detectors (waiting on some faster op-amps for precision rectifier) to calculate the two peaks.Does this method sound good?

author: JackOfVA
author_rank: Unknown
timestamp: June 04, 2013, 11:18:56 pm
content: The patent on the technique used by General Radio in its DigiBridge instruments has long since expired, so you might want to study how it was done in the GR 1658 and others in that line.Manuals are available in the Internet if you search a bit. I think it was also covered in the General Radio Experimenter house magazine, also available on line. And, the patent should be available, invented by Henry Hall.As I recall the circuit, it uses an op-amp as a virtual ground point to sample the current (amplitude and phase) through the device under test. A test voltage is applied to the DUT to generate the current. The current amplitude and phase is measured and this yields the X and R.However, don't hold me to the details on the DigiBridge circuit as it's been a few years since I looked at the manual.It was implemented in all discrete TTL and it would be much easier to do now with a microcontroller and updated op amps.I have a 1658 that sees daily use and it's a first class piece of test gear.As with all precision instruments, there's a great deal of work required to make the real world parts play like the perfect ones in the simulations.

author: thefatmoop
author_rank: Contributor
timestamp: June 05, 2013, 04:22:21 am
content: Thanks for the response! I know i kept it real generic. Yeah i've been working to get all the measurements spot on using precision rectifiers and just an lm339 and nand network to get the phase. I just took the approach of analyzing it using nodal analysis (ignoring ESR) and building the circuits to measure all the variables - then ensuring that my micro's measurements all match the scope.Are there any documents discussing this method?http://youtu.be/fs2MfTW4o_Q?t=3m18sI've been reading a bit intohttp://www.ietlabs.com/pdf/application_notes/030122%20IET%20LCR%20PRIMER%201st%20Edition.pdfDid a bit of searching for general radio's technique and that's the most relevant thing i found so far

author: JackOfVA
author_rank: Unknown
timestamp: June 05, 2013, 06:07:11 pm
content: GR 1657 DigiBridge manualhttp://www.ietlabs.com/pdf/Manuals/1657_im.pdfFor the basic DigiBridge patents, use Google and select Patent search from the "more" listing. Patent no. 4,196,475 and 4,342,089

author: thefatmoop
author_rank: Contributor
timestamp: June 06, 2013, 03:54:16 am
content: so what is Vp, Vq, Ip, Iq? And would a differential amplifier using a single op amp be enough, or do I need to take dave's route and use something like the AD620?

author: JackOfVA
author_rank: Unknown
timestamp: June 06, 2013, 11:39:30 am
content: A complete GR 1658 manual with a detailed theory of operation and schematic can be found athttp://bama.edebris.com/manuals/gr/1658/