Cheap current source to measure mounted coil resistance? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 29, 2022, 12:29:16 pm
content: Hi,We need to measure the very low resistance (~ few milliohms) of transformer coils when they are mounted into a PCB. This is to see whether or not they are properly soldered into the PCB. Its impossible with eg Triple insulated Litz wire, to be sure that solder pot dunking has stripped every strand, as well as the triple insulating jacket…..best way is to solder the coil in to PCB….then shove known current source ~20A through the coil, then measure voltage across coil….hence resistance.The attached ~20A current source is I believe the cheapest way to do this. Would you agree?Is there anything cheap offtheshelf that does this?LTspice sim and PDF scm as attached(Its duty cycled to reduce dissipation......555 time constant would be longer than seen here)(Will use an IRS21271 for the bootstrap driver, and will just tie the CS input to hi-side GND))https://www.infineon.com/dgdl/Infineon-IRS2127PBF-DS-v02_00-EN.pdf?fileId=5546d462533600a4015356768e7a27c0(stripping enamel from wires has been discussed already, here...https://www.eevblog.com/forum/manufacture/stripping-enamel-from-copper-wire/https://www.eevblog.com/forum/manufacture/small-cheap-solder-pot/

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2022, 08:45:17 am
content: There is no need to use 20 A to test for a resistance in the mOhms range.Depending on the inductance one may have to do the test at a relatively low frequency / with long enough pulses. Here less current helps to keep the self heating under control - it just needs a little more care with the voltage measurment, but this is usually easier than high power.With quite some inductance the current source may also need some care and maybe quite some RC damping element (e.g. 1 ohms and 100 µF) in parallel to the DUT to avoid instability.

author: f4eru
author_rank: Super Contributor
timestamp: August 01, 2022, 01:46:24 pm
content: Quote from: Faringdon on July 29, 2022, 12:29:16 pmRule number 1 for soldering QA :Never ever electrically test for solder quality. It does not work. A solder joint has to be inspected optically.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 01, 2022, 02:08:41 pm
content: I agree with the others here.Measuring resistance is not a suitable method for determining solder quality.It's quite possible that all strands are soldered, but only on a tiny spot, and this can easily lead to failure over time, due to vibration or thermal espansion / contraction cycles. Optical inspection works much better.

author: Faringdon
author_rank: Super Contributor
timestamp: August 01, 2022, 03:25:49 pm
content: Thanks yes youre right, i have this current source regulating the input current, to avoid such problems, then just tweak a pot on that input current error amp till the output current is roughly whats wanted.Also, AYK, there wasnt need for a low value sense resistor when the sensing/regulation was done with input current as opposed to output current....since Iin is much lower than Iout.Its thought we woudl struggle to optically analyse each strand of Litz optically.

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2022, 06:02:01 pm
content: If you really need a current source with mediocre accuracy for this application, why not use a power resistor and a voltage supply?

author: Faringdon
author_rank: Super Contributor
timestamp: August 01, 2022, 09:02:44 pm
content: QuoteThanks, yes, i cant afford one at this point, the voltage source capable of 20A, -not affordable at this point.........The Buck one of the top post, AYK, runs off a 30W 12v wall wart, and since vout is so low, , Iin is low....the wall wart is sub £10.The 20A Buck PCB cost me about £55 for 5 PCBs. (incl postage). And i can re-use it at other times in future........components on it are common components that i mostly already have for the 2kW SMPS i'm doing.Eagle Layout scm etc and gerbers of the 20A current source are linked below for anyone who wants it.20A current sourcehttps://drive.google.com/file/d/1pqWFq2QaixQQRHS1_i_BMrFB_1T4G0Pu/view?usp=sharing

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2022, 10:18:21 pm
content: Quote from: Faringdon on August 01, 2022, 09:02:44 pmAs mentioned above by others, you don't need 20 A to measure a resistance. If you have a "few mohms" of resistance, than 1 A will give you a "few mV" of voltage, which can be measured with a cheap DMM.(Assuming that the resistance measurement is relevant to your requirement.)