Why does my shunt have blobs of solder on it? :p - Page 1
Forum section: electronics / testgear

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 04, 2013, 06:03:09 am
content: Says EX530 on the boardDo these meters have a current calibration? or is it assumed in the factory that the shunts are calibrated, so it could be reading wrong now?

author: staxquad
author_rank: Frequent Contributor
timestamp: November 04, 2013, 06:38:49 am
content: Quote from: AmmoJammo on November 04, 2013, 06:03:09 amwe've seen shunts filed down, pinched with cutters to increase resistance to adjust the shunt, so I guess adding solder would decrease resistancedoubt the solder has the temperature stability of manganin though (adding non-linearity), so would be a quick fix only (the manufacturer doesn't care about the quality of their product)proper companies usually start with more metal than needed and file it down or pinch it till the resistance is correct

author: amyk
author_rank: Super Contributor
timestamp: November 04, 2013, 06:46:40 am
content: As odd as it looks, it's for calibration.

author: kizzap
author_rank: Unknown
timestamp: November 04, 2013, 06:48:00 am
content: wouldn't it be more likely that the solder on the shunt is an incidental part of the manufacturing process, and was simply placed there by a clumsy hand?It looks like one of the solder joints wants to break off too.-kizzap

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 04, 2013, 06:52:42 am
content: one is a blob... the other looks more like an accidental smearThis is a Digitech meter, sold by Jaycar, its the same as the Extech EX530... however, I dont think its the same build quality as a lower spec meter from the same range I bought ~5 years ago...

author: pickle9000
author_rank: Super Contributor
timestamp: November 04, 2013, 06:52:48 am
content: That one pad looks very poor.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 04, 2013, 06:56:07 am
content: which one?edit: the solder must just be for calibration... the other meter I just pulled apart again has had the shunt pinched/cut to calibrate it.

author: BravoV
author_rank: Super Contributor
timestamp: November 04, 2013, 07:54:14 am
content: Quote from: AmmoJammo on November 04, 2013, 06:56:07 amNope, that is a shitty solder joint, what meter is that ?

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 04, 2013, 08:03:00 am
content: Nah, that solder joint is fine

author: Stonent
author_rank: Super Contributor
timestamp: November 04, 2013, 08:39:45 am
content: During one of Dave's multimeter reviews there was a shunt that looked like someone had hacked away at it with an axe.I believe it was discussed in the "shaved shunt" discussion on the forum.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 04, 2013, 09:01:33 am
content: Quote from: Stonent on November 04, 2013, 08:39:45 amI found this:https://www.eevblog.com/forum/testgear/teardown-extech-ex570-my-first-but-also-last-extech/Must say, I'm quite happy with the construction quality of my meter now

author: kizzap
author_rank: Unknown
timestamp: November 04, 2013, 09:10:58 am
content: I can comprehend and completely agree with the fact that a saw or snips can increase resistance on a shunt, being that it gives a smaller path for the current to flow. I can't though comprehend how solder on a seeminglyrandomspot, with no care taken to ensure the solder joint is uniform will decrease the resistance, especially when ideally the cross-sectional area of the original shunt would ideally be uniform. If the original shunt has a uniform cross-section, wouldn't the resistance be the same along the length of the shunt?I would believe that it would help if some "subtractive adjustment" had been done on the shunt and it was a kind of surgery to fix it...but still...Dave, do an experiment for us?

author: codeboy2k
author_rank: Super Contributor
timestamp: November 04, 2013, 10:03:38 am
content: Quote from: kizzap on November 04, 2013, 09:10:58 amThe blob of solder makes the cross sectional area at the point of the solder no longer uniformly the same as the cross-sectional area everywhere elseThe formula for resistance of a uniformly circular wire iswhereis the resistivity of the medium,is the length of the wire in meters andis the cross sectional area in square meters. Thusis the radius of the wire.This works because it's uniform, and you can multiply it by the length. But the solder blob adds a cross sectional area that is changing along the length of the wire, now you have to integrate it. And the resistivityis different at that point too, so you would have to do the two regions separately to get the total resistance, and these two regions are in parallel. The math for an unknown blob of solder would be hard, at best. But the resistance will still decrease as you increase the total cross sectional area at that point where the solder blob is. Basically, it's adding a small parallel resistance at that point, it has its own resistivitydifferent from that of the base shunt wire, and it has its own tempco, and its own total resistance along its length, which is hard to calculate because it's oddly shaped.Adding a blob of solder may not seem precise, but they are tuning it at the factory, so they are watching the measurements as they tune it.As an example that is easier to visualize, consider the image below, I've soldered 4 chip resistors together, the size doesn't matter, assume they are all value 100 ohms. You have no trouble believing that the structure in the image is 250 ohms: 100+50+100. without the top resistor, it is 100+100+100 = 300 ohms. So adding the top resistor lowers the resistance because there is a parallel resistance there. You can visualize increasing the length of the lower section with more and more 100 ohm resistors in series, but the section with two in parallel will always be less resistance, i.e. 50 ohms, at that spot.QuoteYes, Dave can prove it with an experiment

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 04, 2013, 10:14:52 am
content: I'd say that this meter needed the resistance of the shunt reduced, due to the long negative WIRE running from the ground terminal on the multimeter, to the shuntWhereas, most other meters need the resistance increased to calibrate it (bits ground/cut out).

author: kizzap
author_rank: Unknown
timestamp: November 04, 2013, 11:34:20 am
content: Quote from: codeboy2k on November 04, 2013, 10:03:38 amAwesome explanation, thanks for taking the time there. I completely forgot that resistance is also dependant on the length of the conductor

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 08, 2013, 09:55:15 pm
content: This meter iiiiissss ssoooooooooooo ssssssllllllllloooooooooooooooooooooooooooooooooow...

author: Harvs
author_rank: Super Contributor
timestamp: November 09, 2013, 12:15:21 am
content: Can anyone tell me why they would bother trimming the resistance of the shunt?First thing that would happen is the shunts voltage would be digitized (probably with some initial analog gain and CMR.) Then offset and scale would be applied in software.How far off would the shunt have to be for this not to be easily compensated in software? Like 20%?

author: retiredcaps
author_rank: Super Contributor
timestamp: November 09, 2013, 12:19:23 am
content: Quote from: AmmoJammo on November 08, 2013, 09:55:15 pmYes, that is what Martin commented on as well.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: November 09, 2013, 01:01:43 am
content: Quote from: retiredcaps on November 09, 2013, 12:19:23 amI know... but its not till you actually use it that you realise just how incredibly slow it is!I might end up getting a lower spec meter for daily use, and just use this when I need high resolution...