Effect of the shielding inside of cheap DMMs - Page 1
Forum section: electronics / testgear

author: kalel
author_rank: Frequent Contributor
timestamp: July 05, 2017, 12:01:34 am
content: In the cheap meters, sometimes you will find only a tiny piece of shielding on the back side of the case.That prompted me to ask about the effect of the shielding, especially when it comes to cheap DMMs (like the DT series and similar) - for the more expensive ones with higher accuracies and resolutions, I expect that every little thing has some effect.But what about the cheap ones, is the almost total lack of shielding making any difference, and so if you do have a meter with such shielding, could it help to add some aluminum or copper sticky tape and increase the area of the shielding?Of course, any answers that relate to effects of shielding on more expensive meters are also welcome, but I wanted to make the purpose of the question clear. For instance, the effect might be smaller or even minimal on the cheap meters similar to the DT series, but huge on an expensive one with high accuracy and resolution.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 05, 2017, 12:23:22 am
content: Like the free ones we get from Harbor Freight? These don't typically have any shielding.As tot he effect of shielding or lack of, on more expensive meters, I think that Gossen Ultra I looked at should answer that.

author: kalel
author_rank: Frequent Contributor
timestamp: July 05, 2017, 12:29:35 am
content: Quote from: joeqsmith on July 05, 2017, 12:23:22 amYes, those should be pretty similar to the DT models (there's the slight modification of on off switch which seems great). We don't get them for free, but from China they start as low as about $2.5 with shipping (the non buzzer model) to about $4-5 as a buzzer + temperature model with a thermocouple.I mean the shielding on the inside of the meter, on the back side (there's probably special additional shielding for more expensive meters and not just that), usually there's a spring touching that shielding and there's a small piece of it (how small, depends on the exact meter, I have seen a tiny rectangle just a bit larger than the spring, and a bit larger). It should be pretty easy to get some aluminum/copper or other conducive tape there and make the surface as large as possible, but I'm not sure if it's pointless or not.I will check your Gossen videos.

author: David Hess
author_rank: Super Contributor
timestamp: July 05, 2017, 01:44:00 am
content: The foil shield also blocks electrostatic or capacitive coupling between the environment including your hand when you are holding the meter and the sensitive high impedance input circuits.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 05, 2017, 02:38:10 am
content: Quote from: David Hess on July 05, 2017, 01:44:00 amding ding ding ... We have a winner!!Then there is that whole problem with these latching relays Gossen decided to use that can change states by a simple magnetic hanger...Of course, Gossen has that BT radio built in and sealing off the entire meter may make that feature worthless.

author: kalel
author_rank: Frequent Contributor
timestamp: July 05, 2017, 06:16:55 am
content: Quote from: joeqsmith on July 05, 2017, 02:38:10 amNot that I care about BT/Wifi that much (I would be happy with a USB data logging system if I was ever affording a data logging meter), but what would be the solution?Say some DT manufacturers are looking at this forum and trying to avoid similar mistakes in their future data logging models... okay, that's not very likely. But would some kind of external BT card that you can plug in be a better solution? Of course, once it's plugged in, there's still going to be some possible issues. But at least, when you're not using the external card, it can be fully shielded.

author: BravoV
author_rank: Super Contributor
timestamp: July 05, 2017, 06:32:20 am
content: Prolly related, will a layer of conductive carbon sprayed evenly has same effect ? worst or better ?As sprayed result has much better cover than just sticking with sheets of metal foil.

author: Gyro
author_rank: Super Contributor
timestamp: July 05, 2017, 08:46:16 am
content: Just be careful to offer up the back side of the case to the front and check the gap between the screening you add and the high points of fuse holders, back of jacks etc.You wouldn't want to compromise the safety ratings (assuming the cheap ones have any), safety anyway. Where already fitted on meters, the foil us usually shaped to avoid these areas. It's probably even more relevant to overspray if you use sprayed carbon, mask carefully.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 05, 2017, 09:40:25 am
content: Quote from: kalel on July 05, 2017, 06:16:55 amThe entire meter is not shielded.https://youtu.be/XvpGa8ieit0?t=1425

author: joeqsmith
author_rank: Super Contributor
timestamp: July 05, 2017, 09:46:26 am
content: Quote from: Gyro on July 05, 2017, 08:46:16 amSeen that on a few meters during testing where breaks down air gap to shield and finds it's way home through the spring.

author: David Hess
author_rank: Super Contributor
timestamp: July 05, 2017, 03:48:25 pm
content: Quote from: kalel on July 05, 2017, 06:16:55 amThe problem with USB is that galvanic isolation needs to be added. The problem with wireless is poor reliability compared to a wired connection.Quote from: BravoV on July 05, 2017, 06:32:20 amAbsolutely this will work; they make conductive sprays for exactly this purpose. The old Apple ][ computers implemented EMI shielding this way. Electroless plating on plastic will also work.QuoteThe foil works well enough and is probably less expensive.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: August 26, 2018, 04:01:08 am
content: Some do. I just opened an old yellow one this morning and it is like the OP said. There is a spring on the pc board that contacts the foil inside the back cover.Quote from: joeqsmith on July 05, 2017, 12:23:22 amSent from my SM-G900V using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2018, 09:08:36 am
content: I don't know about graphite spray exactly (aquadag or similar?), but yes, there are conductive paints to do that.Had a project about four years ago, customer wanted a system with a power supply, control box, and IO fanout box(es). They insisted on putting USB inside unshielded cables. We warned them about that...Actually, the cable was shielded, because pretty much every multiconductor cable has foil in it already, but it's a lot more work mating that up to a metal connector, versus using cheap plastic connectors and not bothering with shielding at all. (We're talking IP rated circular connectors here.)Long story short, we changed the connectors to metal shell, connected up shields and grounds, and put in shielded boxes where possible. The customer wanted a $50 IO/fanout box, we delivered a $200 box.Very nice box, it was; IP66 (or thereabouts) rated, lined with conductive paint, including the rubber gasket, which surprised me! Nice low resistance, fraction of an ohm, contact resistance dominating the measurement I think. Good for at least 40dB shielding, which was what we needed to pass immunity. Just expensive as sin, like $80beforemachining...Tim