Line filters for your Calibration gears and dmm - Page 1
Forum section: electronics / general

author: opa627bm
author_rank: Regular Contributor
timestamp: September 26, 2022, 06:58:16 am
content: Hi all,I believe i have finally fixed my 731A i have replaced the 2 resistors at the gain opamp and 3D printed a small shell around the reference ic and opamp blocks.However during the data recoding, i am getting about 30uv jump from time to time and not sure where it is coming from then i realized it is in sync with my light switch and outdoor acunit.I have slapped a keithely harmonic reduction module that feeds both my 34470 and dmm7510 but can only minimize the effect. 731A is on battery so i think it is ok..Is there anything else i can do to minimize this?RegardsLi

author: strawberry
author_rank: Super Contributor
timestamp: September 26, 2022, 07:18:40 am
content: test lab in steel cage or undergroundradio amateurs turn off all devices to not spoil their expensive receivers (sensitivity <0.1uV)

author: Messtechniker
author_rank: Frequent Contributor
timestamp: September 26, 2022, 07:20:11 am
content: Debounce the switch(es) causing the interference.

author: strawberry
author_rank: Super Contributor
timestamp: September 26, 2022, 07:46:30 am
content: Quote from: Messtechniker on September 26, 2022, 07:20:11 amif it is not SCR

author: Andreas
author_rank: Super Contributor
timestamp: September 27, 2022, 05:38:52 pm
content: Quote from: opa627bm on September 26, 2022, 06:58:16 amHello,Did you already try a 100nF capacitor across the 10V output pins?Eventually you will need a 10-100 Ohms series resistor to prevent the output from oscillating.With best regardsAndreas

author: opa627bm
author_rank: Regular Contributor
timestamp: September 27, 2022, 07:38:50 pm
content: Quote from: Andreas on September 27, 2022, 05:38:52 pmHi Andreas,I dont have any output caps or series resistors, there is a 1K series resistor inside the Fluke 731A , but I will try it tonight.will sliver-mica cap works ?Regards,Li

author: Andreas
author_rank: Super Contributor
timestamp: September 27, 2022, 08:48:15 pm
content: Hmm,short wiring is more essential than the material.At maximum I would use a polypropylene capacitor.with best regardsAndreas

author: miro123
author_rank: Frequent Contributor
timestamp: September 28, 2022, 06:25:45 am
content: Hi,I end-up with this configuration.- line filters for room input grid cables.- optical ethernet cables- big metal plate for the test setups- Reliable PE- last but not least - setups runs on many 200Ah/LiFePO4 cells. See picture below for one +-12V setup configuration. Battery packs are independant (floating). It reduces the headache with grounding.============All it begun 30 years ago with introduction of switching power supply in home appliance.It started with TV, computers and end with PV inverters, washing machine and LED lighting, WiFi did not make life easier too.PS: Isolation transformers did not worked for me. They had huge leakage capacitance

author: dietert1
author_rank: Super Contributor
timestamp: September 28, 2022, 07:46:10 am
content: For one of the setups here i am using a CONet shielded transformer that received an additional 3-phase line filter like these:" alt="" class="bbc_img" />(2x phase and PE). The transformer shield is connected to secondary guard, as well as the metal crate. At high frequencies that works as an inductive divider. With the line filter inductance of 5 mH and the cabling inductance of some uH one gets about 60 dB attenuation. A 100 V switching spike of a nearby device passes with 100 mV, still not a lone island. So one needs to work on devices that emit those spikes as well.One day i intend to make a voltage regulator for that transformer. CONet also made a similar shielded transformer with voltage regulator (CEK 300), but that one switches primary coil steps with triacs.Regards, Dieter

author: srb1954
author_rank: Super Contributor
timestamp: September 28, 2022, 10:18:40 am
content: Quote from: opa627bm on September 26, 2022, 06:58:16 amCheck whether you noise is coming through your input wiring or via the mains supply by applying a short to the inputs of your DMMs and seeing if the noise spikes still appear. If they are significantly reduced that indicates that the noise is arriving by your input cables and you need to look at shielded cables and also make sure that ground loops do not exist between pieces of equipment or in the mains power connections.

author: tggzzz
author_rank: Super Contributor
timestamp: September 28, 2022, 10:35:17 am
content: One easy to try technique is to take the mains lead of the interference source, and wind that through a ferrite toroid several times.If it helps fine, if it doesn't then try something else!

author: miro123
author_rank: Frequent Contributor
timestamp: September 28, 2022, 12:55:37 pm
content: If you are going to play with filters.Assumption that higher Inductance is better does not work. The damping equation is D=Z1/Z2 not D=L1/L2Look at Wurth video - 5:50min.It is worth to follow the whole Wurth series "askLorant ". I wish those videos ware available before. Unfortunately I have learned this on hard way.

author: dietert1
author_rank: Super Contributor
timestamp: September 28, 2022, 04:28:48 pm
content: The idea is to use the isolation transformer at low frequencies, the 3-phase filter shown above at medium frequencies, and a snap-on ferrite at higher frequencies, let's say above 50 MHz.If one also works on the emitters, one can get pretty far. In my case i found and removed a problem in an old russian made stereo microscope, where the illumination runs from a transformer and the mains switch of that transformer was sparking.In spring two Würth application engineers visited our lab and i bought from them these coils, as i thought they might serve with a similar circuit as in the 7000 reference. I would use a two layer board in between with a shielding pattern on each side, or maybe two thin ones with air space in between.https://www.we-online.com/katalog/datasheet/760308101302.pdfRegards, Dieter

author: miro123
author_rank: Frequent Contributor
timestamp: September 28, 2022, 09:24:40 pm
content: Yes, triple stage filter do the jobs pretty good.one advantage of those pi filters is that they are not perfectly symmetric. On such way they serves as common mode and partially as differential mode filters.Thanks for sharing the link of wurth component. The idea is great!