DIY megger insulation tester recomedations (not safety holywar tread!) - Page 1
Forum section: electronics / testgear

author: 001
author_rank: Super Contributor
timestamp: September 07, 2019, 12:35:44 pm
content: Hi!It is not safety holywar tread!So, I`m looking for ideas to build my own meggerI need it to test my HV coilsThe idea is to use standard autotransformer and transformer from neon sign to get 2000-3000 volts and analog milliampermeterIs it ok to test isolation with AC instead DC?How I can protect milliampermeter from overrange?(Any industrial-made schematics are highly requested)

author: Rerouter
author_rank: Super Contributor
timestamp: September 07, 2019, 12:39:29 pm
content: Better to test with DC, this way your only measuring its insulation, not its parasitic capacitance.Have some diodes across your mA meter, your source should be current limited, but it will have to survive any external capacitance,Your device should behave like a ideal current source, no matter what it is across it should have less than 1mA on its output.

author: Gyro
author_rank: Super Contributor
timestamp: September 07, 2019, 01:41:48 pm
content: I would hope you would have a microammeter in there rather than a milliammeter (go for a 50uA one).Yes diode protection on the meter. Put a limiting resistor in series with the meter and then a diode across the pair, that way you can control the maximum overload current through the moving coil better when the diode clamps (you should get away with a single diode).If you want to be able to measure higher leakage currents, then you can always switch a shunt resistor across the meter - you can't go the other way (mA meter -> uA) without adding a gain stage.

author: 001
author_rank: Super Contributor
timestamp: September 07, 2019, 04:01:36 pm
content: Quote from: Gyro on September 07, 2019, 01:41:48 pmDid You mean simple diode across meter to clip burden voltage at 0.7v?

author: Gyro
author_rank: Super Contributor
timestamp: September 07, 2019, 04:48:25 pm
content: Yes.Include a resistor in series with the meter too (the diode is across the meter and series resistor together) so that you can set the maximum current that the meter will be exposed to at 0.7V.

author: Hexley
author_rank: Regular Contributor
timestamp: September 07, 2019, 07:12:37 pm
content: Rich Measures did a nice design of a high-pot tester "for non-destructively measuring the voltage withstanding ability of relays, capacitors, transistors and silicon rectifiers."There might be some tips and tricks to borrow from his design, even though it is much higher voltage than the OP mentioned.http://www.somis.org/BVT.html

author: 001
author_rank: Super Contributor
timestamp: September 08, 2019, 01:08:24 pm
content: Quote from: Hexley on September 07, 2019, 07:12:37 pmAwesome link

author: 001
author_rank: Super Contributor
timestamp: September 08, 2019, 07:48:48 pm
content: Looking for Australian articlehttp://archive.siliconchip.com.au/cms/A_112015/article.html(no longer avaliable at website)

author: Gyro
author_rank: Super Contributor
timestamp: September 08, 2019, 09:31:46 pm
content: I know it's cheating, but 5kV insulation testers (GM3125) from China are starting to turn up on ebay for around the £120 mark - other shopping sites may be cheaper.Of course shipping to Antarctica may be an issue as usual.

author: SaleB
author_rank: Contributor
timestamp: October 29, 2022, 01:53:29 am
content: Quote from: Hexley on September 07, 2019, 07:12:37 pmReading some content on DIY insulation testers has led me to this old topic. This link is still available throughWayback Machine, but I have also made a PDF print of it for my collection that I'll attach to this message.