Isolation transformers and reference standards - Page 1
Forum section: electronics / testgear

author: unimorpheus
author_rank: Regular Contributor
timestamp: February 28, 2017, 09:52:29 pm
content: Since I have acquired some many pieces of used test gear lately I have been looking into calibration options. Someone on the forum had mentioned the cheep Chinese devices and products fromwww.voltagestandard.com, particularly the DMMCheck and DMMCheck Plus. Is seems each of these products are discontinued. I reached out to Doug from voltagestandard.com via email as to why the products were discontinued and he sighted lack of sales and market interest. While I am interested in his 0.003% 10V reference it still leave me lacking for current and resistance standards. I really don't trust the Chinese eBay stuff.I also have been looking into isolation transformers for DUT isolation and I have really only found the TrippLite products readily available. From what I understand the TrippLites have to be modified to be true safety isolation devices. What are the details of this mod? Does this just involve disconnecting the ground reference on the secondary? If so then where should the secondary ground attach? I would think chassis ground should attach to mains earth ground, does everything on the secondary float including the secondary ground lead? It would appear, at least to me that the ground reference would be unnecessary on the secondary side if hot and neutral were truly isolated. Everyone warns of the dangers of common ground in an iso but no one seems t provide the preferred circuit configuration. Thanks and sorry for the nub questions.

author: bitseeker
author_rank: Super Contributor
timestamp: March 01, 2017, 12:53:24 am
content: I don't remember the TrippLite configuration exactly. I believe there was a discussion in one of the myriad threads here.If the neutral on the secondary is connected to ground, then removing that connection would be the first level of isolation. Ground on the output would still connect to earth ground (i.e., ground is ground is ground).The next level of isolation is to also disconnect the ground line on the output. It all depends what you need and, of course, you have to be aware of the implications of each setup.Some people install switches to be able to easily change the configuration of the isolation.

author: unimorpheus
author_rank: Regular Contributor
timestamp: March 01, 2017, 02:30:54 am
content: Quote from: bitseeker on March 01, 2017, 12:53:24 amThat is precisely what I am trying to nail down. What are the different configurations and their implications. I will do a bit more searching to see if I can maybe find a circuit diagram or two.

author: mtdoc
author_rank: Super Contributor
timestamp: March 01, 2017, 02:38:57 am
content: Todd Harrison did a pretty good video on modifying a Tripplite for bench work a while back. There are lots of things to understand when modifying and more importantly using an isolation transformer correctly.

author: bitseeker
author_rank: Super Contributor
timestamp: March 01, 2017, 04:46:20 am
content: Unless you really need to disconnect the DUT from ground, I wouldn't disconnect the ground completely. Just break the ground-to-neutral connection on the secondary. Todd's video covers that particular configuration.

author: unimorpheus
author_rank: Regular Contributor
timestamp: March 01, 2017, 05:20:28 am
content: Quote from: bitseeker on March 01, 2017, 04:46:20 amI would think I still have a ground to neutral connection at the main breaker panel. In an apartment it is hard to say. I am definitely going to test these configurations.

author: Bryan
author_rank: Frequent Contributor
timestamp: March 01, 2017, 08:27:32 pm
content: I had a similar question here if you want to follow the thread.https://www.eevblog.com/forum/projects/safety-isolation-transformer-question/msg1143993/#msg1143993