Build external power solution for UNI-T UT71C DMM - Page 1
Forum section: electronics / testgear

author: mdijkens
author_rank: Regular Contributor
timestamp: February 16, 2017, 08:23:20 pm
content: Hi,I am still very much a beginner in electronicsI have a UNI-T UT71C DMM with the possibility to do datalogging to the PC.But when using this to log some measurement for hours or days, I run out of 9V batteries quickly.Now I am thinking of connecting it to an external 9V power supply but I'm not sure if there are things I have to watch out for?1. Does the powersupply need to be of a certain quality or type to keep the same accuracy/precision as with the battery or can I use simple buck/boost type?2. Do I need to worry about isolation? Groundloops? etc?3. Other things to watch out for or be aware of?Thanks in advance

author: jeroen79
author_rank: Frequent Contributor
timestamp: February 17, 2017, 10:41:14 am
content: Regarding 2: Yes, you should keep the meter isolated.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 17, 2017, 10:54:08 am
content: I'd use a 9v AC wall wart transformer and a small DIY rectifier/smoothing/LM317 board to give 9v for this. That will give you nice clean DC (non of that switching noise) and full isolation.Some of the DC wall warts are switchers and some of them have an earth connection to the connector shell so you can't trust them with isolation properly.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 17, 2017, 12:09:47 pm
content: I was playing around with a CEM brand meter doing a fair amount of data logging with wireless. I would guess about 40 - 50 hours on a fresh battery.The problem I don't like with the AC is the isolation. You may very well get away with some cheapo adapter. I have no way of knowing what you are going to do with the meter. There are different transformers that have better isolation. You will see these listed as a medical grade parts. Also you may end up putting a hole in the meter to run the cord or mount a connector to which may be a leak point. Leaving the cover off may be a concern too. Really depends.Another option may be to just run it off a bigger battery. This is the pack I use for 9V. I leave the battery cover off when I use it. For the bench, its fine.

author: The Soulman
author_rank: Super Contributor
timestamp: February 17, 2017, 12:35:38 pm
content: Quote from: joeqsmith on February 17, 2017, 12:09:47 pmWhen measuring a 1000 Volts the internals of the entire meter (including the battery) could be at a 1000 Volt potential.What's the insulation resistance on those battery pack leads?Or on the electrical tape? (Yes I know whom I asking..)Best is to use rechargeable battery's or fit a dc/dc converter with a proper insulation rating inside the meter.Or if you fully understand the risk of something like Joe did AND only use the meter at low voltages (lower than 50) you should be ok.Same goes for external power adapters.

author: mdijkens
author_rank: Regular Contributor
timestamp: February 17, 2017, 12:50:50 pm
content: Quote from: SingedFingers on February 17, 2017, 10:54:08 amI have a whole box of thes wall wart transformers, but how can I determine for sure if they are switching or not?About isolation, what resistance should I measure at minimum to know they are isolated?

author: karoru
author_rank: Regular Contributor
timestamp: February 17, 2017, 12:56:37 pm
content: Quote from: The Soulman on February 17, 2017, 12:35:38 pmEven with dollar store-type electrical tape I had to lick it (to answer possible questions - not simultaneously with taking a reading) to get any reasonable insulation leakage on 1000V megger. Nowadays everything will work up to 1 kV, unless you soak it in water!:) Don't quote this as a safety norm though.

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 17, 2017, 12:58:43 pm
content: Quote from: mdijkens on February 17, 2017, 12:50:50 pmThe really heavy ones are usually linear ones. Most are switchers these days as they are cheaper to ship from China (not for any other reason!). I'd check continuity between earth and the jack shell. If it's open circuit then it's probably at least 1kv isolation and above. Again it depends on your particular isolation requirements but for low/medium voltage stuff that'd be fine.

author: tszaboo
author_rank: Super Contributor
timestamp: February 17, 2017, 01:00:43 pm
content: Quote from: mdijkens on February 17, 2017, 12:50:50 pmIt doesn't work that way. These wall warts dont have the required isolation distances and safety features to continuously operate isolated on high voltages. Meaning it will work at 10V or maybe at 230V and become charcoal at 300V.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 18, 2017, 12:01:59 am
content: Quote from: The Soulman on February 17, 2017, 12:35:38 pmRight, understand the risks.