Is the Jaycar 'MP3840' power supply actually mains isolated?  How can I test it? - Page 1
Forum section: electronics / projects

author: EvilSOB
author_rank: Newbie
timestamp: August 29, 2020, 04:52:49 am
content: I have a Jaycar 0-30V bench supply as seen in the below link.https:/ /www.jaycar.com.au/0-to-30vdc-0-to-5a-regulated-power-supply/p/MP3840Can anyone in the know tell me if it is actually FULLY mains isolated?Or is there a way I can SAFELY and SURELY test it myself?I dont remember if it 'said' it was, but Im not likely believe advertising blurb anyway...I ask because I have a 'USB Oscilloscope' that connect to my home PC, and I want topower a project with the above bench supply and test it using my Oscilloscope,WITHOUT frying my PC or my Oscilloscope...Mains isolation has always been an area I cant seem to grasp, so I want to play itvery safe, so I may be being overly careful, but I can live with that...Thanks Guys

author: bob91343
author_rank: Super Contributor
timestamp: August 29, 2020, 05:25:25 am
content: First you measure the resistance between the output of the power supply and the input mains (disconnected from power of course). It should read very high resistance, thousands of megohms. Also measure between input power leads and chassis, again thousands of megohms. (Ordinary test equipment cannot measure this high resistance.)Open the unit and inspect the primary wiring. It should have only a power switch and fuse and transformer primary connections. Sometimes a neon pilot lamp.The hard part is if there are other components such as capacitors. Generally speaking, they won't compromise isolation unless they are leaky. In older units there are deliberate connections between the mains and chassis but the most common resulted in about 3 Megohms from power line to chassis.You can also use a hypot tester between mains and output but be careful not to use too high a voltage else you may break down insulation.If you have a schematic diagram you can see immediately if it's isolated.

author: tombi
author_rank: Regular Contributor
timestamp: September 03, 2020, 01:32:13 pm
content: Also it is current limited so can’t you just turn the current limit right down (100mA day) and then short positive to ground? If there is still a voltage between positive and negative it is floating. Now try negative.I have the older model and it has a floating output.

author: Circlotron
author_rank: Super Contributor
timestamp: September 04, 2020, 02:13:34 am
content: What do you mean when you say FULLY mains isolated? No connection to the mains at all, that is, isolated by a transformer, or floating outputs as well. Seems a bit overpriced. I got one with similar rating off eBay for AUD$55.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 04, 2020, 03:12:53 am
content: If you buy direct from china, there is no quality control, or at best it's "unknown".When buying stuff locally that is gone through "normal procedures" you can be reasonably sure that it at least confirms to some standards.If you short both pins of the mains input plug, and then measure resistance between the primary and secondary side, you should measure Lot's of Meg Ohms, If it fails this test, then, well it failed.If it passes this test, it does not mean much. A real test is done with a "Megger", which applies several kV between the primary and secondary side (3kV, 5kV or even 10kV, depending on standards etc.)Small transformers often have a plastic spool with 2 chambers, and the primary and secondary windings are physically separated. Bigger transformers often have the windings over each other, with an extra layer of insulation material in between.My advise it to open the box and look what's inside. This is an electronics forumNote that due to physical closeness of the primary and secondary windings, there is some capacitive coupling. Transformers specified for "medical" use have:* Extra high isolation voltage.* Low capacitive coupling between primary and seundary.* made in such a way that if they fail, they fail in a safe way.Watch some youtube vid's about transformers, or take some old transformers apart if you're curious.

author: Circlotron
author_rank: Super Contributor
timestamp: September 04, 2020, 03:31:02 am
content: My power supply, I always use it with one side of the output, usually negative, grounded to the case with the shorting link. I checked incoming mains ground is properly connected to the metalwork. I feel that’s the safest way with power supplies of unknown pedigree. Hopefully it would reduce common mode output noise as well.