Bench Supply Ground - Page 1
Forum section: electronics / general

author: Slowmo
author_rank: Newbie
timestamp: December 04, 2013, 04:29:35 am
content: Got one of the cheapest power supplies for a very simple task and nearly damaged my scope and circuit.http://www.mastechpowersupplies.com/variable-regulated-power-supply-hy1803d.htmlThis PSU does not have a ground terminal, but it is not "floating". The thing is that positive terminal is grounded and I found it out while connecting ground lead of scope to my circuit's ground. I had set the PSU to very low current so fortunately nothing bad happened.So, is it normal that positive terminal is grounded? Can I modify the power supply by "ungrounding" it? Usually bench supplies are floating anyway with a separate ground terminal.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 04, 2013, 07:17:32 am
content: depends, open it up, look where the power comes in.Does the power essentially just come in, go through a fuse and switch and then into a transformer?

author: Dawn
author_rank: Regular Contributor
timestamp: December 04, 2013, 04:26:40 pm
content: Your profile doesn't indicate what country you're in, but I assume from the ad that you're in the states. Before blaming the power supply, have you checked your outlet? Many older houses in the states, DIY wiring, and upgrades rarely paid attention to the proper polarity of the hot and neutral, especially on two wire outlets as it meant very little to average users and could go unoticed for decades. That's been a forever headache in the states with hot chassis.SOP whenever we went to field locations was to use one of those little testers to make sure the outlet was wired correctly when control stations or remotes were installed. Even in relatively new offices, the figure was probably 30% that either had flopped hot and neutral or an open ground. It's been demonstrated here that especially on some Chinese knockoff products, the wiring is at fault, so it's a possibility too. Best advice is to be wary, check everything, and assume nothing.It wouldn't be a bad idea to rig up a standard ground fault probe test as part of your bench and a cheap 500ua or so meter movement in a box and make a test probe out of an exhausted, stogie sized magic marker with a momentary test button and series standard resistor for leakage current. Plenty of examples on the net for a standard circuit referenced to a cold water pipe or ground rod ground. You'd be very surprised how many exposed metal parts on older equipment fail the minimum standards for leakage due to insulation breakdown or leaky caps. A surplus isolation transfromer as part of your bench is a good idea too. Prices of 4A variac/isolation transformers made by Sencore and BK have dropped into nothing recently, often under $100 for a perfectly working unit. I bought one for $28 in working shape. Nothing goes on my bench that connects to the mains without a safety test first.

author: SLJ
author_rank: Frequent Contributor
timestamp: December 04, 2013, 05:09:16 pm
content: They have a forum to ask questions:http://www.mastechpowersupply.com/forum/index.php

author: Slowmo
author_rank: Newbie
timestamp: December 04, 2013, 06:57:02 pm
content: No, I'm from Europe. The ad was the first link that popped up while searching for the model.European plugs can be plugged in both directions. I did a quick continuity test and discovered that the positive terminal is connected to ground. Ground and neutral wires are connected together in a fuse box where the power cable enters my house.By the way, Dave has a video about this subject:So almost always the negative line is grounded. And because Dave said it does not have to be the negative line, Chinese listened to it and grounded the positive line of my power supplyI will take it apart and it should probably be quite easy to remove the grounding form the secondary side.

author: sync
author_rank: Frequent Contributor
timestamp: December 04, 2013, 07:19:45 pm
content: Quote from: Slowmo on December 04, 2013, 06:57:02 pmI think your unit is faulty. It should be floating.

author: Slowmo
author_rank: Newbie
timestamp: December 05, 2013, 12:49:57 am
content: Yes, you were right. It was kind of suspicious that positive line is grounded and I found out exactly why it was.The screw holding the metal cover is rubbing against thick power trace which happens to be our positive line. Why couldn't they put the screw hole a little lower and why is power trace extended all the way to the back of the board. I don't see a point in that as no electrical connection is made at that end (that is if we don't take into account the screw).

author: SLJ
author_rank: Frequent Contributor
timestamp: December 05, 2013, 01:19:38 am
content: Quote from: Slowmo on December 05, 2013, 12:49:57 am"Got one of the cheapest power supplies"Wack that end of the run off. Looks like you could just cut a slot in the trace.

author: Slowmo
author_rank: Newbie
timestamp: December 05, 2013, 05:04:31 am
content: I put a thick patch of electrical tape where the bolt is touching the trace. There's still warranty, so I'd better not be cutting any traces for now.

author: SLJ
author_rank: Frequent Contributor
timestamp: December 05, 2013, 01:20:56 pm
content: The least you should do is tell them they have a defective product design and show them.But then again an importer of this crap probably doesn't care as it will cost them to send them all back.

author: JoeO
author_rank: Frequent Contributor
timestamp: December 05, 2013, 02:49:50 pm
content: Looks like the manufacturer has a design and test issue to fix.

author: SLJ
author_rank: Frequent Contributor
timestamp: December 05, 2013, 03:02:11 pm
content: Maybe going to their forum isn't such a good idea:"Dear All,Due to excessive number of spammers, we have to restrict the forum access. If you need to post anything, please register on the forum and send us an email to support@mastechpowersupply.com. We will manually activate your membership so that you can post on the forum. we apologize for the inconvenience.Best regards,Admin"Send them an email showing the problem. support@mastechpowersupply.comLet's see if and when they respond.

author: rsjsouza
author_rank: Super Contributor
timestamp: December 05, 2013, 03:33:01 pm
content: I had a similar problem with mine (checkhere) and blew up a PC motherboard due to this problem.Once you get burned one time, you will never again take for granted that grounds are at the same potential... I test everything now.

author: Slowmo
author_rank: Newbie
timestamp: December 05, 2013, 05:37:30 pm
content: Ok, sent a short e-mail to support describing the problem. Lets see what they have to say if they reply at all.

author: con-f-use
author_rank: Unknown
timestamp: December 05, 2013, 09:07:05 pm
content: Quote from: Slowmo on December 05, 2013, 05:04:31 amScrews have relatively sharp edges that rotate (occasionally). Don't think the electrical tape is gonna cut it (more the other way around). Why don't you glue something more sturdy on top of the trace?

author: ConKbot
author_rank: Super Contributor
timestamp: December 09, 2013, 06:18:18 pm
content: Offtopic, but if you use that supply to charge a battery, unhook the battery before you turn off the supply. If you dont, it blows up the output transistor because they didnt put a reverse diode from output to input in the supply. Or find a convenient spot to bodge in a beefy diode backwards across the output transistor.

author: macboy
author_rank: Super Contributor
timestamp: December 10, 2013, 04:07:24 pm
content: Quote from: ConKbot on December 09, 2013, 06:18:18 pmThis doesn't need to be a beefy diode, any little 1N400x will work. Any current through it should be quite small.A reverse-biased diode across the output, however, should be rated to handle the output current of the supply.

author: SLJ
author_rank: Frequent Contributor
timestamp: December 10, 2013, 08:03:50 pm
content: Quote from: macboy on December 10, 2013, 04:07:24 pmWhy wouldn't it need to be rated to handle the output current of the battery?

author: headwes
author_rank: Newbie
timestamp: December 10, 2013, 08:38:44 pm
content: It looks like the screw is quite a bit longer than it needs to be. Why not just shorten the screw?

author: minime72706
author_rank: Frequent Contributor
timestamp: December 13, 2013, 05:01:07 pm
content: An Amazon.com review from just over two years ago echoes your aforementioned issue:QuoteCould you try measuring the voltage yourself? Forgive me if you have already - I did not read the whole thread yet. Assuming it behaves the same when turned off, you might also measure the resistance; however, if it gave a spark, I imagine it's rather low ... unfortunately.Yeah I saw this particular supply at Elexp.com, my main local mini-distributor (mainly for education), and it frightened me for some reason. Make sure to get a power supply with both a (-) terminal (black) and earth ground terminal (green) so that you have the option of having the supply float or not.I'm kind of interested in playing with it because I bet you can fix it. It just seems absurd that this would happen since it has a transformer. EDIT: I skipped over the photo with the screw initially. I get it now ... still just as absurd.

author: TheBay
author_rank: Unknown
timestamp: December 14, 2013, 03:32:46 am
content: That's a really bad design! Who thought to put a screw there, I wonder if it's a genuine Mastech or a clone with different internals?So no physical earth on that metal chassis PSU? does it have any form of floating ground?

author: hofig
author_rank: Newbie
timestamp: December 14, 2013, 08:32:26 pm
content: My unit seems similar but due to toleranses it is barely not making contact.Cutting trace, adding some insulator on pcb, shorter screws and couple washers to raise the pcb a bit should solve the problem. (Or drilling and tapping a hole a bit lower)The problem may not be too obvious when you open it up and there is no screw installed.