Ground loop blowing dc-dc converter? - Page 1
Forum section: electronics / repair

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 12:11:32 am
content: Hi all,I've recently had a problem were I've had a couple of QFN-16 DC-DC converters blowing inside a microscope cameraand I'm thinking ground loop issue?The setup is.. Almost everything I'm powering is run from one wall outlet, which has 3 multi plug blocks connected to it with about 20 outlets available.. Don't panic.. Themax consumption is about 800W if everything is switched on (which it never is.) I've tested the house wiring with a socket tester and it shows thatall the Earths are good (also in the plug block extensions.)I have an HDMI lead from my computers capture card that goes off to the microscope camera, the camera is also powered from a separate 12v DC PSU. When the HDMIlead and the 12V DC connector are plugged in together and the microscope camera switched on, the DC-DC converter on the microscopes internal power supply failspermanently. After replacing 2 of them I knew something was amiss.I've tested AC voltage from the end of the HDMI lead where it would enter the back of the camera to the computers USB and sound ports etc, and I get about 4v Max and 3.5uA.So onto the 12v DC PSU... When testing the end of the DC connector back to the DC PSU's body.. .. On AC I'm getting 100v & 170uA ... And I can see the sparks.. This isblowing the DC-DC converter isn't it? I've tried some other laptop PSU's which do change the uA values a little, but not enough of a difference the warrants swapping the DC PSU.What can I do to remedy this issue please?

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 01:41:23 am
content: Quote from: orbiter on March 12, 2016, 12:11:32 amHiIt sounds like a faulty power strip or a faulty utility ground. It could indeed be a faulty ground pin on any piece of gear involved or a defective power cord. It's "call the electrician" time.Bob

author: Monkeh
author_rank: Super Contributor
timestamp: March 12, 2016, 02:04:46 am
content: Sounds more like an ungrounded adapter, not a fault.Replace the 12VDC supply with one with a three-pin inlet connector.E: Hang on.. why does the PSU have a metal body?

author: MickM
author_rank: Regular Contributor
timestamp: March 12, 2016, 02:09:28 am
content: I use a thing like this:http://www.amazon.com/Sperry-Instruments-GFI6302-Outlet-Tester/dp/B000RUL2UU/ref=sr_1_4?ie=UTF8&qid=1457748395&sr=8-4&keywords=electrical+testerThis one was the first hit for "electrical tester" on Amazon.It will show open grounds, reverse wiring etc.There is also a CFCI one.Mick M

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 02:11:54 am
content: Quote from: MickM on March 12, 2016, 02:09:28 amHiProbably not the right gizmo for the UK ...Right idea though.Bob

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 02:14:36 am
content: Quote from: Monkeh on March 12, 2016, 02:04:46 amIts actually a proper dc power supply im using albeit a cheap one, and its input is connected via live, neutral & earth.I tried the cheapo adapter that came with the microscope camera and that has the same effect, in that the dc-dc converter pops in the microscope camera once the hdmi cable is connected.Out of the many many devices I connect here daily its only the microscope camera that has an issue.Sent from my Nexus 7 using Tapatalk

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 02:19:52 am
content: Quote from: orbiter on March 12, 2016, 02:14:36 amHiOk, so if the only thing that has the problem is the microscope, then it's power adapter is defective. You either need one that is not improperly done or an isolation transformer to float if from your local utility connection. Since it appears to be defective, the transformer is not the ideal way to go. Since you have tried multiple adapters, its likely defective by design rather than by a manufacturing issue. The only real answer may be another brand of adapter (or microscope).Bob

author: Monkeh
author_rank: Super Contributor
timestamp: March 12, 2016, 02:22:42 am
content: Quote from: orbiter on March 12, 2016, 02:14:36 amRight, and is the ground output on the power supply earthed, or is it floating? Simply earthing it will likely resolve the issue.While you're at it, check the HDMI cable for the same issue (voltage to earth).You might also consider a little protection on the DC/DC input.

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 02:34:04 am
content: Its actually a proper dc power supply im using albeit a cheap one, and its input is connected via live, neutral & earth.I tried the cheapo adapter that came with the microscope camera and that has the same effect, in that the dc-dc converter pops in the microscope camera once the hdmi cable is connected.Out of the many many devices I connect here daily its only the microscope camera that has an issue.Sent from my Nexus 7 using TapatalkRight, and is the ground output on the power supply earthed, or is it floating? Simply earthing it will likely resolve the issue.While you're at it, check the HDMI cable for the same issue (voltage to earth).You might also consider a little protection on the DC/DC input.Its floating.. IIRC Its an 80w DC power supply.. Metal cage type from china.. Similar to this...https://www.google.co.uk/search?q=12vdc+100w+power+supply&prmd=sivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjGgpiIkrvLAhVCnRoKHV3zBeYQ_AUICCgC&biw=600&bih=960

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 02:37:20 am
content: Quote from: orbiter on March 12, 2016, 02:34:04 amHiIf it's truly floating, then it should be no problem. It should not pop or bother anything else.Best guess: it's not grounded *and* it has voltage with respect to ground. That would cause things to pop. Easy check: measure the voltage between case and ground. For the brave, switch to current and measure the *current* between case and ground (only do this if you have spare meter fuses in stock).Bob

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 02:41:38 am
content: Monkeh...HDMI cable to computer chassis etc is ok... 3v Ac and 3.5uAWhat is advisable for dc-dc input protection?Sent from my Nexus 7 using Tapatalk

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 02:44:30 am
content: Bob... Those details are in the first post my friendSent from my Nexus 7 using Tapatalk

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 02:51:07 am
content: Quote from: orbiter on March 12, 2016, 02:44:30 amHi... and thus the comment .. quite a ways up ... it's defective by design.Bob

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 02:56:57 am
content: Could you perhaps suggest a solution please for the dc-dc converters protection?

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 03:11:09 am
content: Quote from: orbiter on March 12, 2016, 02:56:57 amHiStrap all the points that *should* be ground together. As long as your current numbers are correct this should not be a problem. If at turn on or turn off, you have a spike ... be careful. If your DC/DC converters are suspect, put a nice big zener diode on each of the to absorb any surge.Bob

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 03:19:17 am
content: Thanks Bob..The dc-dc converter I fitted is new, from Farnell.. I've bought a few of them due to this issue.. I actually thought I'd somehow killed the microscope camera but nope... Just the tiny lil' qfn converter.The grounds all meet back at the single wall socket.. Is this what you mean by tied together?

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 03:23:27 am
content: Quote from: orbiter on March 12, 2016, 03:19:17 amHiIf things are popping and sparking, then *something* is wrong with the way they are connected. Are things still popping and sparking?Bob

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 03:29:45 am
content: Whats popping (failing) is just the tiny dc-dc converter.. As for the sparks these are tiny arcs i can see in the dark when testing between the 12v dc psu chassis and the 12v output with my sharp dmm probes against psu chassis.I can also feel the ac tingle on my forearm if i touch it with both the HDMI & 12 dc connector.I cannot test right now Bob.. Its 3.30am here and im in bed, so that stuff will have to wait till morning

author: uncle_bob
author_rank: Unknown
timestamp: March 12, 2016, 03:36:16 am
content: Quote from: orbiter on March 12, 2016, 03:29:45 amHiThe ground of the HDMI *should* be solidly connected to the ground of the DC/DC converter and to the ground of the microscope and to the ground of the PC. Everything you have done suggests that the power mains plug connection is not accomplishing this. Simply plugging things into the strip connects the ground on the plug. That does not appear to actually ground the various components together. This suggests that either you need to fix the missing "ground on the plug" to "ground on the device" connection *or* put in another wire to make the ground connection to another grounded object (like the PC chassis).Bob

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 03:49:01 am
content: The hdmi lead to computer is ok bob if not connected to microscope & 12v dc. It tests fine giving me only 3v ac and 3.5uA ac between end of the hdmi lead & mains earth. If I measure computer chassis to mains earth i get the same readings. Only when 12v dc connector and hdmi lead are connected to microscope camera does the issue occur..Will do some more testing tomorrow and come back to you.Thank you for the help & advise so far, its much appreciated.

author: Monkeh
author_rank: Super Contributor
timestamp: March 12, 2016, 12:14:50 pm
content: Holy crap, the quote walls. Stop that, please.What you are seeing is capacitive coupling onto the output of your DC supply. Connect the V- terminal to earth at the supply terminals.

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 01:12:38 pm
content: Thanks monkeh.. I'll do that a bit later on.So is this due to the psu's being rubbish.. In that the capacitive coupling is coming across from the ac side of the psu.. through the suppresion capacitor onto the dc side.. Thus giving ac on the dc side output?

author: TurboTom
author_rank: Super Contributor
timestamp: March 12, 2016, 01:37:12 pm
content: Orbiter -it's common to on SMPS to have an RF low-impedance path from primary to secondary in order to reduce RFI. This is usually a ceeramic capacitor of a few nanofarads, rated ad tested to several thousand volts (since it's safety critical). This is usually placed between primary DC rail positive or negative and output common (GND). Hence you can measure about half the line voltage with a high-impedance AC voltmeter and a tiny current in AC amps mode. Nothing wrong with this and the equipment should easily be able to cope with this.I would rather relate your findings to something being wrong either with the HDMI port on the camera or on the computer. If you check the HDMI pinout, there's a 5V supply line on pin 18 specified there, maybe somehow current is flowing from the camera to the PC (since the 5V line voltage on the PC may be slightly lower than in the camera), thus overloading the DC/DC converter and damaging it. I would try to modify an HDMI cable with this line disconnected.Cheers and good luck,Thomas

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 01:50:15 pm
content: Thanks for that info Tom.. However I think it more likely that the AC im seeing between the DC socket and mains earth would be from the microscopes dc power supply. The only reason i say this is that my computer has a very good 1200W Enermax Revolution PSU which I would think far superior to a china DC PSU. Plus loop testing between the HDMI cable and mains earth shows only 3v ac and 3.5uA.Its a good idea to mod the HDMI cable though as I only need perhaps its GND and signal connections.

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 02:19:24 pm
content: Appologies.. The threads multi quotes were of my doing.. Tapatalk didn't show that mess up.. Anyway, removed the ones I was able to now.Perhaps you could do the same with your quotes too please Uncle_Bob, just to tidy thread up a bit

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 12, 2016, 02:56:34 pm
content: Have you considered using Clip-On Ferrites on the cables?I would think it is possible that there is equipment connected to your computer that acts like an antenna and picks up all kinds of stuff that is causing a voltage between the HDMI-Cable shield and Protective Earth.When I had a similar problem with my Xbox One and my HDMI-Capture Card, I could almost completely solve the problem by using a VGA-Cable that had big ferrites on it. Using the HDMI-Cable from the PC to the Monitor -> Color-Artifacts, Video-Artifacts, complete collapse of the video-stream. Same symptoms if I touched the PC-Case btw.

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 03:37:49 pm
content: Ok so far today... On the 12v DC microscope PSU I've bridged V- and Mains Earth temporarily .. But unfortunately it made no difference.. Getting 100v ac & '240uA' ac today between PSUDC output and it's chassis.I then did what I should have done initially really, & disconnected everything from the wall socket apart from the 12vDC power supply that powers the microscope camera. Guesswhat? The problem immediately disappeared... Current between the output of the 12DC power supply and it's chassis went from 240uA down to 2uASo that rules out the mains earthAND a fault with the DC Power supply I guess.So now I've just gotta track down something with an earthing issue.@ SaabFAN.. Yes there are large ferrites on all the power cables thanks.I'm going to leave this thread hanging for a few days now guys just whilst I do some tracking down ok, I am disabled ya see so crawlingaround whilst plugging and unplugging everything in a small space, then testing etc will take me some days.I do thank you all very much for the help and advice so far though, I've picked up some valuable tips whilst trying to discoverthe issue here

author: orbiter
author_rank: Frequent Contributor
timestamp: March 12, 2016, 08:07:39 pm
content: Just an update guys... Quite embarrassing but I discovered the grounding issue...It was a faulty 12v DC adapter.. However NOT the one I thought it was initially. The microscope camera was being powered by a totally different adapter thats wires went out ofsight around the back of the bench and through a slight birds nest of wires. I'd been following and testing what I thought was the correct DC jack but it was from adifferent & faulty DC power supplyAnyway the microscope camera is operating perfectly now, and on the correct PSU too.. Only a very slight current flows now between the DC jackand it's PSU chassis .. 1.9uA as opposed to 170uA +The troublesome psu lives no moreThanks guys