Ground pole in a DIY bench power supply from scratches - Page 1
Forum section: electronics / beginners

author: Francesco Gozzo
author_rank: Contributor
timestamp: June 10, 2018, 02:29:41 pm
content: Goodmorning everyone, I’m new here so hope to write this discussion in the right place, I’m also new in electronics but I thought it was time to start learning some serious electronics apart from arduinos and similar.I would like to know where the ground output plug in the front panel should be wired in a bench power supply made from pre existing boards and supplies. Sould it be wired to the ground pole of the input ac power, should it be wired to the metal case of the wired power supply or somewhere else?Would also ask what are the main advantages of having the ground output plug other than allowing the connection in sequence of two power supplies.Thank you very much for your attention.Again sorry for the lacks in knowledge and terminology. Hope to fill them with some serious study of books and experience.

author: rstofer
author_rank: Super Contributor
timestamp: June 10, 2018, 03:50:01 pm
content: The green ground jack should be connected to earth ground. So should any exposed metallic surfaces. So, bring the earth ground inside the cabinet and bolt it to the chassis. Then run a wire from the jack to the same place.Look inside a piece of commercial equipment and see how they did it. Look at some of Dave's 'tear down' videos. He always goes over the grounding.

author: Nusa
author_rank: Super Contributor
timestamp: June 10, 2018, 05:26:39 pm
content: The ability to put power supplies in series requires that the power supplies be properly isolated from their power source.Proper Lab/bench supplies usually are properly isolated. Whether or not you make use of the ground terminal is irrelevant.PC and generic device supplies typically are not. Do not try to put them in series.

author: Francesco Gozzo
author_rank: Contributor
timestamp: June 10, 2018, 06:04:36 pm
content: Quote from: Nusa on June 10, 2018, 05:26:39 pmThank you very much for your help! May I ask what kind of insulation you mean?Sent from my iPhone using Tapatalk

author: Francesco Gozzo
author_rank: Contributor
timestamp: June 10, 2018, 06:10:19 pm
content: Quote from: rstofer on June 10, 2018, 03:50:01 pmThank you!! Sure I will! I found a picture of a quite cheap power supply and noted the wiring you say but being unbranded I preferred to ask here to be sure. Thank you again!Sent from my iPhone using Tapatalk

author: Ian.M
author_rank: Super Contributor
timestamp: June 10, 2018, 06:34:38 pm
content: Additionally, by design, most isolated switched mode power supplies (SMPSUs) have relatively high line frequency leakage currents from their primary to secondary side - 0.5mA isn't exceptional, driven by a potential of half the RMS mains supply voltage, so easily enough to blow the gate oxide of any MOSFET that isn't protected by clamping diodes, and damage sensitive bipolar devices.Its is *possible* to design a SMPSU that doesn't have a high leakage current but it increases the cost significantly so you'll only find them in special applications.That means that if you are assembling your own bench power supply and you are starting with a SMPSU to get the DC bus to feed your voltage and current regulator circuit, if you don't want to risk blowing sensitive load circuits, you probably shouldn't use it to build a floating output PSU.Additional to rstofer's comments on bolting supply ground to the chassis or cabinet, that bolted connection must be 100% reliable. It has to be able to carry a fault current sufficient to fuse or trip the supply over-current protection on *ANY* mains supply it may normally be connected to, and must not become disconnected (e.g. due to solder melting) while carrying the fault current before the over-current protection activates. For these reasond its generally preferable to use a crimped ring terminal on the grounding point, not a soldered one. Any soldered connections should pass the wire through an eye on the terminal so it cannot easily come loose, and be covered by heatshrink so it stays in place long enough to activate the over-current protection even if it overheats. Also the grounding point should be a dedicated screw or bolt, preferably not capable of being loosened from outside the case, and should not be a mechanical fixing for any other part.

author: Francesco Gozzo
author_rank: Contributor
timestamp: June 10, 2018, 08:37:56 pm
content: thank you, it found a lot to read online thanks to your reply. The power supply i'm triyng to assembly is composed by a Transformer feeding 24v AC to a kit board shown in link below that as i can tell from my researches is a "linear" power supply (Not 100% sure about that).Is there the possibility to reduce those risks from current leackage by usign:- a linear power supply with floating ground instead of a SMPS?- a SMPSU with a proper ground instead of a floating one?If thats correct i understand it would be an advantage of using a ground connection to return the power from say the hypotetical breadboard circuit.if this is one of the advantages would be helpful in the comprehension because searching around i can't find advantages of grounding plugs in psu other than security in case of fault(Just to clarify, as what i have researched a floating ground device is the one that leaks of the ground connection returning to the wall gound just having a positive and negative plugs, am i wrong?)https://www.tomtop.com/it/p-e0329.html?currency=EUR&Warehouse=CN&aid=gplaitcd&mid=10000018478&utm_source=SEM&utm_medium=Google+PLA&utm_campaign=TT_PLA_CD&utm_content=3707&gclid=Cj0KCQjwmPPYBRCgARIsALOziAN9cYAtXOUQHr_9bz6WnX7AVEBkBoM65Qpcu-UYfL98niw_q40tLTIaAi0EEALw_wcB

author: David Hess
author_rank: Super Contributor
timestamp: June 10, 2018, 10:40:08 pm
content: Distinguish "ground" from "common".The ground post should be internally connected to the power supply chassis and the AC power ground.For a floating power supply, the ground post then may be connected externally to any one of the floating output posts. For a bipolar supply with three posts, one will be "common" between the two outputs.

author: Francesco Gozzo
author_rank: Contributor
timestamp: June 10, 2018, 11:26:09 pm
content: Thank you for the clarificationAfter a while I finally found something and would like to have feedback from you.An advantage of having separate ground plug consist in the fact that the power supply itself is completely insulated from the ground, as you suggested,and this allows the psu to be connected in sequence with another with a voltage amount not over their output?Sent from my iPhone using Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2018, 02:25:07 am
content: That is right although the voltage limit between "ground" and "common" or one of the floating outputs is usually something like 50 volts.

author: Francesco Gozzo
author_rank: Contributor
timestamp: June 11, 2018, 11:15:13 am
content: thank you for your help it has clarified a lot on something i have not been able to properly understand alone. I feel sorry if this subject is really a basic one compared to the other beginner ones .