Connecting secondary winding 0v to mains earth in the uk - Page 1
Forum section: electronics / beginners

author: multime
author_rank: Regular Contributor
timestamp: August 09, 2024, 09:24:28 pm
content: I remember in the eighties and nineties that it was common practice to connect the 0v of a transformer secondary to mains earth. A lot of the power supply project in various publications recommended doing this at the time, is this the same advice today.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 10, 2024, 05:09:21 am
content: For this to be answered, please show your intended application.For class 1 applications definitely ground the core of a laminated transformer.Also ground the electrostatic screen, if present.Grounding the secondary winding/s is application specific.X

author: multime
author_rank: Regular Contributor
timestamp: August 10, 2024, 03:59:22 pm
content: Its a variable 0-40v bench power supply i am building using 300va toroidal transformer with 2 x 15v seconderies

author: IanB
author_rank: Super Contributor
timestamp: August 10, 2024, 04:10:29 pm
content: Bench power supplies usually have floating outputs. Grounding is left at the user's discretion depending on application.

author: tooki
author_rank: Super Contributor
timestamp: August 10, 2024, 05:12:45 pm
content: Quote from: multime on August 09, 2024, 09:24:28 pmI don’t recall that being the case back then. And I’ve read a lot of magazines from that era in recent years, and I haven’t noticed any such general recommendation, either.But for a bench power supply you definitely don’t want to do this!

author: Xena E
author_rank: Frequent Contributor
timestamp: August 10, 2024, 07:35:44 pm
content: Quote from: multime on August 10, 2024, 03:59:22 pmIf by that you intend to connect the two secondary windings in series then rectify with a four diode bridge,do notconnect any part of the transformer secondary to ground.If this is a bench PSU and in some instances find you need to ground one side of the PSU output, then provide a separate binding post for the purpose... you do have to be aware of the pitfalls and dangers of doing this if the supply is connected to other equipment, you could easily cause a ground loop or worse, a dead short.Regards,X

author: multime
author_rank: Regular Contributor
timestamp: August 10, 2024, 10:12:42 pm
content: Thanks for the advice and info everybody its appreciated. I have done this grounding connection in the past on advice given at the time but times change

author: tooki
author_rank: Super Contributor
timestamp: August 10, 2024, 10:31:39 pm
content: Quote from: multime on August 10, 2024, 10:12:42 pmNot a “times change” issue, it’s as situational now as it was 40 years ago.

author: Andy Chee
author_rank: Super Contributor
timestamp: August 10, 2024, 10:57:03 pm
content: Here's a random power supply construction project from the 90s.Note that earth/ground is connected to a binding post only, as well as the transformer core, and heatsink rear metal panel (the front metal panel has an erroneously omitted earth connection).The circuit is floating and has NO connection to earth/ground. This is notated on the schematic by the use of different earth/ground symbols.

author: radiolistener
author_rank: Super Contributor
timestamp: August 11, 2024, 10:21:19 am
content: Quote from: multime on August 09, 2024, 09:24:28 pmGood quality products have GND line connected to Earth. For example, oscilloscope BNC connector GND is connected to mains socket Earth terminal. But some cheap PSU don't have Earth terminal on the mains socket and can damage your electronics if you connect some equipment together at powered state.If you see this symbol on your equipment, this is a bad sign...it don't have Earth terminal

author: Siwastaja
author_rank: Super Contributor
timestamp: August 11, 2024, 10:35:02 am
content: As always, ignore radiolistener.--For a bench supply, you get best flexibility by not connecting, but add a PE post next to the negative post so that user can easily bridge them together whenever they want to do that.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 11, 2024, 10:44:00 am
content: Quote from: radiolistener on August 11, 2024, 10:21:19 amThis is irrelevant to the OP, it's not what he asked.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 11, 2024, 10:50:30 am
content: Quote from: radiolistener on August 11, 2024, 10:21:19 amThat is silly. There is plenty of good quality double insulated equipment, and there are plenty of applications where this "good enough" or even preferred above grounded equipment.Connecting the secondary of a transformer to protective earth is not done for low voltage transfromers. If you want a lab power supply, the secondary side is usually floating. As a result you can put multiple supplies in series, and then call any of the three terminals "ground", so with two 15V power supplies you can use it for either +30V + / - 15V or -30V. This would not be possible if the secondary winding would be connected to protective earth. Such a connection would also create strange results if the secondary has a bridge rectifier.Connecting the secondary of a transformer to PE is sort of common for high voltage transformers. Your microwave oven which has 2kV output probably has it's secondary winding directly connected to PE. As a result, that side of the transformer is guaranteed to have a low voltage, and the high voltage with it's several kV potential is the only side that needs thick insulation, warning signs etc.

author: multime
author_rank: Regular Contributor
timestamp: August 11, 2024, 08:51:12 pm
content: Here,s a confusing schematic from a Velleman psu using the same symbol for earth as the 0v line

author: ledtester
author_rank: Super Contributor
timestamp: August 11, 2024, 09:20:36 pm
content: The article from Maplin, October 1992 may be found here:https://archive.org/details/electronics-the-maplin-magazine/Maplin-Electronics-1992-10-058/page/40/mode/2upOn page 43 there is a wiring diagram (figure 4) showing how the earth connection should be wired:https://archive.org/details/electronics-the-maplin-magazine/Maplin-Electronics-1992-10-058/page/42/mode/2upOn the next page there is a wiring diagram (figure 7) for the L and N terminals of the power inlet - but there is no additional wiring shown for the E terminal.It appears E is just meant to be a chassis ground and perhaps they didn't have an alternate ground symbol to use for it.

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2024, 09:33:20 pm
content: Quote from: radiolistener on August 11, 2024, 10:21:19 amThis is 100% nonsense. Ignore this.

author: radiolistener
author_rank: Super Contributor
timestamp: August 11, 2024, 10:15:06 pm
content: Quote from: tooki on August 11, 2024, 09:33:20 pmYes sir! I'm already running to execute your order!You so often like to give orders to others what they need to do and so sure that you are 100% right.Are you by any chance a retired military commander from some dictatorship regime?

author: Xena E
author_rank: Frequent Contributor
timestamp: August 11, 2024, 10:48:22 pm
content: Quote from: multime on August 11, 2024, 08:51:12 pmIt doesn't show an earth symbol at all.It shows the incoming protective earth connection connected to chassis.In this case neither is directly connected to anysecondary winding on the transformerThe topic title being:QuoteCorrectly drawn, if included the standard symbol for a protective ground should only appear once on an electrical diagram if there is only one electrical connection to it.The vellman kit only shows the electrical connector into which the mains cord is plugged therefore is correct not to show the earth symbol.

author: IanB
author_rank: Super Contributor
timestamp: August 11, 2024, 10:57:19 pm
content: Quote from: Xena E on August 11, 2024, 10:48:22 pmThere's a problem, because if you follow the symbols on the diagram, it looks like the negative side of the rectifier is common with the mains earth connection. If this was not intended, they needed to use a different symbol for the two terminations circled in red:

author: Xena E
author_rank: Frequent Contributor
timestamp: August 11, 2024, 11:03:09 pm
content: Quote from: ledtester on August 11, 2024, 09:20:36 pmAgain, the earth symbol is not shown.It only explicitly shows the incoming mains connection.Maplin were careful to show the protective earthing connections explicitly within your first diagram even as far as the front panel binding post that is included for the user to either connect where they need to, or not, as the situation requires.Sorry. However anyone not understanding these nuances, should not be using the equipment, less still trying to build it.Regards,X

author: radiolistener
author_rank: Super Contributor
timestamp: August 11, 2024, 11:07:18 pm
content: Quote from: IanB on August 11, 2024, 10:57:19 pmThis is not a mistake, this is how it's implemented in a usual power supply with Earth terminal on the mains socket.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 11, 2024, 11:08:59 pm
content: Quote from: IanB on August 11, 2024, 10:57:19 pmQuite true.I'm not familiar with the design but I'm assuming that it wasn't intended by Velman to be a floating supply, although it wouldn't cause a problem with an earthed oscilloscope you couldn't float it with that connection that you point out.In any case that's bad practice.Edit:And again it doesn't show the earth connection on thesecondary winding of the transformerwhich as I've pointed out was the topic starter.

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2024, 11:15:19 pm
content: Quote from: radiolistener on August 11, 2024, 10:15:06 pmNope, just one of the many people here who recognizes that you often state things that are completely wrong. So when you do, we point this out so that others, like OP, know that it’s wrong.

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2024, 11:17:34 pm
content: Quote from: Xena E on August 11, 2024, 11:08:59 pmIn the article, they show the front panel of the PSU, which has only two binding posts for the output, + and -, and the - one also has the earth symbol on it. So it’s expressly labeled as being mains earth referenced.

author: radiolistener
author_rank: Super Contributor
timestamp: August 11, 2024, 11:19:25 pm
content: Quote from: Xena E on August 11, 2024, 11:08:59 pmI understand your complaint about the phrase earth connection to the secondary coil of transformer, I also came across this phrase. But technically secondary coil is galvanically connected to the earth through diode bridge on this schematic.

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2024, 11:20:22 pm
content: Quote from: radiolistener on August 11, 2024, 11:07:18 pmWell, it’s actually quite uncommon on lab power supplies.Many, many power supplies, both old and new, are floating. So you can’t say it’s “usual” to be mains earth referenced. (“Usual” means “most of the time”, not just “sometimes”.)

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2024, 11:21:26 pm
content: Quote from: radiolistener on August 11, 2024, 11:19:25 pmNot the same thing. Being connected through a diode bridge is very different from being connected directly.

author: radiolistener
author_rank: Super Contributor
timestamp: August 11, 2024, 11:24:12 pm
content: Quote from: tooki on August 11, 2024, 11:21:26 pmFrom safety point of view it is the same as direct connection. There is no galvanic isolation between Earth terminal and circuit ground.

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2024, 11:25:26 pm
content: Quote from: radiolistener on August 11, 2024, 11:24:12 pmSo what? That wasn’t the question.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 11, 2024, 11:30:25 pm
content: I think the OP needs to post the circuit what he intends to build and then ask for advice.We are arguing over assumptions and semantics here.Regards,X