Connecting Center Tap of transformer to earth mains - Page 1
Forum section: electronics / beginners

author: ghinckley68
author_rank: Contributor
timestamp: April 25, 2016, 07:39:10 pm
content: Any problems doing?

author: KMoffett
author_rank: Regular Contributor
timestamp: April 25, 2016, 07:42:59 pm
content: Why do you want to do that? Please post a drawing of how you are thinking of connecting this transformer...primary and secondary.Ken

author: vk6zgo
author_rank: Super Contributor
timestamp: April 27, 2016, 03:37:48 am
content: It was common practice with tube type fullwave rectifiers to connect thesecondarycentre tap to the equipment chassis,& hence to the Mains Earth.If you,on the other hand,wish to connect the centre tap of theprimaryto Mains Earth,that is not a good idea.In the case of a single phase supply with the Neutral & Earth connected together at the building entry point,you are in effect,shorting out one half of your primary,& applying the full Mains potential to the other half.If you have an earth leakage detector (RCD in Oz speak),that will operate & turn everything off before you can cook your transformer.if you are using the strange North American split phase 240v system,it may be OK,providing both sides of the centre tappedpoletransformer are perfectly balanced.If not,your RCD will again operate.It is unnecessary,& probably forbidden by your regulatory authority.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: April 27, 2016, 04:20:08 am
content: What he said!

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 27, 2016, 06:16:35 am
content: There is no answer to your question as an abstract concept.Certainly there are cases where connecting a transformer center tap to ground/earth makes excellent sense. And even cases where the circuit or system wouldn't even operate without that connection.And conversely there are cases where that would be a terrible idea. And even cases where you will blow something serious if you did such a thing.There is no way your question can be answered intelligently.If you have some particular example that you are asking about, then supply sufficient details so that we know what you are talking about.

author: samnmax
author_rank: Regular Contributor
timestamp: April 27, 2016, 07:17:43 am
content: I have an old AT&T power supply which has the center tap of the secondary connected to ground. It's this one:http://meci.com/electrical/transformers/at-t-power-module-1-power-supply-isolating-transformer-model-3301a.htmlI think it was made to power a printer, I'm not sure. It's 20V AC with two 10V taps, so I use it to test amplifier circuits that need a split supply. I assume grounding the center tap would ground the chassis, just like some bigger printers that use a 3 pin mains connector.

author: jitter
author_rank: Frequent Contributor
timestamp: April 27, 2016, 08:52:34 am
content: Quote from: vk6zgo on April 27, 2016, 03:37:48 amIt is still common practice on earthed devices, why do you think (e.g.) a scope's earth (BNC shields) and mains earth are connected to eachother?The simplest way to make a symmetrical PSU is by using a single bridge rectifier and use the transformer secondary side's centre tap as the common/ground/return.And since that point is connected in one place to mains earth, you are effectively connecting the centre tap to mains earth (aka protective earth or safety earth).Why that connection between mains earth and signal reference? Well, if the case isn't connected to the signal reference, I'm lead to believe that its shielding properties aren't nearly as good. Hence this practice of mains earth referencing on lots and lots of devices (among which is most of the mains powered test equipment we use...).This practice can cause some challenges though, think of groundloops, and be carfel where you clip the probe's groundlead...

author: vk6zgo
author_rank: Super Contributor
timestamp: April 28, 2016, 05:11:29 am
content: Quote from: jitter on April 27, 2016, 08:52:34 amIndeed!However,I was using a simple "two diode" full wave rectifier supply as an example of how such a connection was used in the past,& didn't want to "muddy the waters" by going into more complex cases.Actually,the kind of supply I referred to didn't always have the chassis returned to Mains Earth,as some older radios,etc,used two core power leads.After the end of the tube rectifier era,most single polarity supplies went to untapped secondaries,& either bridge or voltage doubler rectifiers.QuotePretty much so,though there are workarounds,all with their own problems.If you have a large, metal cased device which is Mains powered,it makes sense to connect that case to Mains Earth in case of a short between the Active line & the case.QuoteYep!

author: ghinckley68
author_rank: Contributor
timestamp: April 29, 2016, 05:08:42 am
content: its a torrid core with a +_12volt tap and +_20volt tapI am using to run small amps i am building but then i hook the ouput from my bk persion 4040a to it i get a hum since it BNC is refernreced to earth my amps have no refercen to earth hence the tieing the center tap to earth.