Question about a transformer - Page 1
Forum section: electronics / beginners

author: uk.casmith
author_rank: Contributor
timestamp: September 16, 2018, 09:25:09 pm
content: I have purchased a clock from a shop in the UK. Open it with the idea of stealing the transformer and some components. I have some questions about using the transformer. I'm pretty new at this stuff:FYI: All UK plugs must have a ground (earth) wire and the voltage is 220 volts.The transformer has three wires running out of the transformer towards the circuit board. I've tried measuring the voltage but because it's AC I cannot get an accurate reading. If I measure the voltage after the diode i'm getting around 5.42 volts.My question is if I disconnect the three wires: Two of them are the AC and one is ground.Where do I connect the ground wire?And if someone can give me some safety advice that would be great!Thanks,Cameron.

author: Benta
author_rank: Super Contributor
timestamp: September 16, 2018, 09:59:25 pm
content: You don't connect the ground wire. Small things like your clock are designed to be "floating" and just need phase and neutral. Reinforced insulation from the mains keep them safe for the consumer.A symbol depicting two squares within another somewhere on your clock should show this.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: September 16, 2018, 10:55:39 pm
content: Quote from: uk.casmith on September 16, 2018, 09:25:09 pmHi Cameron, welcome to the forum. I can give you some advice regarding mains voltage and safety, however, based on your question and context, I advise you not to do anything more until you pick up a book and study some basic electric theory. The problem is that anyone giving you advice on this forum would probably put you in danger and will create a liability to you and the advisor.With that said, PLEASE seek out a mentor that can directly guide you before you get hurt or killed. I encourage you to go further and learn more and try things, but under the guidance of a more experienced tutor.I repeat, do not go any further until you have more knowledge and guidance.Hope this helps...

author: Brumby
author_rank: Unknown
timestamp: September 17, 2018, 12:05:09 am
content: Welcome to the forum Cameron.We always welcome anybody who wishes to ask questions about mains related subjects ... especially when talking about transformers and "ground". We get rather nervous sometimes ... mains is not very forgiving, which is why any "remote advice" given here may seem overcautious.I am curious about this statement:Quote from: uk.casmith on September 16, 2018, 09:25:09 pmAC is not (on its own) a valid reason for being unable to get an accurate reading - so I must wonder what it is about your measurement that makes you say this?Also, when it comes to commenting about a transformer, it is always useful to provide a photo showing it and the connections on it. In your case, a shot or two of the wiring from the mains cord to the transformer would also be helpful.With photos - please make special effort to:1. Get the subject in sharp focus2. Have it well lit3. Get as close as you can so that the area you want to show us fills as much of the frame as possible.

author: drussell
author_rank: Super Contributor
timestamp: September 17, 2018, 12:53:25 am
content: Welcome!Firstly, a photo or two of what you're looking at would be very helpful.Quote from: uk.casmith on September 16, 2018, 09:25:09 pmYou should be able to get a proper reading using a meter that is set to the correct range to measure AC voltages. What kind of meter are you using? Perhaps a photo of that, as well.Also, there is usually more that one diode, often what is called a bridge rectifier (which is actually four diodes in one package) with four leads.QuoteTypically when there are three wires on a secondary winding, it is a centre-tapped winding. This means that there is a winding with two "halves", with the third wire being connected to the point between the two halves, where they join. This is a very useful feature for many applications for various reasons and purposes.There should not usually be an actual ground wire, though, since once you are om thesecondaryside of a transformer, you arbitrarily call some point of your circuit "ground" potential, even though it may not truly be earthed. Photo, please.QuoteFirstly, since you are a beginner and don't want to be anywhere near the mains power yet, if you must experiment with a mains connected device, even just a transformer, be absolutely sure that all connections that are at mains potential are fully insulated! Starting by using battery powered circuits or a self-contained wall-wart type power supply may be more appropriate, however, as long as things are truly insulated completely, the potential danger can be made to be minimal.Again, if you provide some photos, we can help to see if this clock is insulated well enough to be in there poking and prodding around when you're just starting out and learning.

author: uk.casmith
author_rank: Contributor
timestamp: September 17, 2018, 07:08:03 pm
content: Hi, Everyone,Thanks for your replies.I've attached the photos as requested.I didn't realise but I needed to set my meter up to measure AC. The results are as follows:Lead on far left to far right: 9.4 to 10 voltsLead on far left to middle: 4.8 to 5 voltsLead on far right to middle: 4.8 to 5 voltsOn the circuit board it labels the black lead in the middle ground but it seems to be giving a center tap value in-between the 9.7 volts, which is what i've read but why name it ground?I hope that helps with more information?I know AC is very dangerous and appreciate everyone's help. I'm going to end up building my project using a AC/DC wall adaptor with a 7805, but I still find this very interesting and would like to do it some time in the future.So any advice is greatly appreciated.Also, I should note I take full responsibility of my own actions.Thanks,Cameron.

author: MarkF
author_rank: Super Contributor
timestamp: September 17, 2018, 09:26:50 pm
content: It's unfortunate that they labelled it Ground. Think of it as a common wire. Most power supplies have a red, black and green terminals. The red and black wires are isolated via the transformer and the green terminal is the ground. This video will help explain them:Your transformer is a center tapped secondary (i.e. the blue wires are the outside of the coil and the black wire is the center of the coil). There are many ways to configure a power supply. Here are four different options. Your picture looks like the top one.

author: Jwillis
author_rank: Super Contributor
timestamp: September 17, 2018, 09:29:26 pm
content: Live and Neutral mains at the primary with no earth ground. At the secondary side ,The black wire is a center tap of the two blue wires .Not an earth ground.The voltages from black to either of the blue wires will be the same approximately4.7VAC .From blue to blue will be approximately double the value at around 9.7VAC .So 4.74 V from black to blue and 9.7 V from blue to blue. After rectification there will be a 3 lines .One will be common ground of zero volts ,one positive of around +4.7volts DC ,and one negative at around -4.7 volts DC. This is because some micro chips require a positive and negative voltage with a common ground to operate.

author: drussell
author_rank: Super Contributor
timestamp: September 18, 2018, 01:23:17 am
content: Quote from: Jwillis on September 17, 2018, 09:29:26 pmOrdinarily, yes, if you were using a bridge rectifier to get symmetrical positive and negative rails above and below the centre-tap, but in this case the layout is just like the old two-diode full wave rectifiers from the tube era using both sides of the secondary winding with just one diode on each leg of the transformer instead of a four diode bridge arrangement.This configuration is still a full wave rectifier but only one half of the secondary conducts at any given time and the output voltage is only half the total winding voltage, .707 instead of 1.414 times the total secondary voltage, plus you have one less diode drop (which can be an advantage in low voltage, high current supplies, for example.)

author: Brumby
author_rank: Unknown
timestamp: September 18, 2018, 01:40:46 am
content: First - a compliment on the photos, the first one in particular. With the two meter shots, you can see reflections can become a problem - but we can see the readings, so that's all that matters. The last one could be better focussed - but we are just able to make out enough to see how the power is set out. Well done.Quote from: uk.casmith on September 17, 2018, 07:08:03 pmUnfortunately, this usage appears all too frequently. As mentioned above, it would be better to see this and think of "common" when assessing circuit topology and function. Whether or not it is actually connected to a true ground or "earth" is a separate question. The same can be said for the term "chassis". To complicate matters even more so, even a reference to "earth" should be checked to see if it is actually connected to an earthing point.