Why can't I connect a transformer like this to the mains? - Page 1
Forum section: electronics / beginners

author: axawire
author_rank: Contributor
timestamp: March 17, 2021, 09:37:21 pm
content: As the AC lines have an alternating current why can't/shouldn't I connect a transformer to the mains power supply like the attached image - with or with out capacitor plate?

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2021, 09:43:41 pm
content: Because a complete circuit is required to flow. The other side of the transformer needs to be connected to the neutral, in order to make the circuit.

author: bob91343
author_rank: Super Contributor
timestamp: March 17, 2021, 09:54:59 pm
content: Either that, or a VERY large capacitor plate. Compute the current flow based on the capacitance, voltage, and frequency.

author: Manul
author_rank: Super Contributor
timestamp: March 17, 2021, 10:05:00 pm
content: Well, you *can*. Should you? Probably no. It is true, that there will be some minuscule AC current running through your transformer primary winding, because of capacitor plate which forms a very small capacitor to ground. It will likely be in the range of a few microamps, almost unmeasurable.Note, that your capacitor plate will be "live", so if you accidentally touch it - bad news.Unless you have experience with electrical safety and you know what you are doing, I suggest to not try such things. Very little purpose (except for pure experimentation).Quote from: Zero999 on March 17, 2021, 09:43:41 pmI believe both wires are live in USA? I think they use middle tap grounded transformers for their 110V distribution to houses. It does not, of course, change the validity of your statement.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2021, 10:21:42 pm
content: The US uses a centre tapped 240V arrangement, but most circuit are 120V. If it's a 120V circuit, the phase wire will be 120V, with respect to earth and the neutral near earth potential.

author: TimFox
author_rank: Super Contributor
timestamp: March 17, 2021, 10:23:22 pm
content: Normal 120 V distribution in the US has one side near ground (neutral) and the other hot (line).For households, the usual connection from the utility is 240 V with a grounded center-tap to give two 120 V circuits.The full 240 V (both hot) is used for higher-power loads such as clothes dryer or central air conditioner, but the lighting circuits are 120 V.In his circuit diagram, one of the two flat blades should be longer than the other, denoting neutral. The third (rounded) pin is ground/protective earth.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: March 19, 2021, 03:05:05 am
content: In the US the neutral wire is bonded to earth in the pannel.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 19, 2021, 03:52:36 am
content: Quote from: axawire on March 17, 2021, 09:37:21 pmWhy on earth would you?If you are somehow thinking about isolation---- that is provided for the circuit operating off the secondary winding by the transformer itself.To operate, the transformer primary need to be connected from live (hot) to neutral.No connection, no operate!Another trap----in your circuit if you touch the free end of the primary, you will get a shock, perhaps a little bit less severe than from the hot side, but a quite nasty one.To the Mains supply, you are connected from live to neutral in series with an inductor, so it is quite happy to deliver you an electric shock.

author: perieanuo
author_rank: Frequent Contributor
timestamp: March 19, 2021, 10:06:57 am
content: Quote from: axawire on March 17, 2021, 09:37:21 pmbecause any modern electrical implementation would have a main differential disjunctor 30mA type on main circuit and this one will cut youwhy? for protecting users for electric shock/death, for protecting electrical company from theftyou got your answer

author: Monkeh
author_rank: Super Contributor
timestamp: March 19, 2021, 03:13:55 pm
content: Quote from: perieanuo on March 19, 2021, 10:06:57 amThat's not, at all, what RCDs are for. And no, not 'any modern implementation' has one.