Discussion in 'FAQs' started by 60103, Mar 6, 2003.

  1. 60103

    60103 Pooh Bah

    I am about to wire in a Transformer and I need some advice. The Transformer is bare bones and I've bought most of the parts to complete it -- plastic box, fuse, switch, line cord.
    Peter Thorne's book shows a wiring diagram where the is a fuse in one line and a switch in the other. Is this right? I think they should both be in the hot side.
    Which is the hot side of the line cord? wide plug or narrow?
    There are 3 output wires. Which ones do I use? It's labelled "16v C.T." Is that Centre Tap? What's that?
    I plan to mount the transformer on the outside of a plastic project box and have all the wiring connections inside. The assembly will be at the back of the layout, out of the way.
    Thanks for all advice.
  2. Russ Bellinis

    Russ Bellinis Active Member

    I'm presuming you are talking about starting with the 110v outlet in a typical house. If you open up a wall socket, you should have a black wire and a white wire. If your house is newer, or has had wiring updated, you may have a green wire as well. The black wire should always be hot. The white wire should be the "neutral" and if you have a green wire, it goes to ground. You are right that any fuse should go on the hot side, but a switch will work on either side of the load. The transformer won't turn on until you have power coming in on the black wire from the plug, and then going through the "load" (primary coil) and back to the wall through the white wire. A fuse should always be on the black wire before the transformer primary coil. The on off switch can be on either side of the coil. I would put a switch on the black wire coming into the transformer, and then put the fuse between the switch and the transformer. That way, you can shut off power to change a fuse or to check the fuse safely, but it will work if the switch is on the neutral wire. Just don't put the fuse on the neutral wire, it has to be before the load in order to offer any protection.
  3. ezdays

    ezdays Out AZ way


    Russ is correct it saying that any fuse or switch should be on the black wire since the white wire is electrically connected to the green (or ground) wire, usually at the breaker box. Now it doesn't matter which wire goes to which on the transformer. Usually both input wires are the same color anyway, except that there could be anther wire for 220 volt input if the transformer was rated at 110/220. Your transformer should be marked as to input and output and other ratings such as amps and volts.

    Oh yeah, the big prong is the white, or common wire.

  4. 60103

    60103 Pooh Bah

    Thanks, Don & Russ, it sort of confirms my feelings.
    I was out at the electronics shops this morning, looking for power cords.
    The only polarized ones they had were three wire; the two wire ones had both prings the same size. A grounded cable seems like overkill on this.
    There are three wires coming out of the secondary. I suspect that the middle is a neutral and the others are a +16V and -16V (or however you call it in AC). I'll have to check with my AC voltmeter when I get it assembled.
  5. jon-monon

    jon-monon Active Member

    Best to put both on the hot side. If you switch the neutral, it will turn off, but it will still have hot wiring after the switch. If you forget to unplug it in that state, and handle the wiring, the juice will try to go through you to get to ground (earth).

    For those in 220V land, you have two hto wires coming in and you should switch and fuse both.

    Getting shocked only hurt for a little while, so I found, a few hundred times.

    Rather than chance going by memory and being wrong, I just grab a voltmeter and measure each side to ground. A hot wire is 110 VAC to earth or the ground on the outlet. This is true of 220V service too, but 220 is hot on both sides.

    C.T. is center tap. It's a wire coming off the center of the coil on the secondary side. It's used as a floating ground point reference if your circuit requires. It will be shown inthe schematic if needed, if not, you can just insulate it with some heat shrink (cut the wire so it is not "stripped", put shrink over the end, but hanging off the end a 1/2", heat it, and squeeze it with pliers while hot).

    The side with two wires, the primary, goes to the 110VAC supply. To find out which on is center tap on the other side, go ahead and wire the primary to 110, then measure VAC between two of the three others. If you get about 8 VAC, you got hte center tap, if you get about 16 VAC, you got the proper secondary wires. Measure them all, just to be sure, the voltage can be off quite a bit. There is usually some color coding to help you out two, like one color for the secondaries, and another for the center tap.


    Probably best to keep it all up off the floor, just in case of flooding.

    Attached Files:

  6. ezdays

    ezdays Out AZ way

    Three wires are not necessarily overkill. If you do use the third (green) wire you should connect it to a metal chassis if you have one. The idea is if the hot wire were to short out to a metal box or enclosure, it would blow the fuse before it blew anything else.
    The third wire should be a center tap. If you get 16 VAC (no polarity) between two wires, you should get 8 volts between the center tap and either of the other two wires.

    One other thing, if you had room inside the box, that's where I would mount the transformer, otherwise be sure the box is not sitting out where it could get bumped around. Sometimes transformer wires are brought out directly from the windings, but most often they are soldered to the windings and are prone to being yanked off if not protected. Not only is this dangerous, but it does ruin the transformer. You could also put some sort of protective box over the transformer.

  7. Russ Bellinis

    Russ Bellinis Active Member

    One last thing to remember, transformers create quite a bit of heat. Keep it away from flamables including flammable scenery materials.
  8. jon-monon

    jon-monon Active Member

    With a plastic chassis, the third wire should go to the frame of the transformer, if it is metal. It will offer limited protection, but the 3 wire cord is better than unpolarized.

    If you use an unpolarized power cord, you loose control over which is hot and which is neutral, so you can blow a fuse on the neutral, and have a hot circuit, and you will want to investigate. Of course you should unplug it anyway, but it's easy to forget.

    Also a good reason to verify all your AC outlets are properly wired; it's not uncommon to find the hot and neutral swapped, which still works, but may defeat safety measures of products in use.

    Out if the transformer will in fact be AC, they are AC devices, incapable of passing DC and they do not convert AC to DC, that will be done by a rectifier (or some diodes after the transformer.
  9. Russ Bellinis

    Russ Bellinis Active Member

    If you haven't got a rectifier, yet. You can get a "diode bridge" from Radio Shack or any electronics supply warehouse for a reasonable price. I'm not sure, but probably under $5.00, certainly under $10.00.
  10. 60103

    60103 Pooh Bah

    The transformer will be powering an add-on transistor throttle.
    It looks like I'll be getting a three wire cord & plug for it.
    I'm leaving the transformer outside the case for cooling, but it will be at the back of a shelf just below the layout.
    Thanks for all the information & advice.
  11. Paul Davis

    Paul Davis Member

    You should defiantely put the transformer inside the case. mounting the transformer to the outside of the case is unsafe. We want our model railroads to be fun not deadly.

    Idealy the box should be metal but plastic is probably fine. If you're worried about the transformer overheating a metal case will help dissipate the heat. a small fan could be added in a worst case scenario. Be sure to use grommets and strain releifs for any holes which have the 110V cable running through.

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