15 volt ac Transformer

[Anonymous]

I have a transformer that puts out 15 volts AC that I want to use for accessories. Most Lionel accessories use 14 volts AC max. Is there a way I can knock down the transformer to 14 volts or less?

[webenda]

Joncoy,

Rectifiers drop .7 to 1 volt each and the voltage drop is almost independent of load. In the circuit below, the voltage across the load is 1.4 to 2 VAC less than the transformer voltage. For your application replace the ammeter with a wire.

[Anonymous]

Wayne.
Have no idea what you said there. I don't have an ammeter. Is there anything I can do at the output terminals of the transformer?

[lionelsoni]

John, I will try to describe the technique in greater detail.

The idea is to connect items called "bridge rectifiers" in series between the transformer and the accessories to reduce the voltage. You have to modify the bridge rectifiers slightly; and the number of them that you use determines the voltage reduction. After you reduce the voltage, it will no longer be the pure sinusoid that comes out of the transformer, but will nevertheless be suitable for practically any accessory that you use it with. Since it is not sinusoidal, the effective voltage that results depends on the kind of load you have and cannot be accurately measured with typical voltmeters. As a rough guide, each bridge-rectifier package reduces it about a volt. The best way to set it up is to try adding and removing bridge rectifiers from the circuit until the accessory performs the way you want.

Radio Shack has a 4-ampere 50-volt bridge, 276-1146, for $2. (The 276-1173 400-volt one for $2.50 will work just as well if they're out of the $2 part.) For a small load, you might be able to use the 1.4 ampere 276-1152, which is cheaper ($1.50) and smaller. But, if you have room, go with the 4-ampere ones.

Each bridge rectifier unit has 4 terminals, marked +, -, ~, and another ~. (The ~ terminals may be marked "AC" instead, or even left unmarked.) To modify a bridge rectifier for this purpose, connect the + and - terminals together on each bridge rectifier. Then use the two ~ terminals to wire the bridges in series with the accessory or accessories.

If you use the Radio Shack 4-ampere bridges, connecting + and - together is a little awkward, since they are the outer terminals. However, you can easily figure out how to bend them around the inner ~ terminals without touching the ~ terminals and solder them together.

After you wire the + and - together, those terminals can be used as an intermediate connection between the two ~ terminals. So you can go up and down by half steps, using the + and - as an intermediate tap on each bridge.

When you try it out, if the accessory seems to need more voltage, move one of the wires, for example, the wire to the accessory, to the + and - terminals of the end bridge rectifier. If you need more, completely remove that bridge and just connect to the ~ of the next one, and so on. If you need less voltage, add another bridge.

[flyingyankee616]

[:D]Hi joncoy,I'm not much on electronics,but if there is a store in your area like Radio Shack,you can take your transformer in and explain what you would like to do. they would probably give you a step by step to make it simple.

[lionelsoni]

I wouldn't hold my breath on that one, Charles. The folks at Radio Shack aren't much on electronics either, in my experience. You can see from this link that there is no requirement whatsoever for knowledge of electronics to work there:
http://www.radioshackcorporation.com/careers/retail.html

[webenda]

Maybe this will help? (Click on image to enlarge.)

[ChiefEagles]

Webenda and Bob [lionelsoni] are right. Bob just helped me reduce the voltage for a smoke unit I put in a dummy. I used the 400 volt ones as they do not get as hot as the 50 vilt ones. If you can use a soldering gun, they are not hard to do.
Here is the photo of the ones I built for the smoke unit.
The ones I used

+ and - soldered together

Finished product.

Now "hooked" to smoke unit.


You probably will only need one. I plan on using them to slow down some accessories [some look better slower than others] that will be running off of a transformer with several attached. Thank you Bob for all the help you have given. Glad to see Wedenda in here too.

[lionelsoni]

Thanks for putting up the pictures, guys.

[Anonymous]

Wayne,Bob and Frank,
Thanks so much for the pictures it helps alot. I had to get the 400amp one because they were out of the 50 amp ones. Now if I use more than one I would connect the ~ leads together and the remaining ~ leads of the last rect would go to the transformer and one to the accessory correct?
Again thanks for the info.
John

[ChiefEagles]

You got it. [I just followed Bob's insturctions. He's the Electrical Engineer]

[lionelsoni]

Yes, connect them into a series string, that is, the first ~ of the first bridge to the transformer, the second ~ of the first bridge to the first ~ of the second bridge. Then the second ~ of the second bridge to the first ~ of the third bridge, and so on; and the second ~ of the last bridge to the accessory. Or you can connect the +- point of the last bridge to the accessory to shorten the string by one-half bridge.

[Anonymous]

Bob,
Ok I've got it now. I was trying to connect the ~ together with the last bridge going to the transformer and the accessory. Again thanks to all.
John

[Anonymous]

It is not uncommon for transformers to read higher voltage when not under a load. Is yours reading 14 volts or does the rating say 15 volts. The one volt difference is not much and the tolerance of the accessories to only one volt will most likely not make a difference.

You have solved the problem. Another way is to reduce the voltage by removing some of the secondary (15v) windings. I have done this on several occassions, two being reducing the voltage of two nice higher amperage 20 volt transformers to 12 v for lights and 14 for switches. I was overseas 20 + years ago and did not have access to bridge rectifiers. The transformers are still working.

Charlie