How to find the correct transformer for a power supply project?

I can find the correct part online from the schematic I am reading for every one but the transformer. The symbol for it states that it is a metal core transformer that one side (2 leads) connects to an outlet plug (117VAC) but the other side has has 3 leads labeled 0V, 24V, and 30V. There is a switch that goes between the 30V and 24V leads to allow you to switch between 15-30V or 0-15V, then connects to a diode bridge in which the unregulated DC heads to the regulator. I can see that it's a tapped transformer, but how do I find such a part at, say, Jameco or DigiKey? What are the search parameters? I find nothing if I search for "117VAC (or 120VAC) to 30V transformer". If I want to find a certain diode, capacitor, or transistor, it's a snap, but I have to back away from projects that require transformers because I'm not sure how to find exactly what I need even if I have a schematic. Any help would be greatly appreciated. Also, if it helps, I got the schematic from page 472 of volume 2 of the Encyclopedia of Electronic Circuits (Rudolph F. Graf).

I don't know either, but someone out there should do a piece on transformers, builds mods and application because for such a simple devise it is very complex as to it's construction, material and use. we have step up, step down, frequency, audio, power chokes inductors and on and on air core inductive ,copper and iron wound aluminum ect ect. I too have tried and tried to find specific ones to no avail and burnt a many trying to improvise so come out there turn loose with the info!! I want to know how many winds of what gage to build a 24Vdc x 24Vdc center tapp with 175 Vamp or how to wind a rodin to replace one or a torroid GEEZ do we have to fry ourselves in our curious quest
Capt. Howdy8 years ago
You sound like me. I gave up and was about to buy a computer power supply for about $30 and a friend of mine gave me a junk pc. I built an lm317t based variable power supply and used the +12vdc from the computer power supply as the input. It's easy and has clean voltage plenty of amperage short of running some 1000 watt audio amp. The LM317T is thermal overload protected and the PC power supply just shuts off if you get your leads together. Plus you can wire up three fixed voltage outputs 3.5-7.5 and 12vdc. My other power supply i used a transformer from a car battery charger a heavy duty rectifier from radioshack and the LM317T variable circuit. The transformer was 15 years old. I used it for 12 years before it died.
Xombee (author)  Capt. Howdy8 years ago
Thanks for your reply, and it's sorta comforting that I'm not totally alone in my frustration. I have found many variable power supply schematics online that sound fantastic, yet the ones with transformers are consistently cryptic when it comes to providing straight-up part numbers or values that I can enter into a part providers search feature. I have found our that I can put 2 transformers of different voltages together to reach the suggested value I need, as long as they share the same values otherwise. Still, it's just curious that there doesn't seem to be the same consistency with transformers as there are with, say, IC's, diodes, or regulators. Maybe the answer will come after I learn a bit more. I will certainly take your suggestion to heart, because I have plenty of computer power supplies from around 130W to 450W. Take care, and have a great day!