help with understanding transformers

[sickbend]

would anyone mind giving me some places to find information on using transformers in projects. I'm more interested in learning how to filter hum from them but a primer would be good. I'm sure im not the only one who could benifit from a basic tutorial. For example i know we can use dc wall warts, but with making a dc power supply is it as simple as ac step down transformer to bridge rectifier and filter caps? Another question how do center taps work and how can you safely test what voltages a transformer is putting out say one pulled from an old radio. I started these questions cause of two things one i want to make a pedal power power supply for a friend and two i want to make a psu for my tube cricket
could i use this http://www.radioshack.com/product/index.jsp?productId=2102494 with a bridge rectifier and a  variable regulator to make a 9 volt pedal board supply to power say 4 or 5  pedals? 
 

[R.G.]

First, go read "Power Supplies Basics" at Geofex, http://www.geofex.com. Better still, bookmark geofex and read it all.

Transformers are just that - they transform a voltage+current on one side to a different voltage+current on the other side. The most important factor is the transformation ratio, generally given as just the voltage ratio. Power transformers will have one side having a winding designed for the local AC power, that being 120Vac and 60 Hz in the USA. A transformer for making regulated 9V will often have a secondary rated for 10Vac to 12Vac, for reasons outlined in the article at GEOFEX.

The voltage ratio is a reflection of the number of turns of wire on the transformer core. The cross sectional area of the core and winding window set the turns-per-volt at which the core needs to work. Smaller transformers need more turns per volt, for reasons to do with the nature of ferromagnetic cores.

The wire used to wind the coils is not perfect, and has some resistance. This gets to being very significant in small transformers which need a lot of turns per volt and therefore thin wires. The wire resistance causes some loss of transformer voltage as current flows through it. So the voltage on small transformers sags, sometimes a lot, under load. This is one reason you need 10-12Vac to make 9Vdc from a small transformer.

Transformers are one of the most perfect parts available, in that they convert nearly all the energy put into the primary into usable output from the secondary. A well designed transformer will "eat" only about 2% - 5% of the energy coming in for its own operation. So the power coming out is to a first approximation equal to the power coming in. That means that if the voltage is transformed in the turns ratio, the current is transformed in the inverse of the turns ratio.

So a 120Vac to 12Vac transformer (i.e. 10->1 voltage stepdown) also gives you 10X as much current on the secondary as the primary. If you need 100ma of 12Vac from this example, the primary only pulls 10ma (plus some small additional to cover losses) from the wall socket.

About now you're wondering "OK, all well and good, but what about filtering and hum?" The reason I haven't gotten to that is that the transformer has next to nothing to do with filtering and hum. That's all done in the rectification and filter caps. Read the article at GEOFEX for that.

Center taps:
Center taps are just that - a wire contacting the exact center of a winding. If you have a 120Vac to 12Vac transformer with center tap on the secondary, then the 12Vac secondary has three wires. There is 6Vac on each side of the center tap. In reality, a center tap is always made by combining two identical half-windings. If you ground the center tap, then each end of the winding swings both above and below the center tap. Read the GEOFEX article.

Safely testing voltages from an unknown transformer
1. Ask yourself whether you can trust yourself to already know how to do the tests safely. If not, get someone who does already know to supervise/help you. You cannot learn all that's needed to do this safely from reading this forum, and I can't type that long.
2. It will involve at least this, and probably more to do it safely:
   (a) Take good notes about how the transformer is already hooked up if you're removing it from equipment, especially which leads go to the AC primary power. Label these well. Likewise, if you can find the high voltage secondary for powering tubes, label them, and the heater supply voltages. If you can't find this out, it's harder work.
   (b) Ensure that every single wire coming out of the transformer is open circuited and not loaded in any way.
   (c ) Apply a low voltage AC voltage to a winding and measure all the other windings. This tells you the voltage ratios, and if you knew which one was the AC primary pair, you can then simply compute the voltages on all windings.
3. Get someone who already knows how to do this safely to help you before you try this on your own.

[sickbend]

thanks R.G.