What is a secondary transformer?

Question

On a number of projects I'm building, there's a transformer between wall power and device power. In the parts lists, the transformers are usually designated by "12v secondary transformer" or "18v 4a Secondary Transformer".

Because the flow goes from wall power through the transformer and then to a rectifier, I assume that it's just converting 120V AC to whatever DC, stabilizing it a bit with the rectifier.

Is that a safe assumption? Instead of finding an actual transformer, can I just substitute, say an 18V 4a laptop power supply or 12v wall wart I have lying about, using that for my power input to the rectifier? Why or why not?

Or is there something more to a "secondary" transformer?

Answer 1

Others have straightened you out on the "12V secondary" part. As far as substituting a DC adapter (wall wart or laptop PSU) for the transformer; this will generally be safe, but a couple of points to note will help avoid surprises :

1) If the project is driving motors, remember that motors are nasty loads that can generate high voltage spikes when running, and 5-10x their rated current for a moment when starting (or permanently when stalled). Transformers are pretty tough and take this abuse well; laptop PSUs may shut down and cheap wall warts can make smoke under the same stress. (If the project has current limiting and noise suppression around the motors, you should be OK.)

2) a "12V" secondary generates 12V RMS at the rated load current. That means the peak voltage is about 1.4* higher, plus 10% or so at no load. Subtract a couple of diode drops and ripple on the resulting DC and you'll get 15V or so there. If the project needs that extra voltage, a 12V adapter won't do the job.

Answer 2

The phrase can also be written as "A transformer with the secondary voltage being 12V" - meaning it's primary is rated for 110V and it will put out 12V.

Yes, you can replace the transformer with a wall wart - but keep in mind that safety ratings can be different so intended use must come into play.

Answer 3

12 V secondary transformer must be read as "(12 V secondary) transformer". So the word "secondary" refers to the indicated voltage, not to the word transformer.

Note that the indicated voltage is the RMS voltage (Root Mean Square), that's the equivalent DC voltage for the same power in a resistor load. That's less than the peak voltage, the voltage which you get after rectifying and smoothing. The peak voltage is \$\sqrt{2}\$ times higher than RMS, so a 12 V transformer will give 17 V peak. You have to subtract two times 0.7 V for the rectifier diodes, so you get 15.4 V DC from a 12 V transformer.