Transformer

Try lobbing a load resistor across the voltmeter. Try, oh, let's say, 200Ω or so. What happens then, and what can be learned?

actually in the process of getting dc trhough rectifiers or diodes developes this problem.i have seen this in my own case where output at no load is nearly 20 volts and at load it reduces to 12 volts.u can correct the difference of 1.2 volts by using suitable value of resistance as per your output current.

Besides the dc diode voltage drop, what you're seeing is the cheap transformer regulation.

Some small transformers can have a regulation of over 25%, meaning that the open circuit voltage is approximately 25% higher than the loaded secondary voltage.

This is primarily because of the internal resistance of the windings and the loss of the core.

To make a tranformer cheap the manufacturers cut down on the core size and increase the current density in the windings, saving them the cost of thicker copper wire.

All these cost savings make the transformer smaller but more importantly cheaper with the added problem of higher running temperatures and loss of efficiency.

I have designed transformers for high security and long life installations and when they are running they almost are cool to the touch, they might be twice the size of a cheap ready made transformer, but they are efficient and running cool helps with lifetimes and reliability etc...

John.

So, what does the key connect? Where is the filter capacitor in all this?

What you describe is perfectly normal for an inexpensive dc supply.

Put really simply the open-circuit (no load) transformer voltage will be higher than the rating (24VDC). When you load the transformer up (by placing a load across the output, such as a resistor) then the load draws current and the transformer voltage drops. The output voltage of cheap transformers will do this (due to poor regulation as others have mentioned) and so they mark the nameplate with the transformers approximate output voltage when the expected load is connected. This has nothing to do with any diodes or capacitors connected to the output of the transformer.

Also, it normally doesn't matter that the voltage is much higher than what the name plate says when the switch is open due to the fact that these transformers are generally built into a specific piece of equipment (such as a lamp) and so the equipment will be designed to handle the poor voltage regulation. Its all just part of reducing the cost of the product to a bare minimum by cutting corners.

I am sure there is a much better explanation on the internet, perhaps wikipedia or google "transformer voltage regulation" or "how a transformer works", etc.

I'm assuming the "key" switches some kind of load on? If so, you are seeing the difference between RMS and peak(no load). Transformers are rated in RMS voltage so under no load conditions the absolute max voltage will be higher. Pls note the output of a transformer with 120vac in will be 24vac out. A rectifier usually follows with a smoothing capacitor to convert to dc. No load voltages may be bothersome, so you could always regulate the voltage first. Life's all about choices