I picked up a couple of little 240v/12v transformers from a junk shop for a project. They have the voltages marked, but nothing to indicate which are the input and which are the output leads. I can’t easily tell which is which from where the wires emanate, but then I don’t know what I’m looking for.
I do notice that one set of wires is thicker, but does that mean they are for connection to higher voltage mains or higher amperage output? Of course I could just apply voltage to one set of wires and see what the voltage is on the other set but I’m concerned that if I get it wrong I may blow it.
Also, is there any way to tell/estimate/guess what the maximum amperage output the transformers might be? They are marked with voltages but nothing else.
Most likely, the thinner wires are the primary winding.
The thicker wires will be thicker to carry more current - in this case, it’s a 1:20 ratio between the primary current and the secondary current, and the same ratio in reverse for the voltage stepdown.
As for guessing at the current capacity, can you estimate the wire gauge on the primary and secondary windings? Failing that, roughly how big/small are they, and how heavy?
The primary (high side) is usually marked H1,H2 and the secondary X1,X2 if that helps any. You’ll need a wattage rating or some more information to accurately know the amperage rating.
As mentioned above, the thicker wires are usually the secondary in a step-down transformer. You can also use an Ohm meter to determine which winding is which - the one with the most resistance will be the Primary (more turns, thinner wire). You can get a general estimate on the wattage of the transformer by it’s physical size - compare it to other transformers of similar rating.
All those are correct. If you have another source of 12 VAC, you can connect that to either wire pair without risk of damage. If you have picked the primary leads you will have 12/20 or 0.6 vac coming out the secondary, and if you have picked the secondary leads you will have 240 VAC on the primary, both of which are OK.
Sounds like the problem of which is which is solved, thanks guys.
Physical size: one is about 1 3/4 x 2 1/2 x 2 inches. The other is about 1 1/2 x 2 x 1 1/4 inches. The problem with rating them by size is that those I have for comparison don’t seem to correlate size and amperage very closely.
Given that I bought these for next to nothing, I’m prepared to just take a rough guess and then try them out in the intended application and just hope that they can cope with the amperage (which won’t be very high anyway). What would the symptoms of overloading them be? Getting hot?
Yes, they will get hot if overloaded. Some transformers (notably ones sold by Radio Shack) tend to be over-rated, and run very hot at their rated load. A good transformer should get more than warm at it’s rated load.
They also have an apparent impedance with respect to the load, so if you overload the secondary, the load gets a lower voltage than you want.
The overheating comes from two sources. One is resistance in the windings. The other is hysteresis loss in the core. I’m not sure what the typical balance is, and how it depends on use. In the limits, without any load the great majority of the heat would be from hysteresis, with a tiny resistive loss in the primary because it is delivering some power to heat the core. With a big load, the resistive losses in the windings would be bigger, in fact I think they’d always be proportional to the current draw. I don’t know, though, how the hysteresis losses would depend on current; I think they’d be bigger with bigger current but can’t quite picture the whole deal clearly enough to have confidence.