How do I identify a transformer with no markings on it?

[Plecto]

Hi. I just scavanged a transformer from an old Panasonic stereo system, but there are no markings on the transformer. It is about 8x8x8cm, weighs about 1kg, has a 1.25A fuse on the primary side and a wattage rating of 156W on the back (back of the stereo system). I'm not sure what these 156W means. The amp is supposed to be 2x50W so I guess 156W power supply would make sence, but those 150VA transformers I've been looking at are usually bigger and heavier than this. I am also curious about the inputs and outputs. It has four inputs on the primary side and 9 outputs on the secondary. Why have they littered the thing with so many outputs and inputs? Surely there's only need for two input pins and three output pins?

 

[godfreyl]

Mains transformers often have two primaries which are connected in series for 240V, or in parallel for 120V. This is so the equipment can be used in different countries.

 

[milesguidon]

Transformers can have multiple windings/outputs, center taps, etc. They probably have a transformer with multiple taps and output voltages, hence the increased number of inputs/outputs.

Check it out:

https://www.allaboutcircuits.com/vol_2/chpt_9/5.html

 

[BradtheRad]

Possible reasons for the numerous output wires:

* to power one or more pilot lamps

* drive a preamp stage at 20 to 30 V, while driving speakers from a lower V supply

* to power left channel independently from right channel

Etc.

 

[Plecto]

I see. I think I know how the inputs and outputs work. They are like three seperate transformers together, right? All I have to do is to measure my way to find out what is what, connect the three together in series and find the center point so I can use it as a bipolar supply? I can see how the primary is connected though so that won't be a problem allthough I find it strange that just two pins are connected (would assume that the two others should be connected in series), but that doesn't worry me

What I really need to know is the VA ratings on it, to see if I can use it for my project. The question is how much I can trust the 156W which is written on the back side of the amplifier.

 

[RetroTechie]

That 156W rating on the label is what the equipment should maximally pull from the mains supply, and is often a high rated very-worst-condition. Like what you'd use to decide what value fuse to put in primary circuit. Output rating (+ a guestimate of *heat* losses in transformer & other parts) are a better indication of what a transformer can deliver. Or put known load(s) on it, do measurements, and see how hot the transformer gets when loaded longer time (say, hours) with that.

Taking that 2x50W figure might mean that at maximum power, 50W is pumped to each of 2 speakers, up to ~156W is taken from mains supply, the difference is losses in various components of the stereo (mostly transformer & end stage of amplifier - read: what gets hot and/or is attached to a heat sink).

I see. I think I know how the inputs and outputs work. They are like three seperate transformers together, right?

Simplisticly speaking: yes. But note that separate windings will be rated for different currents, so taking them all in series might produce a high output voltage but burn a transformer winding when you load it heavily. Wire thickness / DC resistance for a specific winding gives a clue. Like for most 110-230V -> low voltage transformers you have high voltage/low current & thus many windings/thin wire on primary side, low voltage/high current & thus fewer windings/thick wires on secondary side.

Better have a good look at how it's used in the existing circuit, to get an idea about how much current is pulled from each transformer winding (and thus what a suitable configuration could be for a circuit to re-use that transformer).

 

[tpetar]

Hi. I just scavanged a transformer from an old Panasonic stereo system, but there are no markings on the transformer. It is about 8x8x8cm, weighs about 1kg, has a 1.25A fuse on the primary side and a wattage rating of 156W on the back (back of the stereo system). I'm not sure what these 156W means. The amp is supposed to be 2x50W so I guess 156W power supply would make sence, but those 150VA transformers I've been looking at are usually bigger and heavier than this. I am also curious about the inputs and outputs. It has four inputs on the primary side and 9 outputs on the secondary. Why have they littered the thing with so many outputs and inputs? Surely there's only need for two input pins and three output pins?

156W is total power of that transformer. Often amplifier use transformers with higher voltages such 35V-50V-60V to get power but decrease current. Multiple transformer output are needed for various voltage needs in circuits. You can use multimeter set as ohm meter and check resistance and winding links. Also you can use nonius to measure diameter of wires to see what is max current per winding. And finaly you can check voltages on secundar outputs.