Over the years I've collected dozens and dozens of transformers. Most are stripped out of old equipment and most have little to identify what they are so cannot be used without some investigation.

I recently needed a transformer suitable for making a high voltage power supply for a transmitter. Something like 500 volts at 100mA or so.

I rummaged through my collection to find a suitable transformer. Of course lots of transformers are not mains transformers, they are for audio matching and inter-valve/transistor use, etc etc as well as for connecting to mains. The last type may be for UK mains solely, but some are universal for US or UK mains. The first problem is identifying what exactly a particular transformer was designed for.

There may be clues. The obvious one is markings. Clearly if it has 240v or 230v plus maybe some tappings marked on it that's half the battle. One I picked up yesterday had lots of numbers marked on it but these were just sequential numbers from 1 to 26 apart from a pair of unused tags marked "N" and "L" (a good sign).

Here's my method of figuring out what you've got.

First make a sketch of the pin layout and measure DC resistance between each and every pin. Lots of resistance readings will be quite low, say less than 1 ohm, but high enough to judge whether there's a winding involved. Look at the connecting wire to a particular tag or loose wire end if there are no tags fitted and make a note as to whether it's thick or thin enamelled wire or multi-stranded. It may be advantageous to solder a couple of tagstrips to flying leads to avoid breakages. Draw a rough circuit diagram of potential windings based on resistance measurements.

Often you can figure out which connections are those for mains input, but it would be imprudent to just connect these up and plug into a mains supply.

Initially try connecting a low voltage AC supply from a low voltage mains transformer to each potential winding in turn whilst monitoring the current flow. I use a small clamp meter. If the current is too great, say over 1 amp, accompanied by sparks and loud humming try a different winding. Without a clamp meter just monitor the AC voltage as you connect it to a winding and if it drops by only a small amount all well and good, but if it drops significantly move on to another winding. When you find a winding that consumes say half an amp this can be used as a test winding. The choice of test input voltage is important too. You can try 6 volts or 12 volts for starters.

Having established a suitable test input winding you should then measure the voltage at the test winding for reference, then each of the other potential windings that you discovered by resistance tests. The resistance check is vital because with a modern digital AC voltmeter you'll get a reading across any pair of connections whether or not they are both connected to the same winding. Very confusing.

Note each output voltage. This will then give you an idea of what the windings represent. For example, with 6 volts input at the test winding you might see 200 volts on another. You might see 4 volts, or virtually anything. Remember that many transformers don't work as well when stepping up if they were designed for stepping down. The aim is to find the mains input winding.

As an example. I applied 12 volts at 0.4 amp to one winding and measured about 100 volts on another. The latter winding had four windings in series. I was able to measure something like 85, 90, 95 and 100 volts across the set. This set of windings was terminated in a single line of tags. Adjacent to this line was an identical set of tags, across which I measured roughly the same voltages. At the end of one line was an unused tag labelled "N" and opposite was a tag labelled "L". What I was looking at was a transformer designed for US mains OR UK mains. By paralleling the tags and connecting the ends to L & N you have a 115 volt transformer and by series connecting the lines you have a 230 volt transformer.

When connecting windings in series (to develop more voltage) or in parallel (to develop more current) it's essential to connect them the correct way round. Before attempting to parallel a pair of windings, first connect them in series to see if the voltage doubles or drops to near zero. If the voltage doubles the two windings are said to be in-phase. The "finish" of the first winding goes to the "start" of the other. If the "finish" of the first winding is connected to the "finish" of the other the two voltages will cancel out. When you see a doubled voltage you will have identified the start and finish of each winding.You can then parallel the two windings if you want the same voltage but more current. You must always connect "start" to "start" and "finish" to "finish" for parallel connection.

Once a potential primary winding has been identified, being cautious you can connect 230 volt mains via a variac to the likely 230 volt winding, or if you're sure, just connect it to mains.

I tried the latter. There was little or no hum and a quick test showed 12 volts at the winding used for tests so all was well. I also found several other windings. One delivered about 150 volts, others various low voltages, but one to my surprise gave 960 volts. Thinking back into the mists of time I remember salvaging this transformer from an old oscilloscope, hence the high voltage winding. When testing windings it's an idea to use a dummy load to get an idea of the power rating of the winding. Calculate a resistor value and required wattage, connect it across a winding and if the voltage remains say 80% you can assume its good for the tested current. You can also gauge a rating from wire thickness if the wire is visible and enamelled copper but if its a multi-stranded wire connected to a thin wire used for the winding then assume it probably has a low current rating.

If the transformer doesn't like having mains applied you'll hear a loud humming and if you leave it a sizzling sound and maybe smoke. This might happen if you're inadvertently testing an audio transformer.

One transformer in my junk box declared itself to be from the Admiralty and was 38VA. The tags weren't labelled but by testing I determined it was designed for 240 volt and had two 7.5 volt windings when connected to mains. The "38VA" means that each low voltage winding can simultaneously supply 19VA. Assuming the voltage drops to 6.5 volts under load each winding can supply 19 divided by 6.5 = 2.92 amps or lets say it can supply a nominal 6 amps with the windings in parallel or two supplies of 3 amps each. The primary winding will consume 38 divided by 240 = 158mA so can be protected typically by a 330mA time delay fuse.

If you require a high voltage nowadays, say around 200 to 300 volts there is little choice in mains transformers, however you can use a 12 volt output transformer connecting to a similar one reverse connected. Efficiency won't be wonderful but you'll get your HT from a simple diode bridge connected across the original 230 volt primary, now employed as a secondary. In fact you can expect up to 240 x root 2 = 339 volts or so which will reduce to say 320 volts at lowish currents and suitable smoothing arrangements.

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