So I got my new 2x6V transformer to the lab finally and hooked it up. As expected from a wound coil of wires it works perfectly. I was, however, briefly mystified by the plethora of wires emanating from this chunky beastie and wondered why the hell the secondaries were reading half a volt AC when it should have been much higher.
Transformer wiring can be confusing to the novice (or drinker) and generally you don’t want to mess it up like I did once by plugging the (low resistance) secondaries into the mains blowing the breaker and marring the plug where it melted due to arcing.
Like many transformers, this one was manufactured with two sets of primaries and two secondaries, allowing for a variety of connections. The two primaries are meant to select between US and Euro mains (~120V and ~240V respectively), and the two secondaries meant to deliver the stepped-down voltage to be filtered and regulated.
Each set is nothing more than two terminals and a coil of wire wound around an iron core – an inductor. When current is run through the coil, it generates a magnetic field that inducts a current on the secondary side converting it back into electricity. The voltage on the secondary is function of the primary voltage and the ratio of the number of turns between the primary and secondary.
When presented with two sets of primaries, it’s a bit confusing to figure out how to wire it into a two wire plug from four terminals. The best way I find is to think of it like batteries (though the same rules hold true anywhere in electronics): voltages in series add and currents in parallel add.
Practically, this means that if you wire the two primaries in parallel you can double the current output of the transformer, and if you wire it in series you double the voltage and halve the current.
This particular transformer is meant to convert 117V to 6V, and has two sets of these. Coming from it are 8 wires, four primary and four secondary (two for each coil). This is quite customizable and could be wired in a variety of ways depending on your application.
For 240V euro mains, I would wire the primaries in series, essentially combining the two coils into one allowing a greater step down to still get the desired secondary voltage. For North American mains I would wire them in parallel to keep it at 120V.
On the secondary side, I could keep the two coils independent and get 6V each, or wire them in series to get 12V, or wire them in parallel to get 6V at double the current output.
Here’s where the fun comes in: if I were to wire the primaries in series (as for euro) and keep the secondaries in series, what would I get? Instead of the expected 12V, I would get 6V because the number of effective winds between the primary and secondary is doubled.
What I ended up doing is wiring the primaries in parallel (making two 120V primaries) and wiring the secondaries in series (making a 12V secondary).
What I am glad to note is that many transformers are rated at just below the typical mains voltage on the primary side, so when it receives a typical mains voltage, the secondary voltage is likewise a bit higher. This is helpful considering voltage regulators have a dropout voltage that must be overcome to regulate the desired voltage and takes care of any irregularities or variances in your home mains voltage.
Couple this with the fact that when you rectify and filter (using a bridge rectifier and capacitor) the secondary AC voltage, you end up bumping it up a bit so you will always get higher than the expected voltage which (hopefully) overcomes the dropout voltage of your regulator.
In my case, I wired it so that the 117V -> 12V transformer, when fed my home 120V mains, then rectified and filtered with a 1000µF cap, yielded 18Vdc and change. Wow, half-again my expected voltage! This is hardly a problem and will work great for my application.
Just keep in mind two things:
- Your rectified and filtered output will be a higher voltage than expected
- Wire your transformer wrong and you will, at best, get the wrong voltage and at worst, a blown breaker