In yesterday's post we covered the reason why we need a power supply: getting the correct voltage to our equipment and providing safety isolation from the wall socket. Today I want to cover transformers and their size requirements: understanding this will be key to getting a grip on how a Switch Mode Power Supply works.
The size of a transformer is dependent on two main factors: how much power you want to get through it and the lowest frequency you operate it at.
If you'll recall the transformer has two coils of wire: an input coil that attaches to the wall socket and an output coil that connects to our equipment. These two coils sit close to each other but do not have any electrical connection between them - instead they share a magnetic connection. When we want to get some power through our transformer we are generating a magnetic field and the size of that field is dependent on how much power we want: bigger field equals more power, smaller field less power. We control the size of the field by the size of the wire and the amount of steel in the transformer: heavier wire and more steel give more power but make for a physically larger transformer. This is why a power amp transformer is really big while a preamp or DAC transformer is really small.
The second factor in transformer size is frequency. I know this sounds technical but it's actually simple. The wall voltage is AC which means it is constantly changing its direction: positive to negative and back again. This is the same as would happen if you had a battery and you simply flipped the battery around so many times a second.
Homes in Europe and Asia have the voltage flipping around 50 times a second and here in the US it's 60 times per second. To transfer energy between the two coils of our transformer, the magnetic field needs to be moving back and fourth as described. If you just put plus voltage into the first transformer coil, it becomes a permanent magnet - but it won't transfer energy into the second coil. Only the movement back and fourth of the voltage creates a changing magnetic filed and it is this change that makes it possible to transfer the energy from one to another.
The faster you change the magnetic field, the easier it is to transfer the energy - the more efficient the process is. This makes sense when you think about it: no movement means no energy transfer, slow movement means a little transfer and lots of movement equals lots of transfer. This is super important to understand so read again if you don't quite get it.
So in the case where we have slower movement, 50 times a second relative to 60 times a second, we have less energy transfer ability in our transformer. How do we solve this? Make a bigger transformer with more steel which will help make a bigger magnetic field to compensate for the slower movement.
Here's what's interesting - the size of the transformer goes up dramatically with even a small change in frequency. 50Hz transformers are sometimes 25% larger than 60Hz transformers depending on the manufacturer.
Since most hi fi manufacturers like PS Audio sell all over the world and do not want to stock multiple transformers for each product, we simply specify all our transformers to work at the lower frequency of 50Hz. Then, when you put it on 60Hz, the transformer just loafs.
Tomorrow we get smaller.