Discovery: Part 1 - Electricity and Magnetism: Transformers

Consideration of the Linear Power Supply


Doing my best to explain this to you - please help. If you haven’t already followed the thread on charge then I suggest you read it.

Let us consider the behaviour of the input (mains) side of a simple unregulated power supply. The waveforms at the bottom of the above image show the idealised input voltage, the rectified output and the smoothed output.

The application of the laws of electromagnetism need some simplification to get an understanding of what happens. Firstly let us suppose that the output of the transformer is open circuit (infinite resistance) and that the input coil has no resistance. The self inductance of the primary coil when an AC voltage is applied creates an equal and opposite Electromotive Force (voltage). Therefore, no current flows. Otherwise it would blow the fuse! How else could linear power supplies be so very efficient at transferring energy irrespective of output demand.

Now let us consider just the transformer with a simple resistor across the output. A current now flows in the secondary coil to oppose the magnetic field created by the primary coil. So both coils create magnetic fields that cancel out. If 1 denotes primary and n (number of turns), I (current) and V (voltage) then in ideal conditions

I2/I1 = V1/V2 = n1/n2

Taking the current and voltage parts I1 x V1 = Power = I2 x V2

In order words the idealised situation conserves energy.

Power is lost through

  1. Generation of heat in the coils
  2. Generation of heat in the core by eddy currents
  3. Hysteresis loss in the core
  4. Loss of magnetic flux.

The effect of the rectifier bridge and a simple voltage regulator (capacitor and resistor) is to only allow a current to flow in the secondary when the voltage exceeds voltage across the capacitor. So the current in the primary only flows when the output capacitor is able to charge.

The net effect is that a power supply behaves as a very complex electrical device that is very dependent on the current being drawn by the device it is powering. The mains will supply current in a pulsed fashion with complex harmonics based on twice the mains frequency.

Add the fact that each Naim Power Supply has multiple outputs and there are multiple power supplies and you will appreciate the interaction at the mains is very complex. It would be nice to think that all the secondaries started and stopped drawing current at the same instant, but balancing is an almost task.

The reason we have a dedicated mains supply is to reduce the interaction of the domestic devices with the HiFi.


@Darkebear I thought you might be interested in this topic. I wish I had an oscilloscope to take a look at the mains current into the various boxes playing simple music. I fascinated to understand this area as it does have a major impact on what we hear.


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I read an article on measuring AC mains quality posted by @Thomas.

Below is an extract from it about the effects of harmonics, which the previous explains are a consequence of rectification. More posts will follow.

Of great concern in power-quality measurements is the vast subject of harmonics. Harmonics may be seen in either voltage or current measurements of electrical energy in the distribution system. Harmonics are measurable amounts of power that resides in frequencies other than the fundamental, or intended nominal frequency. They are a frequent cause of electrical equipment malfunction and failure, so the task will be to identify and eliminate them.

Because harmonics are caused by non-linear electrical loads, they can generally be eliminated. In today’s electrical environment, non-linear loads are wide spread. Examples are fluorescent ballasts, rectifiers, non-constant-speed induction motors, and equipment such as computers and printers powered by switching supplies that lack power-factor correction.

When these and other non-linear loads are energized, they draw current that is non-sinusoidal. This non-sinusoidal nature gets reflected back through the supply circuitry so the entire system is affected. Harmonics can cause excessive heating in neutral conductors and the overheating of motors, which thrive on pure sinewaves. Correcting the problem can consist of identifying the load(s) causing the difficulty and disconnecting them.


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