eec247 Information on lighting transformers

LIGHTING TRANSFORMERS

Conventional (non electronic) Transformers
Conventional soft steel laminated core transformers consist of a primary winding connected to the mains supply (230 Volts) and a secondary winding, with a 12 Volt output connected to the load. There are a number of methods for keeping the primary and secondary windings separated. The type of interest here is SELV (Separated Extra Low Voltage) as still used in shaver outlets and used for halogen downlighters until the late 1990s.

Modern (electronic) Transformers
Modern electronic transformers use switched mode power supply type technology as used in Personal Computers since the early 1980s. Basically the mains voltage is chopped at high frequency and fed through a toroidal (doughnut shaped) transformer made from high density iron. As the frequencies are much higher than mains frequency, the transformer cores can be smaller, lighter and less complicated to make than conventional transformers. Modern electronic components are also cheap and plentiful. All things considered, modern electronic transformers are smaller, lighter and cheaper to manufacture than conventional transformers.

CONVENTIONAL (SOFT STEEL LAMINATED CORE) TRANSFORMER

The design of conventional transformers use the simple relationship between number of turns of wire on the primary winding (Ns) to the number of turns of wire on the secondary winding (Np) to determine the output voltage (Vout).

Vout = Vin x (Ns ÷ Np).

The thickness of wire determines how much current the transformer can deliver (Iout) and the output power of the transformer is Pout = (Vout x Iout) Watts, assuming it is an ideal transformer with 100% efficiency.

photograph of a conventional soft steel laminated core transformer

Here is a typical soft steel laminated core transformer.

photograph of a conventional soft steel laminated core transformer in circuit with the sections of laminated core clearly visible.

Another type of construction of the conventional transformer is the "toroidal transformer"
where the primary & secondary windings are wound on a doughnut shaped core.
The Toroidal Transformer is slightly more efficient than earlier types of conventional transformer
but is more difficult to manufacture.

The output waveform from a conventional transformer.
The flat top to the sine wave is due to core saturation.

ELECTRONIC TRANSFORMER

Simplified Electronic Transformer circuit.
Basically, the mains input is rectified to DC, then chopped at 40kHz to feed into the primary transformer winding.
The sense winding and Electronic Control adjust the switching signal to maintain the output at the required level.

Photograph of a typical electronic transformer.

Photograph of the internals of a typical electronic transformer.

Output waveform from an electronic transformer.

High Frequency switching waveform from an electronic transformer.
A "zoom in" on the waveform above.

Waveform from an electronic transformer startup.
It takes some time for the electronics to stabilise once power is applied.

The better quality electronic transformers have a slow start to increase lamp life.

Electronic transformer with no load.
A downside of the electronic transformer is that it needs a load (halogen lamp) in the output circuit
before it generates a significant output voltage.
Trying to measure the output from an electronic transformer with a conventional instrument
will give misleading results.

The test gear used to evaluate the transformers

The test equipment and methods used to produce this guide are very specialised
and detailed knowledge & experience is needed to do so safely.

WARNING: Do NOT run a transformer with the internal workings exposed.
Lethal voltages are present and IT COULD KILL YOU.

CONVENTIONAL V ELECTRONIC TRANSFORMERS

Generally, conventional and electronic transformers perform the same task -
converting mains voltage (230 Volts) to a low voltage (12 Volts), suitable for halogen lamps.
Conventional transformers are simple in construction, bulky and expensive to manufacture.
Electronic transformers are more complicated in construction, but cheap to manufacture.
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Output waveforms of conventional v electronic transformer.