Circuit Surgery Regular clinic by Ian Bell
Transformers and LTspice – Part 1
T
his month, we will look at the
basics of transformers and some aspects of simulating transformer circuits in LTspice. A transformer is a passive electronic device that transfers electrical energy, in the form of alternating current, from one circuit to another without using an electrically conductive connection. A transformer comprises two or more coils (looped conductors) in close physical proximity, one of which is used for the energy/signal input (called the primary winding). This coil creates an alternating magnetic field which, by virtual of their closeness, passes through the other coil(s) (the secondary winding(s)), producing a voltage across these windings, which will cause current to flow if they are connected to a load. Transformers come in a very wide range of formats, from tiny surfacemount RF devices to the huge (size of a house) power transformers used in national electrical distribution networks. In between are the transformers used in linear and switched-mode power supplies, pulse transformers used in communications, audio transformers and many other specialist types. The key properties of transformers are that they provide electrical isolation between Magnetic flux
Changing current Induced vo ltage Primary
Secondary
Fig.1. Basic transformer: two coils linked by magnetic flux.
Simulation files Most, but not every month, LTSpice is used to support descriptions and analysis in Circuit Surgery. The examples and files are available for download from the PE website. 58
circuits, they can change voltage levels and they change they effective impedance of a load connected via a transformer rather than directly.
Electromagnetic induction Electric current (DC or AC) creates a magnetic field around a wire. If the wire is wound in a loop the field in the centre of the loop is more concentrated. The fundamental physics of the transformer is called ‘electromagnetic induction’ and was discovered by Michael Faraday and Joseph Henry in the 1830s (for which they were honoured by having important SI units named after them – the farad and henry). Electromagnetic induction is the creation of electromotive force (‘emf’, measured in volts) across an electrical conductor by a changing magnetic field. In a transformer, a changing current in one conductor creates a changing magnetic field which induces an emf in another conductor. A changing magnetic field is required for electromagnetic induction, so although a steady (DC) current creates a magnetic field, AC is required for transformer operation.
Electromotive force Electromotive force is not a mechanical force – it is the electrical action produced by a non-electrical energy source (eg, chemical energy from a battery or the electromagnetic induction in a transformer). It drives current to flow in a conducting circuit or produces voltage across an open circuit due to the separation of charge. For an open circuit, the charge separation creates an electric field which opposes the separation of charge in balance with the emf driving the separation. The open-circuit voltage is equal to the emf.
Transformer action Fig.1 shows two coils in close proximity. Applying a varying current to one coil will create the magnetic field, which is visualised as magnetic flux lines. Some of the magnetic flux will pass through the second coil, resulting in an induced emf (and hence voltage across the coil). Not nearly all of this flux passes through the second coil so this arrangement will
Magnetic flux
Core
Phasi ng dot
Changing current + –
+ Induced – vo ltage
Primary: N1 turns
Secondary: N2 turns
Fig.2. Transformer with core – this is much more efficient than the transformer in Fig.1. produce a poor transformer – it will not be efficient in transferring energy from the secondary to the primary. The situation can be improved by using a transformer core, as shown in Fig.2. If the core material and structure is carefully chosen (particularly its magnetic properties), then the majority of the flux will be contained in the core and it will therefore pass through both coils, resulting in an efficient transformer. In the ideal case 100% of the flux is delivered to the second coil. Transformer circuit symbols usually consist of back-to-back coil/inductor symbols corresponding with the windings. Lines between the coils may be used to indicate the type of core material. Some examples are shown in Fig.3. Fig.2 indicates the direction of the input current and induced voltage. In order to use the transformer correctly it is often necessary to know which way round the connections are. This is indicated by using dots on the device and on the schematic symbols, known as phasing dots. The dotted terminals have the same instantaneous voltage polarity – when the dotted primary is being driven by the positive half of the AC cycle the dotted secondary will have a positive polarity with respect to the non-dotted terminal.
Air core
Ferrite/metal powder core
Metal (iron) core
Fig.3. Transformer symbols – note the cores and phase dots. Practical Electronics | June | 2021
