A schematic diagram is a graphical representation of an electrical or electronic circuit. Schematic diagrams use standard electrical symbols which are generally drawn to represent the types and operation of the components they symbolise. Magnetic coils can take on many forms as an inductor, a winding, a choke, a solenoid, or the coils of a transformer with or without a magnetic core, so the graphical symbol of these components must demonstrate that are indeed coils of wire and not just a sequence of semicircles or loops.
While the schematic symbol of a transformer may look like two coils (called windings) placed next to each other, the magnetic coupling and orientation between these two coils is also indicated within its schematic symbol. Some coil symbols can be labeled with letters and/or numbers to indicate their electrical connections or dots to indicate polarity.
Dot convention markings use dots on the transformer schematic symbol as a way of specifying the winding direction between input and output and therefore the polarity between windings. Phase-indicating dots are marked on each winding of a common core with their position relative to each other showing whether the instantaneous voltages and currents of each winding rise and fall together indicating zero phase shift (0o), or whether one voltage and current is rising while the other is falling, indicating 180 degrees of phase shift (180o) between them.
Either way, the electrical schematic symbols for transformers, inductors and coils offer a simple and visual way of indicating which components are used with in a circuit design. There are many different standard transformer configurations each with its own transformer schematic symbol, but the individual graphical symbols of inductors and transformers given below along with a brief description and explanation are some of the more common ones we use daily.
Schematic Symbols for Inductors
| Schematic Symbol |
Symbol Identification | Description of Symbol |
|---|---|---|
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Air-core Inductor | A fixed value air-core inductor, coil, solenoid or choke which uses either a self-supporting form or a solid or hollow ceramic, plastic, or some other form of non-magnetic material as its inner core for high frequency applications |
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Iron-core Inductor | A fixed value solid iron-core inductor formed by winding the coil around a solid laminated iron core, indicated by the symbols two solid lines, to concentrate the magnetic field generated around itself when energised |
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Ferrite Core Inductor | A fixed value inductor formed by winding the coil around a non-solid compressed powdered ferrite core or bead indicated by the symbols two dashed lines |
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Tapped Inductor | An inductor coil with either one or more fixed value connections called, taps, along its length for impedance matching and tank circuits |
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Adjustable Inductor | An adjustable or continuously adjustable inductor whose self inductance value can be varied from some minimum value to a maxiumum value when adjusted |
Schematic Symbols for Transformers
| Schematic Symbol |
Symbol Identification | Description of Symbol |
|---|---|---|
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Air-core Transformer | Single-phase air-core voltage transformer with two inductive coils wrapped closely together around a solid or hollow plastic non-magnetic core for radio frequency applications |
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Iron-core Transformer | Single-phase iron-core voltage transformer (VT) formed by winding the two coils around a solid laminated iron core, indicated by the symbols two solid lines, for the transfer of electrical energy from one winding to the other changing an AC voltage from high to low or low to high |
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Power Transformer | Single-phase power transformer (PT) shown as two interconnecting circles for the transmission and distrubution of electrical power from high to low or low to high |
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Ferrite-core Transformer | Single-phase transformer formed by winding the two coils around a non-solid compressed ferrite core to decrease eddy current losses, hum and increase the magnetising flux. Used mainly in toroidal transformers |
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Step-down Transformer | Single-phase step-down isolation transformer which converts a higher primary winding voltage into a lower secondary winding voltage by an amount determined by the turns ratios of the transformer |
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Step-up Transformer | Single-phase step-up isolation transformer which converts a lower primary winding voltage into a higher secondary winding voltage by an amount determined by the turns ratios of the transformer |
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0o Phase Shift | Inline dot orientation used to indicate the 0o phase-shift between the primary and secondary windings used to correctly parallel connect transformers together |
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180o Phase Shift | Diagonal and opposite dot orientation used to indicate the 180o phase-shift between the primary and secondary windings resulting in voltage and current inversion |
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Center-tapped Transformer | Single-phase center-tapped voltage transformer with either primary, secondary or both sides divided into two windings allowing for multiple voltage points. Primary center tap allows for dual supplies, while secondary center tap is useful in rectifier circuits |
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Multi-tapped Transformer | Single-phase Multi-tapped voltage transformer either primary, secondary or both allowing for multiple voltage connection and take-off points |
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Multi-load Transformer | Single-phase voltage transformer with one or more magnetically coupled secondary windings to supply individual loads, or the secondary windings may be connected in parallel for a greater current, or in series for a higher voltage |
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Dual-winding Transformer | Single-phase voltage transformer consisting of two transformers on the same core, with the primary and secondary windings of each transformer wound on the same magnetic core. For use in both low and high voltage supplies and psu applications |
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Iron-core Autotransformer | Single-phase step-down autotransformer with one single coil for both the primary and the secondary windings wrapped around a magnetic iron-core and one or more fixed tapping points giving a secondary voltage equal to or less than the primary voltage |
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Iron-core Autotransformer | Single-phase step-up autotransformer with one single coil for both the primary and the secondary windings wrapped around a magnetic iron-core and one or more fixed tapping points giving a secondary voltage equal to or more than the primary voltage |
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Variac | Single-phase variable autotransformer called a variac with one single tapping point which can be adjusted to produce a variable secondary voltage. Does not provide isolation |
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Current Transformer | Step-down current transformers (CT) wound, toroidal or bar type which provides electrical isolation between the high-current carrying conductor and metering device |
