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Transformers Back Objectives: Definition of Transformer Uses of a Transformer Step-Up Step-Down Isolator

Transformers Back

Transformers Back PrimarySecondary NPNP NSNS : : Symbol A transformer typically has two separate windings with a Varying number of turns (N). They are the primary winding And Secondary winding

Transformers Back The two lines in the center of the two coils indicate this Transformer has a core. (No lines indicates no core) PrimarySecondary: NPNP NSNS : Symbol

Transformers Back There is another type of transformer called a center-tap transformer (photo). It has many uses in today’s electronics PrimarySecondary: NPNP NSNS : Symbol

Transformers Back The benefit of center-tap transformers is the availability of several voltage levels on the secondary winding. PrimarySecondary: NPNP NSNS : Symbol

Transformers Back Virtually every electronic devices which uses AC has to convert the power to a usable level.

Transformers Back PrimarySecondary: Symbol A transformer is a typically a pair of coils where ones magnetic field acts on the other

Transformers Back PrimarySecondary: Symbol The flow of current through the primary causes current to flow in the opposite direction in the secondary.

Transformers Back PrimarySecondary: Symbol The direction of current is designated by dots on either end of the windings.

Transformers Back RECALL current flows through the primary winding, a magnetic field is produced that causes a current to flow through the secondary winding. PrimarySecondary: Symbol NPNP NSNS :

Transformers Back The current flowing through the primary winding causes the current to flow in an opposite direction through the secondary. PrimarySecondary: Symbol NPNP NSNS :

Transformers Back The current direction is indicated by the dots on each coil. Having the dot on the same end tells us the output is in phase. PrimarySecondary: Symbol NPNP NSNS :

Transformers Back Output Voltage is directly proportional to the number of Turns in the secondary winding (Ns) with respect to the Primary windings(Np). This is called the Turns Ratio. PrimarySecondary: Symbol NPNP NSNS :

Transformers Back The relationship is defined as follows: E p /E s = N p /N s PrimarySecondary: Symbol NPNP NSNS :

Transformers Back Example: Primary winding has 20 turns and the Secondary Winding has 40 turns. The result is the input voltage is Increased by a factor of 2. 10/V(out) = 20/40 >> 400/20 = V(out) = 20 volts. This is a Step-up Transformer NPNP NSNS :

Transformers Back Likewise, if a Primary winding has 40 turns and the Secondary Winding has 20 turns. The result is the input voltage is decreased by ½. 10/V(out) = 40/20 >> 200/40 = V(out) = 5 volts. This is a Step-down Transformer NPNP NSNS :

Transformers Back Sometimes, there is a need to isolate voltage source from the Circuit. This can be done by using a transformer with a 1:1 Turns ratio NPNP NSNS :

Transformers Back NPNP NSNS : You should understand that there is no net power loss In an ideal transformer. That means that: Power (primary) = Power (secondary) I p* E p = I s* E s

Transformers Back NPNP NSNS : Moving I p and I s to one side of the equal sign and E p and E s to the other yields: I p /I s = E s /E p (Inversely Proportional) Current (in)Current (out) Voltage (in) Voltage (out)

Transformers Back NPNP NSNS : Transformers and the secondary circuit have a loading Effect on the source voltage that is proportional to the Turns ratio N p /N s x R 1 = R L

Transformers Back Example: Primary winding has 20 turns and the Secondary Winding has 40 turns. If R1 = 10KOhms the Load Resistance (RL) = (20/40)x10k = 5kOhms. Note the difference in expected current needed to drive the circuit. NPNP NSNS :

Transformers Back Summary on Transformers Formulas: V in /V out = N p /N s I in /I out = N s /N p P in = P out N p /N s x R 1 = R L The Formulas above of are based on the assumption that The transformer is ideal and no power loss occurs in the Transformer.

Transformers Summary Back Transformers take a waveform and scale the waveform according to the turns ratio of the primary coil with respect to the secondary coil. If the turns ratio is greater than 1 the waveform is increased, if the ratio is less than one, the waveform is decreased. Sine wave input Scaled Waveform Transformer

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