A transformer as an Isolation Device

transformer as an isolation device

Here, we can use transformer as an isolation device. Electrical isolation is said to exist between two devices when there is no physical connection between them.

In a transformer, energy is transferred by magnetic coupling, without electrical connection between the primary circuit and secondary circuit. We now consider three simple practical examples of how we take advantage of this property.

Transformer as an Isolation Device

First, consider the circuit in Figure.(1). A rectifier is an electronic circuit that converts an ac supply to a dc supply. A transformer is often used to couple the ac supply to the rectifier. The transformer serves two purposes.

First, it steps up or steps down the voltage.

Second, it provides electrical isolation between the ac power supply and the rectifier, thereby reducing the risk of shock hazard in handling the electronic device.

transformer as an isolation device
Figure 2. A transformer used to isolate an ac supply from a rectifier.

As a second example, a transformer is often used to couple two stages of an amplifier, to prevent any dc voltage in one stage from affecting the dc bias of the next stage.

Biasing is the application of a dc voltage to a transistor amplifier or any other electronic device in order to produce a desired mode of operation.

Each amplifier stage is biased separately to operate in a particular mode; the desired mode of operation will be compromised without a transformer providing dc isolation.

As shown in Figure.(2), only the ac signal is coupled through the transformer from one stage to the next. We recall that magnetic coupling does not exist with a dc voltage source.

Transformers are used in radio and TV receivers to couple stages of high-frequency amplifiers. When the sole purpose of a transformer is to provide isolation, its turns ratio n is made unity.

Thus, an isolation transformer has n = 1.

transformer as an isolation device
Figure 2. A transformer providing dc isolation between two amplifier stages.

As a third example, consider measuring the voltage across 13.2-kV
lines. It is obviously not safe to connect a voltmeter directly to such
high-voltage lines.

A transformer can be used both to electrically isolate the line power from the voltmeter and to step down the voltage to a safe level, as shown in Figure.(3).

transformer as an isolation device
Figure 3. A transformer providing isolation between the power lines and the voltmeter.

Once the voltmeter is used to measure the secondary voltage, the turns ratio is used to determine the line voltage on the primary side.

Transformer as an Isolation Device Example

Determine the voltage across the load in Figure.(4).

transformer as an isolation device
Figure 4

We can apply the superposition principle to find the load voltage. Let vL = vL1 + vL2, where vL1 is due to the dc source and vL2 is due to the ac source. We consider the dc and ac sources separately, as shown in Figure.(5).

transformer as an isolation device
Figure 5

The load voltage due to the dc source is zero because a time-varying voltage is necessary for the primary circuit to induce a voltage in the secondary circuit. Thus, vL1 = 0. For the ac source,

transformer as an isolation device Hence, VL2 = 40 V ac or vL2 = 40 cos ωt; that is, only the ac voltage is passed to the load by the transformer. This example shows how the transformer provides dc isolation.