The Basics of Isolation Transformers and How to Select and Use Them

Traditional single-phase power wiring consists of a hot wire, a neutral wire, and a ground wire. When multiple, physically separated devices share a common power line, it is possible to create ground loops due to devices having different ground potentials. These ground loops are especially problematic in medical devices and can be troublesome during device test. For designers, it is difficult to measure ground loops with devices that use rectified line voltages. Grounded test equipment, like oscilloscopes, can inadvertently short power supplies in these devices. Also, high frequency noise can ride on AC power lines causing problems for sensitive transducers and instruments.

All these problems can be avoided by the proper application of isolation transformers between the power input and the device.

Isolation transformers provide separation from the power line ground connection to eliminate ground loops and inadvertent test equipment grounding. They also suppress high frequency noise riding on the power source.

This article will discuss the characteristics, selection criteria and application of isolation transformers using example devices from Hammond Manufacturing, Bel/Signal Transformer, and Triad Magnetics.

How isolation transformers work

Isolation transformers provide galvanic isolation between the AC power lines (mains) and the powered device. That means that there is not a DC path between the two windings. They serve three main purposes:

• The first is isolating the secondary from ground (earth)
• The second is to provide step up or step down of line (mains) voltages
• The third is to reduce line noise being transmitted from the primary to the secondary or vice versa

Isolation transformers, to begin with, are transformers, and they share the common characteristics of transformers (Figure 1). Primary and secondary windings are wound on a common ferromagnetic core.

Figure 1: The schematic of a simple power transformer consisting of a primary winding of N_P turns and a secondary winding of N_S turns on a common ferromagnetic core. (Image source: Digi-Key Electronics)

In the figure, the primary winding has N_P turns wrapped around the core and the secondary winding has N_S turns. The relationship between the primary (V_P) and secondary voltage (V_S) is shown in Equation 1:

 Equation 1

If there are more turns on the primary than on the secondary, then the voltage on the secondary will be less than that on the primary. This is a step-down configuration. If the number of turns on the primary is less than the number of turns on the secondary, then the secondary voltage will be higher than that on the primary, resulting in a step-up configuration. Most isolation transformers have the same number of primary and secondary turns so that the primary and secondary voltages are the same.

Energy is conserved in transformers so, if we ignore losses, the product of the V_P and the primary current (I_P) will equal the product of the V_S and secondary current (I_S). Transformers are rated by the product of the RMS voltage of the primary times the RMS primary current. This is the “apparent power” and it is measured in units of volt-amps, or VA.

The dots on the schematic are phasing dots which show the primary and secondary current directions. Current flowing into the primary dot side of the winding results in a secondary current coming out of the dot side of the winding as shown in the diagram. This is important if windings are to be placed in series or in parallel. Failure to observe the phasing of the winding can result in errors.

The Faraday shield is an electrostatic shield that reduces the capacitance between the primary and secondary windings and is generally grounded. The shield reduces the amplitude of common-mode noise and transients through the transformer.

The primary and secondary windings in the isolation transformer are highly insulated to minimize direct conduction between them. The measure of the effectiveness of this insulation is the leakage current. Most isolation transformers are also tested using high potential or hi-pot testers. These apply a high voltage across the insulation while checking for current leakage.

The physical structure of the isolation transformer can take several forms, including a shell type structure (Figure 2). Here, the primary and secondary windings are wrapped concentrically with an insulating layer, and the Faraday shield inserted between the two layers.

Figure 2: Cutaway view of an isolation transformer using a shell type construction where the primary and secondary windings are wrapped concentrically with an insulating layer, and the Faraday shield inserted between the two layers. (Image source Digi-Key Electronics)

The Faraday shield can be implemented as a foil layer or as a closely spaced winding, as shown. Grounding is generally on the primary side, to an earth ground. Since the primary and secondary windings already use enameled wire, this construction is called “double insulated”.

Alternatively, the windings can be placed side-by-side on the core, in what is termed a “split bobbin” construction, or wrapped on a toroidal core.

Commercial isolation transformers

Isolation transformers may be distributed with open frames or may be enclosed in a shielded structure (Figure 3). Hammond Manufacturing’s 171E isolation transformer uses a shielded enclosure. The end cap shields contain the magnetic field of the transformer, and also serve to minimize pickup from fields external to the transformer. This 500 VA, 1:1 transformer also includes pigtail, NEMA, three-wire grounded input and output connectors, and an integral overload circuit breaker.

Although the ground is wired to the secondary output connector, it will not be used in most isolation transformer applications. This transformer has less than 60 microampere (µA) leakage current between the primary and secondary at its rated input voltage.

Figure 3: An example of an isolation transformer with shield covers over the end caps of the transformer. (Image source: Hammond Manufacturing)

The DU1/4 from Bel/Signal Transformer is a 250 VA isolation transformer that uses open frame construction that has a dual set of multi-tapped windings. There are two primary and two secondary windings (Figure 4).

Figure 4: The Bel/Signal Transformer DU1/4 is an open frame isolation transformer with a dual set of tapped primary and secondary windings. (Image source: Bel/Signal Transformer)

The primary and secondary windings are identically rated at 0, 104, 110, and 120 volts. This permits series or parallel connections on either primary or secondary. Therefore, a nominal 1:1 ratio can be maintained for inputs of either 110 or 220 volts. A step-up transformer from 110 volts to 220 volts or a step-down transformer from 220 volts to 110 volts can also be configured. Additionally, the multi-tap windings allow intermediary voltage ratings such as 208 volts, 214 volts, or 230 volts (Figure 5).

Power connections for this transformer are by means of screw terminals.

Figure 5: The dual winding of the DU1/4 permit many possible wiring configurations including 1:1, 2:1, 1:2 voltage ratios. (Image source: Digi-Key Electronics)

If the primary and secondary are each wired in series, the transformer is a 1:1 voltage ratio for a 220 volt input. If the primary and secondary are each wired in parallel, the result is a 1:1 voltage ratio for 110 volts with twice the available current compared to a single winding. If the primaries are placed in series and the secondaries in parallel, the primary voltage is stepped down by a factor of two. If the secondary is wired in series and the primary in parallel, then a 2:1 step-up is realized.

Medical isolation

Isolation transformers intended for medical applications have to meet more stringent requirements in regard to leakage currents. There are maximum leakage current specifications for ground or earth leakage, enclosure leakage, and patient leakage. Ground leakage refers to leakage currents in the ground lead of a device. Enclosure currents describe currents that flow from an exposed conductive surface to ground via a conductor other than the ground lead. Patient leakage is current that flows through a patient to ground when connected normally to the device. Most devices in this category are certified under UL/IEC 60601-1.

Triad Magnetics’ Model MD-500-U is a 500 VA isolation transformer rated for medical applications (Figure 6). This transformer is certified by Underwriters Laboratories (UL) under specification UL 60601-2 and has a leakage current of 10 µA typical and less than 50 µA maximum.

Figure 6: The MD-500-U is a 500 VA isolation transformer rated for medical applications. It has a leakage current of 10 µA (typical) and uses a toroidal transformer to keep it compact and minimize stray fields. (Image source: Triad Magnetics)

The MD-500-U uses a toroidal transformer that minimizes stray fields and maximizes efficiency while minimizing its size. Like most standalone medical transformers, it is securely contained in a steel enclosure with integral fuses and a thermal cutoff switch.

A typical isolation transformer application

The most common application for an isolation transformer is to isolate a device from the AC line ground. As an example of why this may be necessary, consider the switched mode power supply (SMPS). A typical line powered SMPS presents several safety related concerns (Figure 7).

Figure 7: The schematic of an SMPS showing circuit areas that are ground referenced, and those which are not. (Image source: Digi-Key Electronics)

This is a line powered supply utilizing flyback topology. The primary side of the circuit, shown in the yellow highlight, full wave rectifies the line (mains) input and applies it to the primary rails. This means that the voltage levels occurring between the high and low voltage rails are about 170 volts for a 120 volt line, and about 340 volts for a 240 volt line. This rectified line voltage is stored in the primary storage capacitor, C2.

Note that the primary and secondary sections of the supply are electrically isolated by the flyback transformer, L2, and the optically isolated coupler, Q4. While the secondary section is connected to ground at the negative (-) output terminal, the primary is ungrounded. This condition becomes problematic when employing grounded input instruments such as oscilloscopes for troubleshooting. Connecting the ground connection of a scope probe to the components in the primary side of the supply may result in a short circuit with attendant damage to the primary components as well as the oscilloscope.

The low primary rail in the supply is connected to the AC line neutral. Although the neutral line is connected to ground at the service entrance, by the time it reaches the SMPS input it may be several volts above ground, making it an unsafe connection point for a scope probe ground.

The purpose of the isolation transformer is to electrically isolate the primary section of the SMPS. Once isolated, it is possible to connect the ground side of a probe anywhere in the primary circuit. This places the ground reference at whatever point the ground clip is connected to, eliminating the possibility of shorting the primary.

This same ground isolation ability makes isolation transformers useful in diagnosing and correcting ground loops when multiple devices, each having its own ground return path, are connected together.

The transformer(s) permits isolating the grounds to see which devices are the source of the ground leakage current.

Isolation transformers also reduce high frequency noise being transferred either from the line to the connected device or from the device back into the line. This is due to the series inductance of the transformer and the grounded Faraday shield which reduces capacitive coupling across the transformer.

Conclusion

By isolating devices connected to its secondary winding from the AC source on the primary, isolation transformers permit a redefinition of the reference plane on the secondary devices. This also permits redirecting and controlling leakage currents. At the same time, they minimize the transmission of high frequency harmonics and noise. They are immensely useful for testing power related devices.