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Toroidal Transformers

Posted by P&A International
P&A International
P&A International is China based company formed by partners with engineering background in 2008. Our main focu...
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on Saturday, 12 December 2015 in General Talk

Why choose a Toroidal Transformer?

Over the years, technology has made a lot of advancements and as a result we have been bestowed with a variety of power saving inventions. In the past, only the most complex power application could justify the use and cost of a toroidal transformation as the machine was extremely costly because of the winding toroidals used. Luckily, technological advancements in engineering have lowered the cost of production of winding toroidals, making them more accessible. The availability of toroidal transformers have become a sought out and cost effective alternative to other forms of laminated transformers in a wide variety of applications.

Benefits of Toroidal Transformer design

The highly effective design of the toroidal transformer is cost-effective and saves energy. Here are some of highlights of toroidal transformer’s design:


The unique and excellent toroidal design of transformer offers great efficiency for its given size and weight. The toroidal core comprises of a powerful magnetic circuit that eliminates inherent air gaps unlike those found inside conventional laminated-bobbin transformers. The toroidal transformer features higher flux density due to full utilization of the core area. This results in the use of smaller components that take up less space. The overall efficiency of the toroidals typically ranges from 90% to 95%, however, with a custom design; the results can be much more favorable. The high quality and tightly wound, grain oriented silicon steel core means maximum utilization resulting in very low core losses. In addition, off-load magnetizing currents are also achievable resulting in higher efficiency.

Low noise and low stray flux field

Because of the highly efficient magnetic circuit and uniform distribution of the windings over the core, the toroidal transformer is surprisingly quiet in operation. In fact the toroidal transformer results with low or even zero mechanical hum caused by magnetostriction. With toroidal transformers you no longer have to worry about noise-inducing stray magnetic fields as it produces waves that are 8 times lower compared to laminated stack type transformers. The toroidal transformer is thus the best choice for sensitive electronic systems including high-gain preamplifiers and instrumentation.

Good regulation

Toroidal transformers guarantee low leakage inductance due to winding configuration. The inductance is created when a certain percentage of the magnetic flux produced by the primary source is not consumed by the secondary source therefore not generating any voltage. This is considered as a major problem in laminated stack transformers. Because of the tight coupling, toroidal transformers are able to utilize the all flux. As a result toroidal transformers produce off-load secondary voltages that are lower to laminated stack type transformers. In addition, toroidal transformers prevent power from being wasted as heat due to lower copper loss.

Ease of mounting

Unlike conventional laminated steel stack type transformers, most toroidal transformers are fixed using only one central screw. This speeds up production time and lowers the number of parts required in mounting of hardware.  However, for custom designs, advanced mounting techniques are used to improve efficiency.

Packaging versatility

Unlike conventional laminated stack type transformers, toroidal transformers with specific characteristics may be varied in terms of height and diameter according to the product-design requirements.  This allows the transformer to meet enclosure space constraints. In order to meet retrofit space requirements tall cylinders or flat discs can be made for low profile applications.


P&A International Standard Toroidal Transformers obey the world’s major safety standards and are built according to the construction files of UL and CSA recognized designs. Custom designs can be produced to meet other standards whether European, medical, commercial or military. All P&A Internationaltransformers are CE marked.

Guide to Transformer selection and specifications

P&A International provides standard general-purpose transformers that offer dual primary windings for operation at 50/60 hertz with either series or parallel connection. P&A International incorporates dual secondaries that may use series or parallel in ranges of voltage and VA ratings in order to suit common power and electronic applications.

toroidaltransformer selection and specification guide

The unique combination of windings results in one component to be specified for both domestic and international use i.e. exports to other countries that have different lines of voltage as well as for providing several secondary power configurations.

If your application requires a power supply configuration does not meet P&A’s standard range of transformers then they will happily customize their services to tend to your needs by designing a custom transformer that matches your requirements. This may range to a slight modification to a completely new custom design.

Below are general guidelines that throw light to the criteria that should be considered before specifying a custom transformer:

Power rating:

Transformers are regarded in Volt Amps (VA) which is the product of rms AC voltage and rms current for a principally resistive load.

Example 1: A heating appliance needs 4 amps at 24 V AC and is to be driven from a 115 or 230 voltage, 60 hertz mains supply.

A 115 + 115v to 24V step down transformer is required along with a VA rating of 96 VA (4x24) for this, 100VA will be suitable.

Example 2: Two Halogen Lamps of 50 watt are connected in parallel and require 12 v AC for maximum brightness. An 115v fan also runs at the same time, which draws 182 milliamps. This combination is to be driven from 115v 60 Hz supply.

For this an 115v step down transformer with 2 separate secondary windings is required.

First Secondary: Should be rated 12v, 100 VA (8.33 Amps)

Second Secondary: 115 x 0.182 = 20 VA

The total transformer rating thus adds up to 100 + 20 = 120 VA

A transformer that supplying reactive or rectifier loads needs to be rated according to the load characteristics. P&A’s engineers are always happy to offer their guidance when it comes to the ratings of any appliance. More can be found out on the section on rectifier transformers.

Duty cycle

In case the load is much shorter than the thermal time constant (the time required to reach a steady on-load temperature, this may take several hours) and is not continuous, a small transformer can thus be specified. The following formula is used to calculate the rating required:

duty cycle calculation for transformers

Operating Frequency

The size and weight are two things in which the operating frequency of the transformer depends upon.  The higher the frequency, the smaller the transformer whereas the lower the frequency the bigger the transformer. Therefore a transformer designed for 60 Hz operation will be smaller and lighter as compared to a transformer that has been designed for 50/60 Hz operation, but in this case the size reduction is only very slight as there is only a difference of 10 Hz. However, for a transformer that has been designed for 400 Hz the size may be up to 80% smaller than a 50/60 Hz transformer. It is thus important to specify the minimum expected operating frequency of transformer as operation may be possible above the designed frequency but would fail to operate at a lower frequency.

Primary Voltage

The primary voltages that have been mentioned in the standard transformer specification (or the ones that have been specified in customer inquiries) are the nominal voltages. However, the supply of these voltages may vary when the line supplies are at heavy loads (this may be during dinnertime or other peak timings) or lighter loads (when you and your family are asleep) the drop varying in the rise and fall of voltages may vary as much as 10% of the nominal depending from country to country. The change of voltage is redirected on the secondary voltage (on and off load) by the standard transformation relationship.

transformer primary voltage calculations


A toroidal transformer is rated at 12v while the primary is rated at 117v 60 hertz. The line supply regulation is started as ± 6% by a utility company.  When the line is at the lowest, the secondary voltage will be:

transformer secondary voltage calculations


The transformation regulation is the measure of voltage rise on the secondary due to off-load or light load conditions with the primary input voltage remaining constant. This is measured as a percentage of the secondary voltage; for example a transformer consisting of a regulation of 10% and rated at 12 Volt at full load will have an off load voltage of 13.2 Volt.

The regulation is calculated as:

transformer regulation voltage calculations

The design characteristics of transformers thus inversely depend on the power rating (VA) and are approximately linear for any given load on the secondary. In the above example, we can deduce that the load was 50% of the full load, thus the voltage would then be 5% higher or 12.6v. This regulation needs to be kept in mind in the process of designing rectifier power supplies etc. This would affect the rating of the reservoir capacitors, voltage regulators etc.

However, in this case the custom transformers can be designed with very low regulation figures; however, this depends on the expense on the size and weight of the transformer. Larger cores and wire gauges would be required.

typical toroidal transformer regulation chart

Temperature rise

P&A’s  standard toroidal transformer are able to withstand a temperature rise of maximum 60 ̊C and have a material rating of Class A 105̊C  while the winding wire that has a rating of Class F (155 ̊C) for further reliability. The temperature rise may be above the ambient temperature which is 30-35 ̊C.

If the surrounding components are able to tolerate high temperatures then a reduction in transformer size may be advised. P&A Internationalis qualified enough to manufacture transformers up to most standard temperature classes, however, the cost of manufacturing the transformer may vary depending on the materials used. So, it is important to furnish expected ambient temperatures in a custom inquiry for any temperature class. The running temperature of the transformer of transformer will be the ambient temperature in addition to the temperature rise.

Some standards temperature classes

Y=90̊ C

A=105 ̊C

E=120 ̊C

B=130 ̊C

F=155 ̊C

H=180 ̊C

Capacitive shielding

By nature transformers are wide band devices when it comes to stray signal coupling. A shielding layer is interposed between the primary and secondary windings  if incase a transformer is required to operate  in an electrically noisy environment, this minimizes the capacity between the two windings, helping to eliminate common mode noises however its effectiveness highly depends upon the noise characteristic including the transformer’s overall surface area. The shield may at times be required to successfully satisfy safety regulations and circuit configurations. In addition capacitive screens are also needed, they add layers and cost to a toroid’s overall build. If capacities screens are required then a larger core may have to be specified which would require a larger inner diameter to support the windings.

This kind of sheading should supplement and not be used to replace the line filters and suppressor networks required to operate circuits or EMC compliance.

Magnetic Shielding

While toroidal transformers only emit minimal stray magnetic fields by nature, a certain amount is always found in all sorts of magnetic devices. Compared to most appliances toroidal emission is considered as very low in order to have any effect on circuit operation.  However, some appliances may prove to be more sensitive than others.

This may include high gain instrumentation, high-end audio, wideband, high resolution CRT circuits. However, for these cases the magnetic shielding may be applied around the toroid in the shape of a highly permeable metal band. In most cases Silicon Steel or Mu Metal are used for such sensitive operations. On the other hand, for even more sensitive applications, total encapsulation in steel can or case may be the only option. There are many ways in which you can reduce emission by altering the design that is before any protection is added to the transformer) if your circuit is this prone to magnetic interference then you should provide specifications when requesting a quote.

In rush currents

Toroidal cores create an excellent magnetic circuit and carry high remanence, which is a result if the high number of square hysteresis loops that these cores possess. The high inrush currents can be encountered when switching on large toroidal transformers. This produces better results when compared to conventional laminated stack transformers and can last for a few half cycles of the mains voltage. This is mainly caused as a result of the core saturation though for only a split second. This is considered very normal.

However, this means that larger toroidals which are of 1.5KVA or even higher should not be switched on without taking necessary precautions. In such cases it is recommended that slow-blow type T fuses be used while making primary circuits especially when using transformers that are over 100 VA. For even larger transformers, NTC thermistors or circuit breakers that have been designed for motors and transformers may be used.

In addition normal relay-switched resistor soft start circuits may also be used to produce efficient results, however, a delay may be caused of about 30 to 300 millisecond. Keep in mind that some relays have pull-in delays of approximately this time. Soft start circuits should be used in addition with and should never be used to replace the proper circuit protection provided by fuses or circuit breakers.


The traditional method of mounting toroidal transformers to a chassis or by using a dished steel washer, the transformer needs to be interposed between two cushioning gaskets so that the hardware is held in place using a single bolt that is passed through the central hole of thee toroid. The mounting kit is supplied with standard transformers while other mounting options may also be considered, just do your research.

Warning: Keep in mind that the metal chassis should not be in contact with both ends of the mounting bolt. This may cause a shorted turn that would cause the transformer to overheat leading to its destruction.

Varnishing and vacuum impregnation

Full vacuum and pressure varnish impregnation are provided by Avel, along with envelope dipping with mold-resistant polymers and additional protective barriers. Keep in mind that the process of impregnation and dipping cannot be carried out with the help of either solvent based varnished or solventless epoxy varnishes. For further information you may contact Avel.

Thermal Protection

Thermal protection can be provided to toroidal transformers using thermal sensitive fuses and switches, commonly known as thermostats. These protectors are generally attached to the primary winding and are thus in close contact with the windings. Keep in mind that thermal fuses may not resettable can cannot be reused after blown. Thermal switches are designed to operate according to temperature change, the open at a set temperature and close when the temperature goes down, reforming the primary circuit. The protectors used are present to satisfy certain safety approvals. For additional thermal protection, you may contact P&A International however keep in mind that this may increase costs.

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P&A International is China based company formed by partners with engineering background in 2008. Our main focus is manufacturing of skill and technology intense parts and components in China. Been a small player in the market we work very hard to establish ourselves by providing our clients personalised service, often going outside of our scope to support them.

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