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Duty Cycle

The duty cycle of a transformer is specified as the percentage of time for which the load is drawing its full rated current, in other words, the fraction of time that the transformer is in an "active" state.

A lower duty cycle allows a physically smaller transformer to be specified because the load will only be used for a short amount of time. If a duty cycle is not indicated, a continuous rating (or 100% duty cycle) is used.

Note: Duty Cycle is only effective if the load is intermittent and the "on" time is shorter than the transformer thermal time constant. Thermal time constants for transformers are dependant on physical mass of the transformer.

Duty Cycle Formula

Where: Power Nom. = Nominal transformer rating
Power Load = Actual power in the load
t on = Load on time
t tot = Total cycle time

Temperature Rise

To Understand Transformer Temperature Rise, Several Items Need to Be Identified:

Temperature Rise: The difference in temperature between a non-operating transformer ("cold" condition) and one at full load equilibrium point ("hot" condition) is called temperature rise. It usually is measured in degrees Centigrade

The temperature rise of a transformer is due to the power loss dissipated by the transformer in the form of heat. The power loss of a transformer consists of core loss and of coil losses.

Coil losses (or copper losses or I2R losses) occur in both the primary and the secondary windings, due to the resistance of the wire in the windings. The average winding temperature rise is determined by measuring the resistance of a winding when it's "cold" and again when the winding temperature has stabilized (“hot”) under full load. From the difference in the resistance readings, the average temperature is calculated for each winding.

Core losses are related to the material grade choice and flux level chosen for the transformer core.

Hot Spot Temperature - The average temperature rise is determined for the entire winding. In reality, the inside of a winding is hotter than its outside. The hottest spot (or hot spot temperature) is inside the coil, determined by the manufacturer on prototype units. The hot spot temperature is typically expressed as a temperature increase over the average temperature.

Insulation Class: represents the maximum temperature permitted in the hottest spot in the winding. This is the maximum temperature that the insulation can reach without degrading. Once this temperature is exceeded, the life expectancy of a transformer will be significantly reduced. Insulation class is usually specified by degree Centigrade rise

Insulation Classes:
Y = 90ºC
A = 105ºC
E = 120ºC
B = 130ºC
F = 155ºC
H = 180ºC

Ambient Temperature: The surrounding temperature of the environment where the transformer will be located to determine the maximum allowable temperature rise of a transformer, take the Insulation class, and subtract away the ambient temperature and the hot spot rise. Toroidal transformers are typically designed for a temperature rise of 55ºC-60ºC maximum, and have a material rating of Class A (105ºC). This temperature rise is above a typical ambient temperature of 30-35ºC. (see below)

35ºC maximum ambient
60ºC maximum average winding rise
10ºC maximum hot spot in winding
105ºC ultimate temperature at hot spot

Transformers can also be derated, and perform at a lower temperature rise, as seen in the chart below. If required, toroidal transformers can be provided with a thermal protector to automatically turn off the transformer at a set temperature

Temperature Rise Graph

Inrush Current

Inrush current refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. When a transformer is first energized a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles.

Toroidal transformers have many advantages over conventional transformers. Unfortunately, one of the consequences of a toroidal transformers’ superior magnetic properties is that the transformer "remembers" what polarity the primary voltage had immediately before the power was last shut off. Whenever the voltage has the same polarity when the transformer next is turned on, the core will saturate for part of a half-cycle, and a high in-rush current will flow in the primary of the transformer.

Large toroidal transformers (1.5 KVA and higher) can have up to 80 times larger inrush than conventional transformers, because the remnant magnetism is nearly as high as the saturation magnetism at the "knee" of the hysteresis loop.

This means that larger toroidals (1.5 KVA and higher) should not be switched on without some caution and consideration for how to deal with the inrush current


An ideal transformer would have no energy losses, and would be 100% efficient. Unfortunately, there are losses in transformers. Efficiency is a function of a transformer's power losses, and two factors account for nearly all of these losses, winding copper loss and core loss.

Pin = Pout + Wloss
where Wloss = Wcu + Wfe

Wcu = watts dissipated through copper losses in the windings

Wfe = watts dissipated through magnetic losses in the core

The toroidal transformer typically requires only 10% of the magnetizing current required by laminated transformers. Higher flux densities are permitted because the direction of the magnetic flux is the same as the steel core grain. High flux densities allow fewer turns of copper wire, reducing the DCR of the winding. The graph shows the efficiency that can be expected in terms of VA power ratio.

Voltage Regulation

Transformer voltage regulation is a measure of the voltage rise on the secondary due to off-load or light-load conditions with the primary input voltage remaining constant. In a transformer, both the primary and secondary currents produce voltage drops across the resistive and reactive components of the windings. This voltage drop causes the primary voltage to be less than the supply voltage, and the secondary voltage to be less than expected.

Voltage regulation of toroidal power transformers is expressed by the following equation:

%Regulation = [(Vnl-Vfl)/Vnl] x100
Vfl = Full load voltage
Vnl = No-Load voltage

Regulation may be improved by using large diameter wire in the windings or a larger core. This technique is accompanied by a slight increase in size and cost. This chart shows voltage drop at full load for standard cores at 50/60Hz.

Thermal Protection (optional)

Thermal protection can be built in to toroidal transformers. Thermal protection is typically added in series to the primary winding, and is in close thermal contact with the windings. Thermal protection may satisfy certain safety approvals

Thermal Sensitive Fuses –Thermal fuses are not resettable and once blown cannot be replaced.

Thermal Switches (thermostats). –Thermal switches are designed to open at a set temperature and will close again upon cooling, reforming the primary circuit (with slight hysteresis).

Thermal protection typically adds to the cost of Toroidal transformers and can affect the transformer geometry by adding a slight bump.

Mounting Methods (optional)

Metal Disk (with gasket) Mounting

• Description - standard method of mounting toroidal transformers.  Mounted to chassis with a dished steel washer and cushioning gaskets holding transformer.  Hardware is held in place by a single bolt passed through the center of the transformer.
• Transformer < 15 lbs (7 kg
• Can be mounted horizontal or vertical in a stable environment

Center Potted Mounting

• Description - The center of the transformer is filled with epoxy potting, and a center hole is drilled through the potting for easy, one-screw mounting. The potting can be recessed to hide a protruding screw head. If requested, threaded inserts can be installed and foam or rubber pads attached to the bottom of the unit.
• Transformer > 15 lbs (7 kg)
• Rough handling environments
• Low profile applications or critical height applications

Cylindrical Enclosure

• Description - The transformer or inductor is potted into a cylindrical enclosure.
• Transformer < 15 lbs (7 kg)
• Provides an attractive appearance
• Rough handling environments
• Mechanical protection from debris

Flange Mount

•Description - If vertically mounted, transformers can be supplied with square mounting brackets to ensure secure installation


Static Shielding (optional)

In extremely noisy environments, a static shield may be required to reduce the capacitive coupling between the primary and secondary. The static shield is typically copper foil laminated between polyester tape inserted between the primary and secondary windings. Since the shield adds layers to the transformer winding window, a larger core size may be necessary to provide adequate inside diameter.

In general, as core sizes go up (or as KVA increases), there is a decreasing need for static shielding

Magnetic Shielding (optional)

Due to the Toroidal cores geometry, the primary and secondary coils are wound concentrically to cover the entire surface of the core keeping the core completely surrounded by windings. This allows toroidal transformers to emit minimal stray magnetic fields. However, a certain amount will always be present (as with all magnetic devices). For the vast majority of applications, toroidal emissions are far too low to affect circuit operation, but there are some applications that are especially sensitive.

Typical applications that can be sensitive to noise include measurement instrumentation, high-end audio, and medical. With proper design, many of the stray magnetic fields can be reduced, or eliminated. In these cases where proper design does not reduce the fields enough, magnetic shielding can be applied around the toroid in the form of a high-permeability metal band. This can be Silicon Steel for the majority of cases or Mu Metal for more sensitive applications. For extremely sensitive circuits, total encapsulation in a steel can or case may be the only option.

Operating Frequency (options)

The operating frequency of a transformer helps determine the transformers size and weight. In general, a higher frequency means a smaller transformer.

A transformer designed for 60 Hz operation only will be smaller and lighter than one designed for 50 Hz or 50/60 Hz operation. A transformer for 400 Hz operation is significantly smaller than a 50/60 Hz transformer.

Primary Configurations (options)

Options Available For Primary Configurations:
•Single Primary

•Dual Primary
  →Separate identical primaries to be used in series or parallel.

•2+2 Primary
  →Two separate sets of identical primaries to achieve common tapped voltages

•Multi-tapped or ladder primary.

Winding Configuration


The RMS voltages and currents for each rectified output must be estimated to determine the equivalent VA, which is much greater than the DC watts output. This is mainly due to the high peak current draw during the short conduction time, which causes the RMS current to increase. See the graph below for FWCT (full wave center tapped, 2 rectifiers) and FWB (full wave bridge, 4 rectifiers) to estimate the equivalent VA due to rectification.


Approximate values for non-rectified AC output regulation are shown below. Calculate the % load (your equivalent VA divided by the model VA) to calculate your approximate regulation.

Actual Regulation = Model Regulation

AC REGULATION               Note: This curve shows regulation @ Rated VA = Actual VA

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