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Custom Design

Custom Design

Our experienced design group uses sophisticated software programs to calculate the optimal and yet conservative solution of your transformer or choke requirements. The software programs that have  proven themselves and been refined for 20 years eliminates much of the “human factor” in your design.  Our programs also allow for highly accurate calculations based on your specified DC data out of a power  supply, which make our prototypes meet your specifications the first time around with no time wasted on  trial and error. With over 35 years in the toroidal power business, you can be confident entrusting Toroid  Corporation with your most complex designs. If you are a novice in the magnetics field, we will walk you  through the specification phase, making sure that you get a design that will work for you.

 Contact us now to get started, or provide us your specifications here and one of our experienced designers will get back to you with a prototype design solution!

Custom Design


The figure shows typical curves for efficiency as a function of output power in relation to nominal output power for different core sizes.


Custom Design

Duty Cycle

A smaller transformer can be designed if the load is not constant and the duty cycle is much shorter than the transformer's thermal time constant (which has a magnitude of hours). Note that the voltage drop increases linearly with the instantaneous current.

Custom Design

Temperature Considerations

Operating temperature is an important safety factor. Our transformers, built for Class A (105°C) operation, are normally calculated for a temperature rise of 60-65°C. Actual increase will depend on how and where the transformer is mounted and how well it is cooled. When higher temperature ratings are needed, we offer UL recognized (UL1411 and UL544) transformers to class B (130°C). Transformers rated for class E (120°C), class F (150°C) and class H (180°C) are also available.

Using a larger core size will reduce temperature rise. Due to the small core losses (in comparison with a laminated core), the temperature rise will drop drastically when the output power is reduced. At half the load the temperature rise will only be about 25% of the rise at full load.

The figure shows how temperature rise varies with the actual output power (S out) in relation to nominal power (S nom) for a given core size.

Custom Design

Voltage Drop

This chart shows voltage drop at full load for standard cores at 50/60Hz. The voltage drop is slightly higher for a 60Hz transformers. A design with an oversized core will reduce voltage drop and temperature rise.

Static Shielding

Static shielding may be required to minimize capacitive coupling between primary and secondary winding when the transformer is used in an extremely noisy environment. Since the shield adds layers, a larger core size may be necessary to provide adequate inside diameter for the shuttle of the winding machine.

In-Rush Currents

A consequence of the superior magnetic properties of a toroidal transformer 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. This in-rush current is larger than the in-rush current in a conventional transformer, but it is of very short duration, so it will not affect slow-blow fuses or magnetic overload protectors with thermal delay characteristic. (Some delayed magnetic overload protectors act instantly if the current exceeds a certain value. These cannot be used with large toroidal transformers unless a current limiting resistor is used as described below.)

For very large transformers (1,500VA and up),it is common to use a small current-limiting primary resistor to reduce the magnitude of the in-rush current, and a delayed by-pass relay to short out the resistor after 30-200 milli-seconds. This method eliminates external voltage dips caused by the in-rush current, but slow-blow fuses or delayed magnetic overload protectors must still be used, as is the case for all types of transformers.

Size, Weight, and No-Load Losses

This chart shows size, weight, no-load losses of our custom made transformers from 20VA-10,000VA using our standard core sizes. Other core configurations are available to meet your needs.


x H
2317.22.4 x 1.2.7
3830.22.8 x 1.31.1
5745.42.8 x 1.81.4
9072.63.2 x 1.82.0
140112.73.7 x 1.92.6
1901521.03.9 x 2.13.5
2201761.04.5 x 1.93.8
3452761.54.5 x 2.55.5
5504402.35.5 x 2.58.1
9507603.56.4 x 2.612.4
1,4001,1204.77.1 x 2.918.0
2,0001,6506.78.2 x 3.324.2
2,7502,2009.48.2 x 4.032.8
3,3002,6409.29.5 x 3.436.5
5,0004,00016.310.4 x 4.756.0
7,0005,60020.611.6 x 5.070.0
12,00010,00031.713.8 x 6.0118.0
rating (VA) is determined by secondary RMS data. Physical
size may vary from above data depending on number of primary
and secondary windings.

Primary Configurations

Custom Design

Rectifier Circuits

Custom Design

Termination of Dual Primaries and Secondaries in Series and Parallel

Custom Design


Terms and Conditions

  • For larger quantity prices, please contact the factory directly.
  • Prices and data subject to change without prior notice
  • Delivery: Small quantities of standards are normally available from stock
  • 30-Day Return Policy of Undamaged Items - 15% Restocking Fee on Standard Transformers
  • 36 Month Product Warranty
  • Terms and Conditions