Notes:
All models can be customized for different output voltages and currents
60 Hertz VA is approximately 120% of the 50 Hertz VA value
A* Dimension does not include the E Dimensions
D** Dimension is the height of the potted center
E*** Dimension is the expected bulge for the lead wire exits
**** Multiple holes is recommended for mounting hardware
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Standard Power Toroids
In order to optimize your model selection within our standard sizes, check each of the factors below. Calculations for each figure are based on T_{AMB} = 55° C, and expected temperature rise, unless otherwise noted:
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Efficiency
The figure below shows typical curves for efficiency vs. output VA rating and loading:
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DC Output Applications
A transformer VA rating must always exceed the WDC rating due to the equivalent circuit series resistance, rectifier voltage drop and high peak currents during the charging of output capacitor. Typical values are shown below for our standard sizes:
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Full Wave Bridge
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Full Wave Center-Tap Output
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Regulation (AC applications)
As size increase, the inherent regulation decreases. Absolute values depend on the actual VA vs. rated VA, T_{AMB}, and T_{RISE} for any given size.
% Regulation = (V_{NL} – V_{FL}) / V_{FL}
Where NL = No Load and FL = Full Load
For actual VA used, New % Reg = % Reg (VA_{USED} / Model VA) approximately
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Duty Cycle
If intermittent loading occurs on an output winding, the equivalent sizing VA calculations are affected.
VA_{Equivalent} = SQRT ((VA_{1} t_{1} + VA_{2} t_{2} + …..) / T)
Where one output may have one or more loads for on times of t_{1}, t_{2}, etc., with a total duty cycle of time T, expressed in seconds or minutes. As T gets large, the calculated equivalent VA will have to increase due to the thermal time constant for each size (from less than 1 hour up to several hours).
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Temperature Considerations
Efficiency, DC and AC regulation, and temperature rise are all dependant on the T_{AMB} and loading. The figures below will help estimate T_{RISE} for different values:
Where the New TRISE = (Model TRISE) x (Load Factor) x (Ambient Factor)
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In-Rush Current
Our standard models have very low loss, high permeability core material. Because there is virtually no build-in gap, in-rush current is higher that laminated core structures. The residual flux density can be quite high if the transformer was last shut off at peak flux density. If the transformer is turned on with the same voltage polarity, the peak flux generated adds to the residual, resulting in a saturated core, which drives the permeability down temporarily. Inductance of the primary winding is proportional to the permeability, and thus can create a high peak current, which decays over several cycles to the normal operating current. Slow blow fuses or circuit breakers with delay may be needed to withstand the peak currents. For very large toroids, other techniques may be needed in addition to the above protection. Custom cores can be manufactured at additional cost to further limit in-rush current.
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Shielding
Electrostatic shielding between winding can be added with slight reduction in Model VA rating shown. |