Instructions for Specifying a Transformer

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Specify Application: Always tell us what kind of equipment the transformer will be used in, so we can make sure it will meet all requirements for that application. (Examples:"CRT monitor for medical equipment;" "Commercial audio amp.")

  • Ambient temp. is normally 25-40ºC (95-104ºF).
  • Cooling method is normally convection through chassis. Specify only if different.
  • Size/Weight restrictions: Indicate the actual dimensions desired, diameter and height, or the maximum space available. Indicate if their are any weight restrictions.
  • Leads are 8" long magnet wire self-leads except if the wire is smaller than 25 gauge, then multi-stranded leads will be used. Specify only if different.

Fill in the rest of the application data if you know the requirements, but leave it open if you are not sure.


Specify Primaries: If you do not find the combination you want, check off an empty line under the desired configuration, and fill in your data. 2 + 2 primary as listed covers most AC voltages in use worldwide. Click here to see primary winding configurations.

Specify Secondaries: AC data and DC data are "either/or" alternatives. Do not fill in both for the same secondary.

AC Data:

Specify AC when a secondary is loaded by an AC motor, a heater, incandescent light bulbs, or other linear impedance
UP/UPnom: Specify UP/UPnom if US is specified for other than nominal voltage on the primary.
Duty Cycle: Specify Duty Cycle and the length of each cycle if Is is not for continuous duty.
Isolation transformers and step-up/step- down transformers:This type of transformer is normally specified for AC load, even when it is used to supply equipment with rectifiers. NOTE: the RMS current in a transformer with rectifier load varies with the supply voltage, and is also affected by the data for the transformer. Always specify THE LOAD TYPE for such a transformer. If the load is a Switch Mode Power Supply, we need to know the rated power and efficiency for the Switch Mode Power Supply, plus the lowest supply voltage it shall run at.

DC Data:

Specify DC when a secondary supplies a rectifier. Specify either Uc or Ucv, never both of them. See chart.
Uc: DC Voltages as measured by a DC voltmeter.
Ucv: The valley point of DC voltage with ripple. If you specify Ucv, you must also specify the size (µF) of the filter capacitor.
Ic: Ic is the DC current drawn by the load when Uc or Ucv has the value specified in the same line. ALWAYS SPECIFY THE NOMINAL DC CURRENT AND CORRESPONDING DC VOLTAGE. ALSO SPECIFY IF THERE IS A PEAK CURRENT.
Circuit: Specify as FWB, CTB, FWBL, or DHWL. (See OTHER SPECS if FWBL or DHWL were specified.)
CTB circuits provide + Uc and - Uc or equal value, but the currents in the two halves need not be equal. Use two lines with the same Secondary # to specify a CTB rectifier, with "+" for Uc and Ic in one line, and "-" for Uc and Ic in the other.
Load Type: Specify as L,S,A, or R. (See OTHER SPECS if "S" is specified.)
UP/UPnom: Specify UP/UPnom if Uc or Ucv is for other than nominal voltage on the primary.
Duty Cycle: Specify DUTY CYCLE if Ic is not for continuous duty.
Other Specs:

  • If you do not use ordinary silicon diodes, you must specify the type of diodes used. (E.g. "Schottky Diodes.")
  • Specify Resistance and Current Ripple in filter inductor when you specify FWBL or DHWL under CIRCUIT.
  • Specify the value of the filter capacitor (µF) when you specify Ucv (valley voltage)
  • Specify the lowest rated value for UP/UPnom here when you specified "S" under LOAD TYPE and specified UC and IC at UPnom.
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