I have a transformer which converts 220 Vrms to 9 and 18 Vrms (measured with multimeter, with no load at secondary). Assuming that I'm the manufacturer, how should I determine its output current rating?

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About the primary, I know that its DC resistance is 77.3 ohms, and when connected to 50 Hz 220 Vrms, it consumes 3.3 W (no load at secondary.)

As to the secondary, I know that the DC resistance values are about 3.5 and 7 ohm, and the diameter of the wire used is about 0.65 mm.

The weight of the transformer is about 650 grams.

  • \$\begingroup\$ If you were the manufacturer, you would need to know before manufacturing the voltage and current specs and select wire diameter and turns ratio - then it's simply a matter of testing if the selected design worked out according to specs. How about loading the transformer until the output voltage drops to 9Vrms or 18Vrms, if you know those are the rated voltage specs at rated current. \$\endgroup\$
    – Justme
    Commented Aug 20, 2021 at 8:59
  • \$\begingroup\$ @Justme, 9V and 18V are what I measure with multimeter (no load). \$\endgroup\$
    – apadana
    Commented Aug 20, 2021 at 9:02
  • 2
    \$\begingroup\$ I would calculate the maximum output current by slowly increasing the input voltage under some load and recording the input voltage and currents. Eventually, you reach a point where the input current starts raising quickly without affecting the voltage, resulting in higher losses as you reached the saturation limit of the core (the core can no longer contain the magnetic fields and starts behaving as a resistor instead of an inductor) \$\endgroup\$
    – Ferrybig
    Commented Aug 20, 2021 at 9:20
  • \$\begingroup\$ @Ferrybig, what about fixing the input voltage and measuring output current and voltage while changing the load? \$\endgroup\$
    – apadana
    Commented Aug 20, 2021 at 9:23
  • 2
    \$\begingroup\$ How do manufacturers estimate maximum output current of transformers? - they calculate it during the design phase and test it in the production phase against temperature rise values calculated in the design phase. \$\endgroup\$
    – Andy aka
    Commented Aug 20, 2021 at 9:50

2 Answers 2


Just guessing. A 0.65 kg transformer has approximately 30VA, according to similar transformers being sold on the internet.

You have 230V/2x9V transformer. So the nominal secondary current is approx. 30/18= 1.6A.


Essentially, you should calculate parameters as Impedance of transformer, temperature rising... But you need to choose a Power to Design the equipment.

When you have the power [VA], the electrical current rating can be determined as: I=P/V

In general: electrical current limit is the temperature that its materials used can support.


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