I have a transformer (no codes) 240vac to 12vac this is easy enough to determine but what about the maximum current?

1) I could load it until the voltage drops but to what exactly?

2) Is there any way to determine this from measuring the inductance and series resistance or any other measurement?

  • \$\begingroup\$ Assuming a mains transformer, some will use the weight as a guide: somewhere in the range of 35 W to 55 W per kg. (And 50 Hz would be different than 60 Hz.) So you could just weight it, to get a ball-park figure. Then you can place an appropriate resistive load (use wire-wound/ceramic high wattage resistors) on the transformer and measure the resulting voltage across the resistor. Check out the transformer's own heating at the time. If it remains relatively cool, increase the load a little. You'll reach a common-sense decision soon enough as to what it can handle. Or find the datasheet. \$\endgroup\$ – jonk Apr 17 '19 at 19:12

Typically the VA is determined by the internal wire temp rise due to primary and secondary losses. P=Ip²Rp+Is²I

But most small transformers are rated by impedance with a 10% Vdrop from no load to rated load, giving a VA rating for resistive loads and derated up to 30% from reactive loads.

  • \$\begingroup\$ Except that the really cheap and crappy transformers provide sometimes worse than 30% regulation on a simple resistive load. (Of course, there are times when you want that behavior. So I don't mean to reflect poorly upon well-designed transformers where this kind of regulation is desired [neon transformers come to mind.]) \$\endgroup\$ – jonk Apr 17 '19 at 18:59
  • \$\begingroup\$ I have been lucky with XFMR's in the < 150VA range being close to 10% drop from no load. The sure way is to measure short circuit current briefly and open circuit voltage and chose only 90% of Voc and 10% of Isc. to get VA rating. THen verify with that load on temp rise. Voltage Transformers are usually 8 to 12%pu \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 17 '19 at 19:35
  • \$\begingroup\$ @Sunnyskyguy EE75 I was wondering about rectified with smoothing capacitor and maybe a choke - would that be in 30% range. \$\endgroup\$ – onepound Apr 18 '19 at 8:29

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