I want to connect such a low-value resistive load to a transformer that it draws ~1.6x the rated output current but I'll only do so for short periods of time so the transformer won't overheat. Aside from a drop in output voltage, is there anything that could occur that I should be aware of or transformers don't care as long as they stay cool?

I'm DIY-ing a JBC soldering station similar to Unisolder and MarcoReps' design and I have a question about the transformer VA rating that I need. This all started when I designed my station around a 130VA, 20V transformer and it worked really well, but the transformer barely got warm to the touch(IR thermometer clocked it at 36C) even when running at a high duty cycle while trying to heat a large copper bar. My iron works very similarly to JBC's and the above DIY designs where it only passes half or full waves to the iron with no phase control, reducing noise and interference.

This analysis of the internals of one of the JBC stations reveals that it only has an ~80VA transformer despite the station being listed as a 130W station. Further, based on the Vp values measured, the transformer appears to only output ~18Vrms under load instead of the 23.5V listed on the product page. I do see this other post about the power rating, so I understand to an extent how the voltage numbers make sense, but the power numbers still don't.

Based on all this, how would my transformer behave when I exceed the output current limit? My choice of 80VA, 18V transformer has an output current rating of ~4.4Arms. But the 2.5ohm cold-resistance of the tip would result in a 7.2Arms draw at least when the tip is still cold. From my research, the VA rating for large grid transformer applications is based on heating effects which won't be an issue for me since I'll be operating at very low duty cycles for the most part and my time-averaged power will be <80VA. However, could anything else occur due to these larger current draws? I was thinking saturation of the magnetic field in the core could play a role here? Will this just manifest as a slightly lower peak voltage?

  • \$\begingroup\$ ”Aside from a drop in output voltage, is there anything that could occur that I should be aware of or transformers don't care as long as they stay cool?” How short is a short period? A hotspot somewhere? \$\endgroup\$
    – winny
    Mar 9, 2021 at 19:38
  • 1
    \$\begingroup\$ Remember that the transformer will be much hotter at its center than at its surface. 36C at the surface could be hot enough internally to cause damage. \$\endgroup\$ Mar 9, 2021 at 19:53
  • \$\begingroup\$ check to make sure you aren't getting a voltage spike on the primary, esp. with a cold heater \$\endgroup\$
    – Pete W
    Mar 9, 2021 at 20:05

1 Answer 1


As you mentioned, there are two main problems with overloading a transformer: 1 - Overheating 2 - Voltage drop

Apparently you already have an estimate on both issues, but a detail about the transformer is that when you require 160% of load, the losses in the windings will increase to 256% of the rated losses, that is, the transformer can heat up a lot and the life time of the insulation can be drastically reduced. Of course, the time that it is under low load can compensate for this excessive loss of life during the overload.

About core saturation. In general, there are no problems. The flux densities in the core columns depends directly on the voltages, in which case I understand that you will apply the rated voltage. Due to the overload, the flux density can increase in some points of the core (since the stray field is added to the magnetization field), but the voltage drop, in this case, reduces the magnetization field and generally the thing remains far from saturation .

  • \$\begingroup\$ I forgot about the square relationship, so my RMS power would have to be <80VA, not my average power. Given my peak output power is 130W, that would mean my max duty cycle should be ~38% right? \$\endgroup\$
    – c10yas
    Mar 9, 2021 at 21:14
  • \$\begingroup\$ @c10yas, I suggest that you use a circuit like this (electronics.stackexchange.com/questions/473554/…) to calculate the power you will get at the load. About the duty cycle, if you do not want reduce the life time of the transformer (specifically from insulation), it should be noted that the equations for calculating aging rates are exponential. So, if the transformer exceeds 10°C of the rated temperature, it will lose much more life time than it will save if it the temperature is below 10°C the rated temperature. \$\endgroup\$ Mar 9, 2021 at 22:29
  • \$\begingroup\$ I don't really understand how to use that circuit to do that? I don't have the ability to measure the inductance of the transformer I have and the datasheet doesn't provide it either. The winding resistance is theoretically measurable, but that too would require a milli-ohmmeter at the very least since my multimeter definitely isn't trustworthy in the 100mohn range without 4wire measurement \$\endgroup\$
    – c10yas
    Mar 10, 2021 at 23:20
  • \$\begingroup\$ @c10yas, if the data sheet contains the some information of the transformer, you can use them to calculate the resistance and inductance values of the equivalent circuit. You will need: - Load losses to find the series resistances; - No-load to find the magnetizing resistance; - Excitation current to find the magnetizing inductance; - Impedance (short-circuit voltage) to find the series inductance. \$\endgroup\$ Mar 12, 2021 at 10:43

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