I want to know if there's a way to operate a normal (250 VAC primary rating) transformer at higher primary voltage levels, as I have an AC voltmeter project using a microcontroller (currently using Arduino Uno), with over voltage detection.

The controller will perform a specific task for undervoltage case, a specific task for normal voltage case, and a specific task for high voltage case (>250 VAC rms).

I have accomplished the voltage detection using a simple circuit shown below, in which, the output DC voltage has a certain mathematical relation with the input AC voltage.

But my only concern is whether the 250 VAC (rms) primary rated transformer will survive high voltages (upto 400 VAC rms). If not, what are the ways that can help increase the primary voltage rating. For example, combining 2 transformers, or anything?enter image description here

  • \$\begingroup\$ If a 250 V transformer is run at 400 V it will run into saturation, the primary current will shoot up and its protection circuit should trip. What other scenario are you trying to detect? If you are monitoring the transformer secondary then it should be fairly simple to simulate a rise in voltage there. Are you trying to detect transients or long-term? \$\endgroup\$ – Transistor Feb 27 at 14:11
  • \$\begingroup\$ If you combine 2 transformers, how does that increase the voltage across the primary? If you have 2 primaries in series then the voltage is halved. \$\endgroup\$ – Solar Mike Feb 27 at 14:11
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    \$\begingroup\$ Wind more turns on the primary. \$\endgroup\$ – user_1818839 Feb 27 at 14:13
  • \$\begingroup\$ By 'halved', do you mean that at 400VAC, both transformers will receive 200VAC? (And hence, we're still within the 250VAC rating limit)? \$\endgroup\$ – Hamza Shabbir Feb 27 at 14:14
  • \$\begingroup\$ Did you mean 400Vac rms or peak with a pulse width? That makes a difference. \$\endgroup\$ – Tony Stewart EE75 Feb 27 at 14:26

You can use two identical transformers with their primaries in series if you put their secondaries in parallel. This will guarrantee that they share the input voltage equally.

Transformers core sizes are closely matched to the operating voltage. Little increase over the design voltage is needed to saturate a transformer, which will result in an orders of magnitude increase in current being drawn. In the best case, this will take out the fuse in under a second. In the worst case, it will set the transformer on fire in seconds to minutes.

  • \$\begingroup\$ Thank you Neil. It makes sense. Connecting the two primary windings in series will divide the 400Vac rms to 200Vac rms on each winding. Hence, we will still be in the 250Vac rms rating of each transformer. Right? \$\endgroup\$ – Hamza Shabbir Feb 28 at 10:05
  • \$\begingroup\$ @HamzaShabbir ONLY if you connect the secondairies in parallel to enforce voltage sharing. If you leave the secondaries open circuit, or unequally loaded, then they may split unequally. In practice, that's probably OK, as the transformer seeing the higher voltage will saturate, which limits its voltage, but it's still not polite. \$\endgroup\$ – Neil_UK Feb 28 at 12:22
  • \$\begingroup\$ Got it @Neil-UK. Thank you so much! :) \$\endgroup\$ – Hamza Shabbir Feb 28 at 13:07

my only concern is whether the 250 VAC (rms) primary rated transformer will survive high voltages (upto 400 VAC rms).

Most commercial transformers will operate close to magnetic core saturation and so any significant increase in the AC supply voltage may result in fuses blowing or breakers tripping.

Plus, the insulation rating between primary and secondary is chosen to withstand overloads based as a multiple of the peak AC handling voltage so, you are at significant risk of endangering lives by using a 250 volt rated transformer on 400 volts AC.

If not, what are the ways that can help increase the primary voltage rating. For example, combining 2 transformers, or anything?

Use the correct transformer for the job is my advice. Putting two transformer primaries in series does not guarantee that they will share the applied voltage equally due to manufacturing tolerances in the windings (+/- 50% I've seen). You would also need to ensure that they were loaded symmetrically on the secondary.


If the only purpose of the transformer is to detect (in a binary fashion) the higher voltage (not to act as a power supply, for example), there may be other ways of accomplishing that task.

If you put a capacitor in series with the primary that is rated for mains X service that has a low impedance relative to the magnetizing current it should limit the current when the voltage exceeds the normal range.

You could also design something to operate from the mains and use an isolator (opto or some other means) to transmit the information.

Also note that over/under-voltage protection relays are a standard (Commercial, off-the-shelf) industrial product, and will typically handle 400VAC RMS for a 230VAC version.


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