enter image description here

I got this transformer from the AC adapter of an old laptop charger, but upon inspection I could not determine why there were so many pins and also whether or not this could be used as a step-down transformer. The shown side has 4 pins and the other side has 5 pins. I did some research and came across a similar question regarding a similar transformer that was taken out of a computer SMPS.

What I learned is that this is a high frequency transformer and that it most likely cannot be used as a conventional transformer. But in this situation wasn't it being used as a conventional step-down transformer in the AC adapter to step down voltage before reaching the laptop?

  • 2
    \$\begingroup\$ No it wasn't being used as a conventional step-down transformer, not in any laptop adapter less than maybe 30 years old. \$\endgroup\$
    – user16324
    Feb 5, 2022 at 17:32
  • 1
    \$\begingroup\$ Only If you limit your input voltage to about 0.1 Vrms at any normal line frequency in order to not saturate it. So no. \$\endgroup\$
    – winny
    Feb 6, 2022 at 15:31
  • \$\begingroup\$ I don’t know what a “conventional” transformer is… If you feed 50kHz AC into this transformer, you can arrange it so that the output is a stepped-down copy of the input. If you feed 50Hz or 60Hz, it won’t work in a useful way. \$\endgroup\$ Feb 20, 2022 at 3:50

4 Answers 4


In all likelihood, the AC adapter you're referring to uses a switching power supply to step down mains voltage to a low DC voltage.

There are two ways, broadly speaking, to convert a high AC voltage to a low DC one. The first is like this:


simulate this circuit – Schematic created using CircuitLab

Here, a mains-frequency transformer first steps the voltage down, then it's rectified and filtered and either used as-is or regulated to a more stable voltage, whatever the circuit requires. This is how most power supplies were made in the 1990s and earlier.

But mains-frequency transformers, as a consequence of the low frequency at which they operate, have to be quite large, heavy, and expensive. What if you could make the AC higher frequency, to the point that you could use a tiny transformer that weighs only a few grams and costs less than $1? Well it turns out you can, and most power supplies today are much smaller, lighter weight, and more efficient than the ones of yesteryear as a result:


simulate this circuit

Here, the mains input is first rectified to DC, then a switching element (usually a MOSFET) chops up that DC by rapidly (hundreds of kHz or even a few MHz) switching between the rectified mains and ground, feeds that into a much smaller (and cheaper, and lighter) transformer, then it gets rectified again to the voltage that your device needs.

You might think the second method sounds a lot more complicated, and you'd be right--I've actually simplified the circuitry here by a pretty significant margin to make it easier to understand. But all of its components together do add up to be significantly cheaper and more efficient than a mains-frequency transformer, so this is the preferred method today.

There is actually a third common method that doesn't use a transformer at all, known as a capacitive dropper. These are very simple, but should never be used outside of very particular circumstances because they're inherently significantly more dangerous than any power supply that uses a transformer. The transformer gives you galvanic isolation from mains, which limits how badly you can injure yourself or others if something goes wrong; a capacitive dropper lacks this limitation.

  • \$\begingroup\$ That second circuit is wrong - what you have there is a synchronous buck regulator feeding DC into a transformer. Precisely what you don't want. Most small power supplies are flyback converters with one switching device and one rectifier on the output of the transformer, not a bridge. \$\endgroup\$ Feb 5, 2022 at 2:48
  • \$\begingroup\$ @KevinWhite Edited. I must have accidentally hit the power switch to my brain midway through drawing that circuit, I don't know what I was thinking. \$\endgroup\$
    – Hearth
    Feb 5, 2022 at 2:57
  • \$\begingroup\$ Needs a flyback/circulating diode on the transformer primary. \$\endgroup\$
    – Russell McMahon
    Feb 20, 2022 at 1:49
  • \$\begingroup\$ @RussellMcMahon ...let's just pretend the switch was avalanche-rated. Fixed. \$\endgroup\$
    – Hearth
    Feb 20, 2022 at 3:08

Many pins because they use a standard frame. Which is a good thing, it gives mechanical strength.

It's not a step-down transformer, and it's not how it is used in the laptop adapter. In the laptop there is (probably) a flyback voltage converter. It runs a high frequency square wave through the transformer, about 100kHz.

The duty cycle of the square wave is determined by according to feedback that it usually gets through an optocoupler. This is how the voltage on the output is regulated.


Assuming for now that this is a transformer from a forwards converter and not a coupled inductor from a flyback converter.

theres small transformers used for high frequency operation can only handle small number of millivolt-seconds before they saturate, whereas conventional step-down transformer can handle over a thousand

So using this transfmer at line frequency would only work at really small voltages. (like a tenth of a volt, or that ballpark)

If it's a coupled inductor instead of a transformer the above still applies but it will work even worse.


Can this transformer be used as a conventional step-down transformer?


It appears to be, for example, the 50 kHz step-down transformer of a switch mode power supply (SMPS).

Here's the block diagram of a linear regulated power supply.

enter image description here

Both the step-down transformer, that sources the output current, and the filter would be required to be quite bulky as they would be operating at 50 / 60 Hz.

Here's the block diagram of a switch mode power supply.

enter image description here

In this case both the step-down transformer, that sources the output current, and the filter would be quite compact as they would be operating at 50 kHz.


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