I have harvested a small transformer from a tablet's charger,but I couldn't really find the exact model.

After searching the net,it was clear that the transformer belongs to the EF20 series,but when entering the exact inscription,I got useless results in Japanese.

I find it hard to distinguish the part number from the series code when judging the marking on the back of a component in general,if I never heard about that component name before and if there is a mix of letters and numbers which represent different things.

A datasheet I found:EF20 datasheet

Transformer inscription:EF20-0502-SDC JT
Charger model:BSC12-050200-E

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I want to download its datsheet.Please help me find it.

  • 4
    \$\begingroup\$ Those little high-frequency transformers for mass-produced power supplies are very often custom-made by the 'transformer factory' for the 'power-supply factory'. There is usually no publicly accessible datasheet for them. \$\endgroup\$
    – brhans
    Commented Sep 14, 2016 at 13:35
  • \$\begingroup\$ I guarantee you it's custom. You need to undwind it to find out the details. \$\endgroup\$
    – winny
    Commented Sep 14, 2016 at 13:35
  • \$\begingroup\$ You can get some useful information by measuring its primary inductance, leakage inductance, winding resistances and turns ratio. \$\endgroup\$
    – user16324
    Commented Sep 14, 2016 at 13:49
  • \$\begingroup\$ Your link seems to return error 404. \$\endgroup\$ Commented Sep 14, 2016 at 15:11
  • 1
    \$\begingroup\$ @Dmitry I fixed it.It should work now. \$\endgroup\$ Commented Sep 14, 2016 at 16:01

2 Answers 2


That is the high frequency power transformer in the switching power supply. It is probably intended to have up to 400 V on the primary, chopped at a few 100 kHz.

Since this power supply works down to 100 VAC input, and it is too small to probably have PFC, it is probably intended to produce a bit more than 5 V square wave out with a 140 V at maybe 200 kHz square wave in.

Transformers are proportional, so you can feed it a much lower square wave as input and see what comes out. The winding with the highest DC resistance will be the primary. Put whatever your function generator can do at maybe 200 kHz in, and see what comes out of the other windings.

You're not going to find a datasheet, since this is probably a custom transformer made to whatever spec saves the most pennies for this mass-produced power supply. When you buy 1M transformers a year in lots of 100k, you can get exactly what you want at very low prices, especially when cost is the overriding concern, and quality only means it doesn't catch fire or die within a year.

  • \$\begingroup\$ As a general rule of thumb,how many chances do I have to get a transformer out of a power supply and not have it as a custom part made especially for that product?Be able to find the datasheet?It's like a lottery,but is it worth playing it? \$\endgroup\$ Commented Sep 22, 2016 at 14:20
  • 1
    \$\begingroup\$ @Dan: For a high-volume and cheap product, the chance of it not being a custom part pretty much 0. Of course you can guess and measure a reasonable number of specs, so some of these may still be useful for hobby projects. \$\endgroup\$ Commented Sep 22, 2016 at 15:28

That's a typical flyback transformer as used in this kind of product. I note that the product itself lacks any kind of visible safety approval markings, so there is a good chance the transformer itself is sub-standard and therefore potentially dangerous to life and property. Typical places where they would cut corners would be the use of insufficient internal creepage (the primary often sandwiches the secondaries so galvanic isolation is really important) and sub-standard materials such as the use of regular cheap magnet wire where better stuff should be used. Unlisted tape and such like may not be directly dangerous but it's not guaranteed to be safe enough for first-world markets.

In answer to your question- it's undoubtedly custom made for the circuit they are using. If you look up the chip number (assuming they are using a chip such as Power Integrations rather than some self-oscillating junk) you may be able to get a rough idea of what it is.

If it's not burned out, you should be able to measure the primary inductance. It's almost surely a flyback transformer so it will be gapped (have the ferrite machined away- usually in the center post but sometimes in all 3 legs- or with a spacer adhesive to provide an air gap) to allow a higher saturation current. The secondary inductances will give you an idea of the turns ratios (square root of the inductance ratios) and you can guesstimate the wire gauges from the part that's wrapped around the pins. It's hard to get an idea of the exact number of turns non-destructively since the Al of the material with air gap is not known, but nothing a big hammer and box cutter won't solve.

Here is a typical type of circuit (low end supplies may be a bit simpler)

enter image description here

Usually, as shown, there's a high-voltage primary, a low-voltage secondary and an auxiliary power winding for the high-voltage side. The safety-critical isolation barrier (as far as the transformer is concerned) is between the secondary and both of the other windings, which are effectively connected to the mains. If you swap aux and secondary windings it may be more unsafe than it already is.

  • \$\begingroup\$ Speaking of chips,there are three.The optcoupler ,a pwm controller and a shunt regulator. \$\endgroup\$ Commented Sep 14, 2016 at 16:18

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