PC Power Supply Transformers

[MrWizerd]

Are these transformers useful?  I have plugged in MANY of the numbers found on them and can not find any datasheets.  Is there a way to test them to find out there parameters?  I have been salvaging these power supplies for awhile (I have like 4 more to go and have done 6 already) and there are plenty of useful things, but the transformers have multiple input output pins and I don't know if they are all linked or .....   I have a known 24v transformer that I could use to plug into some of the pins and measure, to see if I can get an output but I worry I will overload them or something, I am not positive what there specs are.  Some of the bigger ones I suppose can transform supply voltage but again without any test procedure I am a little in the dark.  Any suggestions are welcome, if pictures are needed I can take tons, I have around 25+ of these in many different sizes.

The only thing I have found that may be relevant in a cursory search of the web was this site.

http://www.voltech.com/Articles/086-627/003/3_4_Testing_Switched-Mode_Power_Supply_Transformers

[bitseeker]

The easiest way to determine all these pins is to check them with a multimeter while the power supply is turned on.

If the supplies were not able to be powered, one thing that would be helpful is taking photos of the PCB that the transformer is mounted into. That way you can easily see which pins are for the primary winding (that's the high-voltage side). Then, the other pins are for the secondary winding(s), which are generally lower voltage than the primary. To know which pairs of pins are on the same winding, use the continuity tester on the multimeter. If a pair of pins is open, they don't belong to the same winding.

To figure out the ratio between the primary and one of the secondaries, you can use a function generator at a low voltage, say 1V or 10V sine wave, and 50-100Hz. Put that signal into the primary and measure the AC voltage at the secondaries you identified earlier.

If you don't have a function generator, you can measure the ratio of the resistance of the primary vs. secondaries since windings with more turns have more wire and, hence, more resistance.

Note that I'm not a transformer expert, so there may be better ways to accomplish this.

[Seekonk]

I can make just about anything out of anything, but have never found anything useful to make out of these.  Review PC power supply schematics to get typical winding arrangements. 

[T3sl4co1l]

It's easier to take note of the circuit they come from, than to reverse-engineer the part (and potentially have to take it apart and count turns!).

A standard ATX power supply uses a half-bridge inverter stage, supplied by 320VDC, switched at 50-100kHz with a TL494.  This puts useful limits on how many turns, amps and how much flux the transformer handles on the primary, and what the output turns are (for 5V and 12V).

Likewise, flyback supplies (typically the standby/aux supply in an ATX, or most power supplies under 100W) may have a 90-250VAC input, or a more limited range, in either case being around 300V or less on the primary side, probably at frequencies of 50-130kHz or up to 400kHz.  Typical circuits for the controller chip (e.g., UC3842, TOPSwitch/FPS/etc.) are almost always recognizable.

There isn't much point using a transformer outside its intended range; you can scale frequency and voltage proportionally (half frequency == half voltage), and for flyback applications, you have some freedom over how high the flyback goes (say +/- 50% from intended application).

Of course, you can always use a transformer backwards, providing your circuit is also backwards.  (Example: an ATX supply has a FWCT rectifier and choke-input filter.  You'd use a 12V supply, series choke, and push-pull transistors to drive the "secondary" (which is now your primary), and instead of a half bridge on the HV side, a full-wave doubler to rectify it.)

Reusing small flyback transformers is a good way to get modest voltages (say 100-500V at ~mA) from battery or low voltage sources.

Tim

[Cliff Matthews]

Take a look at a good cross-section of SMP schematics: http://danyk.cz/s_atx_en.html As Tim noted, circuits are known by the control chip. Most supplies are 2-in-one (main and standby) with the newer ones being able to support standby currents in excess of 3amps (eg: usb charging). I've seen many sub-boards for the stby supply on a separate pcb on stand-offs screwed onto the top of a heatsink.

For reviews and teardowns, be prepared to have opinions changed about favorite brands here: http://www.hardwaresecrets.com/category/power/ and here: http://www.jonnyguru.com/modules.php?name=NDReviews