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I recently bought a small transformer to transform 230V AC mains to 7.2V DC to power a small IC and relay. I wired it all up, but as far as I can tell, the output is AC rather than the DC I expected.

Here is the data sheet for the transformer : http://www.mantech.co.za/datasheets/products/P58.pdf
I have the P01172

I can't see anything on that datasheet that specifies one way or the other. If I don't see anything specified, should I assume that it does not convert?

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    \$\begingroup\$ Transformers inherently work on AC. The datasheet was written under the quite reasonable assumption that someone perusing a datasheet of a transformer would at least know what a transformer is. \$\endgroup\$ – Olin Lathrop Oct 19 '14 at 13:29
  • \$\begingroup\$ What was the IC and relay supposed to do? did you have a fuse in the circuit? Depending on the chip, putting an AC voltage where it expects DC can fry the innards - unlike Hollywood, there isn't always smoke & sparks to warn you of this. \$\endgroup\$ – Alan Campbell Oct 19 '14 at 14:09
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    \$\begingroup\$ You're probably just best to buy a small wall wart / AC adapter that has a DC output. They use a switch-mode supply and will likely be smaller, safer, more efficient and probably cheaper by the time you add a rectifier, regulator and caps. \$\endgroup\$ – PeterJ Oct 19 '14 at 14:20
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I think you have been led astray by terminology. As others have noted, a transformer converts one AC voltage to another AC voltage level, while providing some electrical isolation. In your case, it converts 230V Ac to 7.3V AC.
What you may have been thinking of is a small power supply that sits in a plastic container and plugs straight into a wall. Some people call these "wall warts" and some call them... "transformers". Yes, they CONTAIN a transformer, but they also have rectifiers and (in the better ones) voltage regulators to give you a nice steady DC voltage.

If you want DC, add a good power diode to one of the output pins, then place a capacitor across the result. If you use a big capacitor, they have a polarity marking.

WARNING: You are now playing with circuitry powered by mains electricity! While 7.3V sounds tame, you have a 230V input, and that is DANGEROUS.
With that in mind:
Buy A Multimeter A decent one will have AC and DC voltage ranges. In fact, having one means you would have been able to answer your own question (AC or DC voltage present).
UNPLUG your stuff from mains if you can - it's a basic safety precaution. Put everything in an enclosure, to keep out curious children / pets / etc.

Electronic goods can be fun, but the mains isn't really the best place to start. You're trying to power something else - a chip and a relay. Whatever they do, you should be focusing more on that, than on mains power supplies as an introduction to electronic goods.

Maybe you know all about safety already. However, none of the other answers I saw addressed this point, and it's really not something we should "assume" everyone "just knows".

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  • \$\begingroup\$ Perfect - thank you. You are absolutely correct that my confusion was a terminology one. In the shop I can buy a 'Transformer' that I plug into the wall and get DC. If I had a mains outlet close to where my unit will be deployed, I would certainly use that instead. Instead I will add a rectifier into my unit. Thanks for the warnings about messing with mains, I am already extremely nervous of it! \$\endgroup\$ – Toby Mills Oct 20 '14 at 10:09
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Transformers always output AC. You need rectification after that to get positive halfwaves followed by a capacitor(s) to smooth it out decently. If you power integrated circuits, you need a linear regulator also, to get smooth DC, say 7805.

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Yes, and also from the schematics labelled with INPUT and OUTPUT you can see that this is just a plain transformer without fancy rectification circuit: it's really just two coils.

Something else that you can see this from is the fact that the datasheet doesn't specify which output is positive and which negative.

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  • \$\begingroup\$ Just two coils and a non-air core as indicated by the stripe between the coil stripes. \$\endgroup\$ – EM Fields Oct 19 '14 at 17:33

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