AC>DC transformers - Page 1
Forum section: electronics / beginners

author: Mufasa
author_rank: Newbie
timestamp: November 10, 2014, 08:20:18 am
content: Sup all, i recently ordered an wall voltage> 20-60w 14v output transformer (power supply) for a project i'm working onit arrived probably a week or so ago, and now that i'm getting around to using it, i realise that it's an AC>AC adapter (project requires DC output)i'm far from an expert on electronics, but my understanding was that a transformer would supply DC output unless special care was used to invert it into an AC currentthat said, is it possible i can just short out some components, or remove some, and get an efficient DC power supply, or no go?i have some rectifiers/diodes i can use otherwise, but i'd rather not have to have an external device attached to it, and having to seal it up and make it look neat and safe otherwisehere is a picture of the open unitand listing on ebayhttp://www.ebay.com.au/itm/390716386103?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649i bought this one, because it was significantly cheaper than a wall adaptor, and since i will be mounting it inside of something, it would be a better solutionany help appreciated

author: German_EE
author_rank: Super Contributor
timestamp: November 10, 2014, 09:02:36 am
content: What you have is almost certainly an inverter for lighting although the output at 14V is a little high. The basic theory of power supplies and transformers is as follows:1) Transformers ONLY work on AC. They take the 230V mains voltage and step it up or down depending on the ratio of turns between the primary coil connected to the mains and the secondary coil. So, if you have 200 turns on your primary and 10 turns on your secondary then the ratio is 20:1 and a 230V input will give an 11,5V output.2) To convert AC to DC you need a device called a rectifier or diode. In theory a single rectifier can be used to do the job but most power supplies use a device called a bridge rectifier that contains four diodes in a single block. This gives you a DC voltage but there will be a lot of hum.3) After the rectifier there is normally a capacitor, and in power supplies it's going to be a fairly large device in a metal can. This capacitor smooths out the peaks and eliminates most of the remaining hum, it also stabilizes the output voltage a little as the load varies.4) Finally, if the voltage across the capacitor is too high you can step it down using a regulator. This circuit is normally a single chip plus a handful of components to ensure stability.The following link shows the complete circuit:http://www.eleinmec.com/figures/016_04.gif5) There are also units called switched mode power supplies that still use a transformer (that's what you have) but they work on different principles and they are not easy to modify as they are more complex.Finally, a request. power supplies involve access to mains voltages and if you're not happy dealing with them or you don't know what you're doing PLEASE ASK FOR HELP. In any event it's a good idea never to work on mains powered circuits alone.

author: Tandy
author_rank: Frequent Contributor
timestamp: November 10, 2014, 09:03:21 am
content: A Transformer is inherently an AC device, the AC is required for the transformer to transform the voltage. The AC power from a transformer requires an additional circuit called a rectifier to convert AC to DC. Most portable devices use DC power so the typical wall wart power supply contains a DC rectifier.You have two real options.1. Buy a new PSU that provides a DC output.or2. Include a bridge rectifier in your circuit so it can be powered from AC.

author: Zero999
author_rank: Super Contributor
timestamp: November 10, 2014, 07:44:52 pm
content: That's not a straightforward transformer. It's an electronic transformer which outputs a high frequency AC voltage, rather than the low frequency 50 or 60Hz from the mains.It works like this: AC =(rectifier)> DC =(oscillator)> high frequency mains voltage AC =(transformer)> 12VACIt's more efficient to convert the mains voltage to a higher frequency and then pass it through a transformer because a high frequency transformer is more efficient and much smaller than a mains frequency transformer. The mains needs to be converted to DC before it goes through the oscillator because electronics require DC to work.Unfortunately this kind of transformer is difficult to use for anything else but lighting because it requires a minimum load in order to draw enough current to allow the oscillator to start properly.

author: Mufasa
author_rank: Newbie
timestamp: November 10, 2014, 10:09:20 pm
content: Ah, fair enoughfor some reason, i had it in my head that the mechanism of transforming a voltage also converted it to DC, but recalling some power supply circuits, that doesnt really make senseI guess i will have to find a new transformer then.

author: djQUAN
author_rank: Frequent Contributor
timestamp: November 11, 2014, 01:40:17 am
content: Here's a video from Mike a while ago about those transformers.