PC ATX PSU transformer question

[AlxDroidDev]

I have been studying how to create my own small regulated PSU to power my arduino, and my first approach is to scavenge a few parts from PC ATX PSU (I have them by the dozens, usually with blown caps). The parts I am most interested in are the rectifying bridges ICs (4-pin diodes) and the transformers. Some of them also have some very interesting ICs, like PWM controllers and op amps.

This topic, however, is about the transformers.

1) I learned that to test a transformer, I need to test the resistance on the primary and secondary, and they should by small, but higher than 1 ohm, and the secondary has a slightly lower resistance. The problem, however, is that the transformers on ATX PSU read 0 ohms on the primary and secondary. How is it so?

2) Of all the power supply schematics I found on the web, the transformer is the very first component, and connects directly to the wall. On ATX PSU, the much smaller transformers are after the diodes, rectfying bridges, etc. I recon they are not the type of transformers the classic schematics for a general purpose PSU refers to. What kinda of schematics to I need in order to use these much smaller transfomers?

3) Why are they so much smaller? How can I use them?

4) What is maximum current an arduino can pull? 1500mA ?

5) How do I properly test these small transformers taken from ATX PSUs?

My idea is to have a regulated variable output (using a 10K potentiometer) + voltage indicator display (already have that, that I got from DealExtreme). The main IC will be a LM317. I have all the parts already, but I lack the knowledge on how to do that using the transformers taken from ATX PSUs.

[DVDdoug]

You probably cannot use the tranformer in a "regular" linear power supply...  Most (all?) computer power supplies are switching designs.    There is an oscillator and the transformers run at much higher frequencies (higher than the 50/60 Hz line frequency).

The problem, however, is that the transformers on ATX PSU read 0 ohms on the primary and secondary. How is it so?

That's the DC resistance.  Transformers run off AC.  With no load, the coils are like an inductor with an impedance (ohms) that's minimum at DC and increases with frequency.

What is maximum current an arduino can pull? 1500mA ?

Not that much...  It should be less than 500mA depending on what you have connected.   The ATmega chip itself can pull a maximum of 200mA if you have the I/O pins "loaded down", and I'm not sure what the USB chip takes.
 

[BulletMagnet83]

It'd be a good idea to research the difference between linear and switched-mode power supplies, and familiarise yourself with the basic "blocks" of each. Components from one cannot easily be repurposed for building the other.

[keeper63]

Also, in case it isn't clear:

1. PC ATX power supplies are all switched-mode power supplies - this is why they are so lightweight (no large iron core transformer).

2. The power supply schematics you see online are almost all linear power supply designs (hence the large and heavy transformer).

Linear supplies tend to be easier to design than switched-mode supplies. Even so, a linear supply isn't a beginner's project, but a small one (like you are wanting) can be a great way to learn the basics first. As others have noted, you won't be able to use any of the transformers from the ATX supplies (as they are meant for switched operation).

Once you have the concepts of a linear supply down, then you can move on to switched-mode supply design (which will likely mean needing a dual-trace oscilloscope as one of your tools, btw).

[AlxDroidDev]

Thank you for the replies, guys.

I can't say those were the answers I was hoping for, but they shed some light on my next steps.

I have a linear (the hefty type!) transformer that is 120V to 15V, but that thing weights almost a ton. Anything built using it will hardly be something portable. It will do the jobs and they schematics are fairly simple and I've understood most of its concepts.

I believe SMPS are a lot lighter and more efficient than linear PS, right? I guess I'll be searching they web for schematics for such projects (simple, small SMPS) so I have something to start with, but if there is one you guys can recommend, it will be very welcome.

About the oscilloscope, I don't have one and I probably won't have one for a few months. They are simply too expensive in Brazil. I plan on getting this one in a few months.

thanks!

[BulletMagnet83]

http://www.youtube.com/watch?v=cM7t1Mpu7s4

I would highly recommend Dave's tutorials on the subject! Good way to get started perhaps?