Misc. Voltages from Transformer

[Zouave]

So, the transformer I purchased came with a 12VAC and an 18VAC output in addition to the 2 45V outputs that were my primary goal. So, I need to get the 12VAC to 12VDC to power some fans I scavenged from a computer I had lying around. Is there an inexpensive DIN Rail transformer that will do the job for me, and possibly even rectify and smooth out the ripple? I now that's asking a lot, but eh...
Failing that, I have a 2200uF capacitor off an old washing machine, its of the linear type (lead coming out each end), and given that the fans are rated at .33A, 12 V, if I'm only running one, that cap should be fine, right?
(80,000 * .33)/12 from the (80,000*A)/V equation to determine the necessary capacitance. Is the .33 a peak value? Could I pretty safely run 2 fans off that same line using the 2200uF cap? At worst, it will, what, have extra ripple? Will that shorten the length of life of the fan?
Any suggestions on how to approach this little side project while I wait for my Geckos and BOB to arrive?

Should I just go with a resistor attached to a heat sink instead of dealing with a transformer?

Alternatively, (and I may be completely off target with this...) Could I use the 18VAC (up to 25VDC when rectified/capped), assuming that I'll have a marginal voltage loss, and run two 12VDC fans with that by splitting the current, or am I stuck at 25VDC at that point?

[domino11]

Eric,
The 12VAC winding will give you about 16.97VDC. This will probably be a little high for your fans. With a toroidal transformer it is not all that hard to wind on 10 turns of wire and measure the output. You can then scale from there to get about 8.5 to 9.0VAC to give you 12VDC. The 2200uF cap will probably work fine for the fan application.

[Zouave]

I am a little hesitant to wrap on a new winding, I was hoping to use the existing ones to accomplish my task. What about removing the plastic covering and unwinding the 12VAC out down till it putting out around 9VAC (which should rectify/cap up to 12VDC)?

What about the other possibility of using the two fans on a 25VDC lead, and using a resistor to burn off the extra volt I won't be using in the circuit? On the plus side, if I could use the 24VDC, and run both fans off of it, the 2200uF cap would be almost perfect for the job, whereas if I'm running two fans off a 12VDC, the calc for that cap puts it at 4400. (.33A fans)

Also, what is the tape that covers the transformer? I'm assuming it is some time of thermally resistant, non-conductive tape, and would need to be applied over whatever windings I might add.

[domino11]

Why the hesitation to wind a new one, but it would be ok to unwrap the existing one? I would leave the transformer as it is and wind the new winding to do what you need. You could add a resistor that would drop the right voltage at your run current, but you would be adding the extra heat from that. If you want to go that route with the 12V secondary winding, make sure your resistor is grossly oversized. At about 5v drop needed, for one fan you would need a 15ohm resistor and for two fans, an 8 ohm resistor. The 8 ohm would be dissipating about 3.5W. I would want to use at least a 10W or preferably a 25W chassis mount resistor.

[bradm]

I second what Heath says, and I've done it. Just take some insulated wire and start wrapping - don't modify any part of the existing wrapping.

[Zouave]

You don't have to take off the tape that is around it? That makes it a lot easier, I was assuming you'd want it inside with the other windings.

My hesitation stems mainly from the fact that I would rather put the existing leads to use, rather than have a bunch of spare wires coming out that I don't need/use. Why add more wire, when I have wire already there, doing nothing... But, if I don't have to unwrap anything, that makes it a lot easier to deal with.

Thanks for the edit, by the way.

I have a fair amount of 18ga, solid core wire lying around. That should be sufficient for the task, correct? Then just wrap it with electrical tape to keep it in place once I have it outputting what I want...?

[Richards]

Eric,

Drive over to Fry's at 4100 Northgate Blvd and pick up a 12VDC power supply. If Fry's is too far to go, look at www.Jameco.com (Belmont, CA) and browse through their 12VDC power supplies. I've often bought parts from Fry's (including the Sacramento store) and I usually stop in at Jameco when travelling through the area. Be aware that some of the sub-$10 units will require special Molex connectors that may be impossible to find unless you drive into San Jose and rummage around Halted Electronics or other "surplus" stores. I normally just buy the "Meanwell" brand from Jameco that has screw connections.

You don't need a regulated switching power supply, but for $10 to $20, it will work. Be sure to buy one that is rated for the current that you'll need. If a 1-amp PS is required, then just buy a 1-amp to 1.5-amp PS. Anything larger will just generate extra heat and electrical interference.

[danilom]

For just powering the fans you can use an voltage regulator, L7812 and connect it to that . If it gets hot put him on a small aluminium plate for heatsink.
http://www.electronicecircuits.com/wp-content/uploads/2010/02/7812-Voltage-Regulator-lm7812-IC-pin-configuration.jpg

[KenC]

I run 12Vdc fan with rectified 12Vac without capacitor cap for years... didn't even bother to measure what is the actual voltage of the rectified voltage...
In your case... its free, every day the fan is still running is a winning day :)

I personally will not recycle capacitor... especially big value electrolytic caps... not a safe move.
Imagine all your expensive electronics covered with electrolyte from the blown cap... Ugly... & expensive...

[Zouave]

Thanks everyone, I managed to secure a 12VDC power supply from a friend of mine with a large drawer of junk electronics, I may go that way, though I'm still looking at running it off the transformer for maximum use of what is already available...

But I just realized something else... I may have wired the transformer incorrectly. Off of Antec's site I was looking at a power supply, and I noticed in their wiring diagram that it says: "115Vac – 2 red wires to neutral & 2 black wires to hot." I had never seen a red wire going to neutral, and the wiring diagram that came with the transformer was ambiguous at best, so I went ahead and put reds to hot, blacks to ground/neutral. Obviously, it looks like I did this backwards. Kind of wishing that Antek sent a better diagram with their transformers, but my bad for not finding a wiring diagram to be sure...

So, would that have any potential impact on my outputs? And did I, in fact, wire it up backwards? (Glad I haven't seen any smoke...)

Moral of the story: Check everything. And then check it again.

[KenC]

Its AC, there are no polarity. so you are safe.