# Help me power my project

#### nuggetz

Joined Sep 28, 2009
15
I'm learning about electronics and have been reading the nice books on this site. Thanks to whoever wrote them.

Anyway, now it's time to get some practice in and I need a little help.

The goal:

Build circuit of (2) DC computer case fans rated at 12v (0.60A) and maybe 2 to 4 LED's.

I've already figured out what I need in terms of resistors for the LED's and how to connect them but I'm stuck on how to approach the fans. I'd like to use my wall wart which is outputting exactly 14.40Volts as measured with my multimeter. Its a 12V, 300mA supply.

I know the fans are rated to run at 12V, but will 2 of them run off the same supply or do I need to be looking at the current (mA) rating of the supply instead? That's what I'm not clear on. Both Fans would need 1.2A of current.If that's the case, my power supply @300mA wont cut it right?

In terms of voltage, again the powersupply is outputting 14, I need 12v. Does that mean that I couldn't use the same wall wart to light up (2) 12v fans or am I on the right track with the amperage calculation above?

The answers to this will really help my understanding of electronics as so far I've been toying with fans and LEDs and once I understand this a bit more I can apply this to building simple circuits.

Thanks in advance. Now where did I put that 2A wallwart

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#### GetDeviceInfo

Joined Jun 7, 2009
1,623

#### Wendy

Joined Mar 24, 2008
21,839
Note, wallwarts are usually not regulated. The moment you put any kind of load on that sucker the voltage will drop. This is a general rule, I've used plenty of wallwarts that were regulated, but your measurement would indicate it isn't. For your application this probably won't matter, but you do need to be aware.

#### nuggetz

Joined Sep 28, 2009
15
Note, wallwarts are usually not regulated. The moment you put any kind of load on that sucker the voltage will drop. This is a general rule, I've used plenty of wallwarts that were regulated, but your measurement would indicate it isn't. For your application this probably won't matter, but you do need to be aware.
Thanks Bill. Now I know what people mean when they say regulated. I just checked the wallwart again and it's really a 9v supply with 14v being measured coming out of it.

In general, would I be able to connect both fans and 2 LED's in series? Still not sure if my current supply will work or not as I'm waiting for an explanation on whether or not I should be concerned with volts or current. I'm guessing current which would mean my crappy wall wart needs to go into the garbage. Maybe I should build a regulated power supply. I feel ashamed not knowing this since I read chapters 1-4 of the VOL 1 -DC book at least 4 times. At some point I just need to play with the components or else things wont truly click.

#### Wendy

Joined Mar 24, 2008
21,839
I don't know for sure, but I would try it. Measure the voltage across the fans while doing this, they can survive for a short period of time if the voltage is too high, but I wouldn't continue if it is more than a ½ volt or so. You can also put capactors across the fans to quiten the noise, but this may be overkill.

You comfortable with LEDs?

LEDs, 555s, Flashers, and Light Chasers

#### nuggetz

Joined Sep 28, 2009
15
I've tested one fan directly connected to this power supply and it works just fine. My question is how to calculate the type and quantity of power I need to drive both fans and LEDs. That's what I really need to know. Not knowing much about all this, I read the lable on the fan which says 12V DC and 600mA. How does one determine what sort of power source he needs to connect this to based on those fan specs?

#### Wendy

Joined Mar 24, 2008
21,839
It needs to be 12V, as measured across the fan. More runs the risk of burning out the fan long term, less may be OK, but will draw more current to compensate.

You can use resistors or simple regulator circuits to achieve this.

Another note: never spin a fan up with canned air (or compressed air). If they spin faster than rated the internal counter emf will burn them up every time.

#### nuggetz

Joined Sep 28, 2009
15
Maybe I'm not being clear or I'm over-complicating this. I understand that if a fan is rated at 12v, it needs a 12v supply. However, if I plan to run 2 fans, do I need a 24 volt supply? Where does current fit in here? That's my dilemma. I know that running a 12v fan with my crumby 14v PS will decrease the lifespan of the fan which is why I wanted to understand the logistics of wiring 2 of them together along with some LED's off a single power source. My plan was to understand this a little better and then include any other components in the circuit as I build it, like capacitors or a voltage regulator if needed depending on what kinda supply is required and the rating of the powersupply I might have sitting around in a dusty box in the basement.

BTW voltage drop across a single fan is 12.34volts

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#### ke5nnt

Joined Mar 1, 2009
384
With a good regulated power supply, wall wart or not, everything you need will run off 12 volts. Like someone else said, if you have a crap power supply that isn't regulated, the voltage will fluctuate as you draw load off it, but mostly only in the downward direction, as in 12volts unregulated supply drawing load might drop to 10V.

To answer your question about 2 fans at 12V, you do not need 24V. With a quality regulated power supply, even with 10 fans plugged in, 12V is all you'll ever need. Current is a different thing. Keep in mind that when it comes to regulated power supplies, quality doesn't necessarily mean expensive. I've found good 12V 500mA regulated power supplies for under $5.00. Now a note about current. You'd mentioned that the fans say 600mA. That DOES mean that EACH fan NEEDS 600mA, so, if you have 2 fans, you will be drawing 1.2 amps total for the fans, and then the current for the LEDs, I think you said 2 LEDs, if they're of the 24mA variety, then 2 would be an extra 48mA added on totaling 1.248 amps. Your power supply for this load at a minimum should say output 1.3A, which will cost a little more but not much, just harder to find in a wall wart or so, at least in my experience. If you find a cheap supply with your current requirements, but its 14, 15, 18, 24, or even 30 volts, you can drop the voltage down with a linear regulator. The LM7812 is a good example. As long as input voltage is 2 volts or so greater than the output, 12volts with the 7812, then it will put out a nice stable stream of 12 volts. However, you have to be sure that you can run 1.3amps through your regulator. The LM7812 I think can do it, but you would HAVE to have a heat sink for that component. Thread Starter #### nuggetz Joined Sep 28, 2009 15 So, I have a spare computer PSU. Could I use that in some way as a regulated power supply? #### Wendy Joined Mar 24, 2008 21,839 The fans need to be in parallel with a power supply rated for 12VDC. Fans are not that picky, and a unregulated 12V power supply may drop to the desired voltage under load (which is what they are designed to do). If you get a regulated wall wart (or brick) you don't need to worry about it. They are relatively cheap, running between$2 to \$15, check the catalogs online or locally.

A computer PSU is major overkill. It may work, or it may not. The problem is they tend to be different from eacy other, and some are pickier about loads than others. A general rule of thumb is all the major power supply outputs need some loading, so to run two fans your going to be generating some serious heat on the side. Your fans will be using around 15 watts from a (minimum) 200W power supply. You could try it and see, usually they will just shut down if their not happy.

What part of the world are you? Members might be able to recommend some sources if you give us feedback.

BTW, KE5NNT, nice avitar! I thought seriously about going for the waving flag when I designed mine.

#### ke5nnt

Joined Mar 1, 2009
384
BTW, KE5NNT, nice avitar! I thought seriously about going for the waving flag when I designed mine.
Thanks Bill, figured it was time to personalize a little since I'm getting to be far more active on the forums.

Bill, your join date says march, 2008. How in 1 and a half years did you rack up over 5,000 posts? That's about 9 posts per day, every day for 19 straight months...

#### Wendy

Joined Mar 24, 2008
21,839
Think of it as addiction. Seriously man! Save yourself before it's too late!

I do a bit of writing on the side for the book, I like it here. I'd been looking for a place like this for over a decade.

I have a couple of former hangouts before I found this place, but they were computer related. Speaking of which, I'm going to send you a PM on them about your computer woes.

#### nuggetz

Joined Sep 28, 2009
15
Thanks for the tips. So, I live in Chicago. I have Radio Shack's nearby. Nothing else really.

So, can someone recommend a nice power supply from somewhere? i just placed an order from jameco yesterday for some resistors, leds, caps, etc and was thinking of ordering one but I wouldn't know what to look for. In fact, I was considering building one myself to maybe run of a breadboard or something. Maybe I need a good schematic.

#### Wendy

Joined Mar 24, 2008
21,839
Chicago is a big town, I would be amazed if they didn't have something local. They can be hard to find, since nowdays electronic components seems to mean stereo equipment.

But as to other sources, there is BG Micro, Jameco (you already know this one), Digikey, Electronic Goldmine, All Electonics Corp, and more. I make a lot of flea markets, and tend to buy them for my junk box.

Don't discount other sources such as GoodWill, Target, Walmart, or hardware stores.

#### nuggetz

Joined Sep 28, 2009
15
So Bill, are we talking about a specialized power supply for this hobby or a wall wart of some kind?

#### Wendy

Joined Mar 24, 2008
21,839
A wall wart is a power supply, it doesn't care if it came from Walmart or Jameco. Things to look out for though, there are some wall warts that are AC power supplies. Other than that pay attention to voltage and current.

#### nuggetz

Joined Sep 28, 2009
15
Ok, so I dug up an old power supply brick from an old dead laptop and was thinking about using it. I believe it's 19v but the amperage rating is pretty high.

So lets see if I got this right:

I can wire up the supply and fans in parallel but need to put a voltage regulator on the circuit to reduce the voltage down to 12v? If the supply is say, 19v, it has the extra 3 volts required by a 12v regulator but like someone said the regulator may not support that many amps running thru it. Would I need to use 2 regulators then? This is getting confusing. I know it was stated that the 12v supply I was referring to in my original post was good enough for 2 fans but is a 19v supply too much? I'm missing an important concept I think.

Also, this laptop power supply brick has a cylindrical component under neath the cable coating near the output. What is that in there? I left it there and snipped the end off to test with a volt meter but wanted to know what it was and whether it should be removed or not.

#### Wendy

Joined Mar 24, 2008
21,839
You can get IC regulators from the local Radio Shack using a 7812 and a couple of caps. Kinda overkill, but it will work. I suspect you'll need two of them.

Best guess, that was a ferrite bead, used to absorb stray RF that would cause interferance. Computers are real RF noise generators, so they need all the help they can get to pass FCC muster. Without a picture it is just a guess, but if that is what it is then it isn't needed.

#### Audioguru

Joined Dec 20, 2007
11,251
Your 12V/300mA wall-wart might catch on fire when it is overpowered with a 12V/600mA fan.