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Power supply question

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BpB

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Im interested in building a stir plate for homebrewing but I have minimal knowledge of electrical wiring and I dont want to blow something up so I wanted guidance from experienced people. The basic build consists of a 12v pc fan, a toggle switch, a rheostat or potentiometer, and a power supply. I have been looking around my house and reading the stickers on some old power cables I have and I realize Im not sure exactly what I am looking for. I found an old cell phone plug that had DC on it and I figured that was probably not going to work as I was only dealing with AC. I have an old dvd player I need to throw out, can I just cut the cord on the back and use that?
 
Your question doesn't quiet make sense, you said you're going to use a PC fan, but those are DC not AC, how is the fan going to act as a stir plate? Typically stir plates are bar magnets on the end of a motor with a coated steel rod inside that does the stirring. Rheostats and pot's are the same thing by the way and generally are a very poor choice to control the speed of a motor because they waste all the power as heat.
 
If the power supply has a mains input then don't mess with the cord to that :).
If you have cord on a scrap appliance then check the label on the appliance. Divide Watts (or VA rating) by mains voltage to get Amps. Any cord with an Amps rating of ~ 1-5 should have insulated cores suitable for interconnecting the fan, switch, pot and low-voltage output of the supply.
I found an old cell phone plug that had DC on it and I figured that was probably not going to work as I was only dealing with AC
Wires don't care whether it's AC or DC. They do care if their current (Amps) rating is not enough for the current you are trying to pass through them! That cell phone plug may not have a high enough current rating.
 
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Thanks for all of your help. When you say "mains input" Im not exactly sure what you mean, is that when they have a block on the cord like most printers do? Or is that when the plug itself is blocky like cell phone chargers? According to the sticker on the back of the dvd player its 120v 60Hz 15w so 15w/120v=0.125A, am I calculating that correctly? If so then I can see it wont have enough amps to power my stir plate. The other cord I found is blocky at the plug end and says Nokia so I assume its a phone charger but its been many years since ive had a Nokia phone so I really dont remember if this was the charger! It says Input 120vac 60hz 50mA and Output 3.7vdc 0.36a so Im assuming at 0.05A thats not nearly enough either. Should I continue to try and find one around the house or should I just buy one and can I get an appropriate one at say radio shack or would it be better to source all of my parts online? And finally, if the rheostat or pot is a poor choice for controlling the speed of the fan what should I use instead? Again thanks very much for answering my lengthy and endless questions!
 
mains input means something that plugs into the wall.

We can't answer your question because you so far haven't mentioned anything about the stir plate itself. Your first post has a brief mention of a PC fan and no other information on how it's constructed.
 
The DVD and charger cords you mention may not be adequate. If that's all you have then go buy a yard or so of flexible mains cord (e.g. 2A or 3A type ) from your local electrical supplier, or get some 'hook-up wire' from Radio Shack or wherever is convenient.
 
Rheostats and potentiometers (pots) are not the same thing.

A rheostat will have 2 terminals on it and will be normally used to control current (where the "rheo" comes from). It will have a pretty high wattage rating around 10W to 1000W and a limited resistance range.

A pot has 3 terminals, a broad resistance range, a limited wattage range (generally 5 watts or less) and different tapers (the relationship between the control shaft position and its resistance ratio).

If you have a 2 terminal rheostat and a three terminal circuit, you won't get very far. OTOH, a 3 term. pot can easily be made into a 2 term. rheostat.
 
The basic idea is glue two magnets to a fan and when it turns it will pull the stir bar in the liquid. There are lots of different examples of people building these and all seem to be pretty similar but here is an example : http://brewiki.org/StirPlate . Is there a smarter way of going about this?

What about a computer power supply cord? I dont know the specs but I do know that I have several and they have always been interchangeable between computers.
 
What about a computer power supply cord?
A bit OTT, but that will certainly handle the fan current.
 
BpB, okay that makes sense now with the magnets glued to the fan. You said the voltage requirement of the fan was 12 volts, what are it's current requirements? If you can answer that question a proper answer can be provided.

I'm still a little confused because you keep talking about this being AC even though you specifically stated that the fan is a re-purposed PC fan which means it would be DC...

If this is truly a basic PC power supply fan like the 3-4 inch case fans that are so common then it won't draw more than 200ma of current. Virtually any old wire you have laying around can handle that current even old phone wire. You can use any 12 volt wall wart that can supply at least that much current. I did a quick simulation with a 60ohm resistor as a load and you'd be fine with a 500ohm rheostat/pot that can handle 1watt (max power dissipation would be 600mw) the exact value/power rating you'd need will depend on the exact specs for your fan.

If you have trouble finding a rheostat/pot that will work you can always simply use a multi position switch to switch in strings of 20-100ohm resistors. The power drop with resistance is exponential so you'd switch in a 20ohm resistor first, then, 40, then 80, 160, 320, 640 to get a roughly linear power drop and stirrers really don't need to have infinitely variable speeds 5-6 speeds should be fine. An audio taper pot/rheostat would be required to get a linear feel to the knob as well but that's just what's ideal not what's required.

By the way Jagura thanks for the correction, I'll try to remember that =)
 
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The fan only says "CERAMIC FAN DC 12V 2400 RPM" how do I determine its current requirements?
 
Take a decent picture of it with something in the frame for scale, we should be able to ball park it's current requirements visually. Ceramic fan seems a bit odd though, I've never heard of a PC fan that had anything to do with ceramic, those are usually in reference to space heaters.

If you don't know, a picture is worth a thousand words.
 
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i make theese magnet stirers alot, but i use small DC motors and cut a thin wooden disk to mount on the shaft of the motor i attach the magnets to that. depending on what you want to stir some form of speed control can be pretty useful. for the stirer itself i often use a small bit of thin metal bar placed inside aquarium airline tubeing then the whole thing is sealed up with clear aquarium sealer
 
Heres the pic, you were right its about 3-4 inches, standard case fan. I also have one about half that size thats 12v as well.
 
If your hook-up wire is as thick as or thicker than the existing fan wires it will be fine.
 
Virtually any wire you use will work fine, that fan probably won't even draw quiet 200ma. If you just splice directly from the end of a DC wall adapter that can supply 200ma you should be fine.
 
The startup current isn't really a problem but the running current really does need to be known for sure. This is easily tested using even a cheap multimeter though.
 
keep in mind the start up current on some theese fans seems to last a fair while! i use 9-12 fans per incubator using controll moduaels that 3v0 designed and built for me, personaly i would take notice of the start up and running current also he needs to take the reading while it has the magnets on as theese fans dont like too much weight on and the current goes up at a pretty quick rate. its only my experiance of them and like i say i run many hundreads of them. interestingly i wouldnt take any notice of the datasheet as ive had fans from the same production batch run at completely different currents and RPM.
 
Remove the fan blades, attach a flat disc and glue the magnets to that, it will run at a lower current. Fans are designed to move air, all he wants to do is move the magnets attached to it. The additional weight of the magnets will increase the starting current, but will not affect running current so long as it's aerodynamically sound, if the fan blades are removed the magnets will more than likely create less drag than the blades did by far hence lowering the running current.

Ghostman, the reason your incubator fans ran at a higher current than listed is more than likely because of back pressure, the running current and CFM of air movement in fans is based on unrestricted air flow on both ends. These types of box fans handle back pressure poorly as vortices form at the blade tips which moves less air and draws more current from tip drag as it's just recycling air at the blade tips not moving it from one point to another. If you have a lot of back pressure to deal with you should use DC centrifugal blowers as they handle the back pressure better.

Here is a link to some very good information about the difference between fans/blowers and compressors. For you Ghostman it will help you but not the original poster.
 
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