Best Way to Convert 120V AC (3A Switch) to 12V DC for 10A Peltier Module and Fans

Hello all,



This is my first post and my first venture into an electrical project that I can't just google my question and get an accurate result. I apologize if I don't follow the etiquette or rules of this forum and please let me know if I have so I can correct myself going forward.
I am building a small portable A/C unit using a 10A (DC) Thermo-electric Peltier module, a 140mm cpu case fan as a blower motor and a 60mm fan/heatsink to dissipate the heat from the hot side of the module. In an effort to minimize the physical size of the power supply, I tried to use a 10A power adapter I bought on EBay, similar to a laptop charger. But it seems to go into a circuit protection mode when the Peltier is connected. I used a car battery and my DMM to determine the module is drawing 5a when both sides are at ambient temp. Current draw drops to as  low as 3.8 amps as it gets cooler so it tells me this EBay power supply is garbage.

I have a robbed a power cord from an old floor lamp my wife was throwing away that has a nifty foot-operated button style switch. The switch states it is 120v AC, 3A. My idea is to convert this to DC voltage capable of powering my A/C unit. But while I have a more-than-average grasp of DC electricity, AC is quite foreign to me. I've calculated that 3A at 120 AC converts to around 33A DC which is more than enough for what I need. Here are my questions though.

1. Can this be done?

2. What component(s) do I need?(Bridge rectifier,voltage regulator, resistors, etc.)
3. Should I be concerned with heat build-up/resistance of any kind?
4. Is soldering 120v considered "frowned upon"?
5. If you're familiar with Peltier modules, are they able to use the "pulsed" freqeuncy of this type of voltage?


Thanks for any advice,
Nate

Nate, the switch has nothing to do with your Peltier element, it just switches the power on and off.  However, 120V at 3A equates to 12V at 30A, so the switch would work with whatever power supply you use.It does seem that the Ebay power supply is not up to the job if your Peltier element works ok on a battery.You COULD just get a transformer and rectifier to produce your 12V, but you'd have to smooth and regulate it as I've read that using raw rectified AC on Peltiers is not a good idea, for various reasons.Doing this yourself will result in a bulky, heavy supply and lots of work for you.  I would bite the bullet and invest in a decent 12V 10A power supply from somewhere.  I would get something like this:https://www.rockby.com.au/catresultsSql_12.cfm?Stock_no=39229
12V, 12.5A.  But this is one of my local suppliers (Australia) and the input voltage would be 240V, so look at places like Newark, Mouser, Digikey in the states, Farnell in UK..  Get something like this in a frame, NOT a wall wart - they will never supply enough current.There's a good FAQ on Peltiers here.Good luck with it!

Hi Nathan-  I agree with David- building a power supply will be a lot tougher than buying one. Look here: Hengfu 12 volt 12.5 amp power supply.  The parts to build your own supply will more than likely be more than the cost of buying one of these switching supplies.If you do decide to build one, the main thing missing from your list is the transformer. You'll need a transformer with a 120 volt primary and a secondary that runs at about 11 volts at 12 amps or more.  The reason I say 11 volts is that the output filter capacitor will charge up to the peak value of the input voltage to the bridge rectifier, minus the forward voltage drop of the rectifier.  Silicon rectifiers generally have a forward voltage drop of .6 to 1 volt.  In a bridge rectifier you'll always have two forward voltage drops, so  11 volts- 2 volts = 9 volts. Now you multiply by 1.414 to get from the RMS voltage of the transformer to the peak voltage of the AC sine wave, so  9 x 1.414 =  12.726 volts. The Peltier device can more than likely stand a bit of overvoltage, so a transformer with a 12 volt secondary will probably be just fine.As for the filter capacitors- you'll need aluminum electrolytic capacitors rated at least 20 volts. You'll need several thousand microfarads to get good smoothing. -Rick

Ok, it seems I am in a bit over my head then as far as building one. I'll just have to buy one.
I should still be able to use the foot switch though to turn on the power supply, correct?
Also, after reading the Peltier FAQ that David linked, it seems my module can utilize up to 16v in order to pull the max 10A that the module can handle. Can standard CPU Fans and case fans handle such voltage or do I need to have a separate 12v rail to power those devices? If so, do they sell power supplies that can deliver split voltages?

Nate... glad our advice has been of use.You can probably get a 15V power supply if you want to push your Peltier a bit more, I would not go above that, and then only if you are sure your Peltier has a max voltage of 16.If you're supplying your Peltier from a regulated 15V, you could use a 3-pin regulator like the 7812 to drop that to 12V for the fans.   Fans would probably be ok on 15V - they'd run a bit faster but should work ok.  And anyway fans are pretty cheap to replace....Yes, you could wire your switch and cord to the mains input of whatever power supply you get.  Most of them have screw terminals for connection. If you'd like to read up more about power supply construction there is a good tutorial here.But believe me, for your voltage and current requirements a commercial switching power supply is the way to go.

Nathan I couldn't find any 15V supplies from hengdu, but here is Newark's page for 15V supplies, the first one delivers 10A and is reasonably priced:http://www.newark.com/w/c/power-line-protection/power-supplies/ac-dc-converters/prl/results?output-voltage-output-1=14v|14.5v|14.5vdc|15v|15vdc&output-current-output-1=10a&sort=P_PRICE

Here's a reasonably-priced power supply from Mean Well.  Two caveats:  First, it has a dedicated 12 volt fan output in addition to the main output.  Second, it requires 20 CFM of air over the heatsink to achieve full output.  If in doubt, go up to the 200 watt version.
-Rick

Good news!A friend of mine had an old 300w computer PSU laying around that he gave to me.
I jumped the Green(PS-On) wire to ground and got the supply to fire up, but I am trying to find a household item(bulb) to put a load on the 5v rail in order for the 12v rail to supply full power. I've read about putting a 0.5A load on it. I used a small 12v, #194 bulb and was able to put a very small load on it (.08A i think), which drove the 12v rail up from 11.69 to 11.74. Is there a common household item I can use or should i just by a 5v resistor to apply the load? I'm not sure about the math of a 12v bulb on a 5v circuit and how watts are calculated. Since Watts is the actual "workhorse", am I looking for a specific watt load or amp load?Desired load: 5v * 0.5A = 2.5W              12v  Load(Same current): 12V * 0.5A = 6WDo I need a 6W load if I use a 12V bulb? Or is a 2.5W load a 2.5W load, no matter how many volts it's riding on?My Math: 2.5w ÷ 12v = .20A          or          6w ÷ 12v = .50A
Also, in an attempt to get as close as possible to 15.4V I was going to use the -12V and +3.3V rails to make roughly 15.3V. The power supply says -12V /0.8A Max, and the +3.3V / 18A Max. 
Does that mean the combined 15.3V / 18.8A Max? 

Update: I picked up a 3.9ohm, 25W resistor and wired it into the 5v circuit and now I have actual 12v on the 12v rail. So please disregard that part of the previous post.

Hi Nathan-Glad you're making progress.  You may still have some hurdles ahead, though.  A 300 watt PC power supply is designed to deliver most of its power on the 5 volt bus, and very little on the 12 volt bus. I doubt you will be able to get more than an amp or two on the 12 volt output.