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12V 12A power supply? (for OpenPCR)
Asked by Tito Jankowski
Posted Jul 01 2010 09:17 AM
Thanks for the awesome responses in regards to my previous question about heated lids.
Now we've got another question that could really use your expertise
For the OpenPCR machine, we need a lot of power -- we're pumping a peltier up to 100C and then dropping it to 50C, and cycling this over and over again at about 2C/s. In terms of a power supply, we started with a standard ATX PS, which is very easy to source and cheap. Problem is, it's giant. We had it as an external unit for Maker Faire, but if you've seen an ATX you know it's not really designed to be external.
So the search began for an alternative. Our power needs are basically a single 12V 12A source, so the ATX was a bit overkill. I then found this Micro ATX power supply which is a better size, 5" x 4" x 2.5" and under $20. Our hope was that we could find something small enough to be easily fit INSIDE the OpenPCR case (roughly 6" x 4" x 5"). The Micro ATX is still bigger than I would like and as someone recently pointed out, we would have to modify our current case to have even more air vents, which I would like to avoid.
Our ideal power supply would be an EXTERNAL one for <$20 and easily sourcable. I'm talking about external like a adapter you would plug an LCD into, or the power adapter that comes with a Macbook.
Any ideas on what would fit the bill? Alternatively, an internal power supply with less cooling requirements than the Micro ATX would be great too!
Answered by Robert.Bruce.Thompson
Posted Jul 02 2010 07:04 AM
This is a project that's dear to my heart. I definitely want to equip my home lab for DIY Bio. I already have a homemade DNA gel electrophoresis apparatus, and PCR is next on my wish list.
As to your power supply needs, I'm not an EE but I do have some experience with DC power supplies. I checked all the usual Chinese suspects, and wasn't able to come up with anything that I'd trust anywhere near the $20 range.
That price pretty much limits you to mass-market stuff, where you get huge economies of scale. I was trying to think of other mass-market 12VDC units, but the only thing I could come up with was the computer power supply you've already come up with. Unfortunately, 144W net is a lot to ask from a power brick. Some notebook AC adapters will meet your requirements, but they're going to cost a lot more than $20. Also, those newer AC adapters are switching, and they're less reliable compared to older power bricks that used simple transformers and coils. I'm not sure what their rated duty cycles are, but I suspect the OpenPCR would kill them pretty quickly.
Although it would require a redesign of your case, I think your best bet is to bite the bullet and do that. Standard microATX power supplies (actually, they're called SFX12V) are completely standardized and interchangeable and likely to remain cheap and widely available for years to come, so you won't need to worry about your source disappearing. Also, people who use the OpenPCR will be able to replace a failed power supply for $20 and a quick trip to their local big box store.
Granted, a PC power supply isn't the most elegant solution. It has lots of voltage rails you won't be using, but who cares? It provides extremely stable +12VDC at the current level you need, and it does so at much less cost than other solutions.
Answered by Windell_Oskay
Posted Jul 02 2010 09:23 AM
That's a tough order.
I don't know about the details of your project, but I strongly suspect that you're being much less efficient than you could. You're trying to use a single power supply not out of efficiency, but out of simplicity.
Think, carefully: Do you have any voltage regulators in your project? Do you have any places that you could use more TECs in series or otherwise "recycle" current? Any large resistors? If any answers are "yes," then you probably are going to be better off with a different power supply solution, and one where off-the-shelf solutions are more reasonable.
If you can give some reasonable accounting of what you're driving with the 12 V power supply, and the power budget for each system, I might be able to help you identify some places for improvement.
Edit: How long does the system need to run for at a time, and what is the average (not peak) current draw while it runs?
Comment by Tito Jankowski : Jul 04 2010 09:33 AM
I talked with Josh about this. The main peltier is drawing 7A, we had budgeted 3A for the lid heater, and had reserved a 20% safety margin. But now that we are using a peltier for the lid heater that will likely use 5V, just need to confirm that with the MOSFET voltage drop and exact lid design/insulation we can hit 110 but think it is likely. So then the 12V needs are dropped to 9A but need probably about 5 at 5V (guess). (So we may make use of a 12V and 5V rail).
We don't have any resistors in the current path, only voltage regulator is drawing around 50ma, and only one peltier for the block (putting it in serial is not a solution anyways as the voltage would be divided so we'd no longer have 12V).
Would love to hear your suggestions, Windell!
Comment by Tito Jankowski : Jul 04 2010 10:23 AM
Also, it needs to run for about 1-1.5 hours.
Might look something like this:
Step 1: Heat to 95C, hold temp for 1 min
Step 2: Cool to 65C, hold temp for 1 min
Step3: Heat to 72C, hold temp for 1 min
Repeat this 30 times
and during all of this the Lid is at 100-110C
Answered by Windell_Oskay
Posted Jul 04 2010 11:28 AM
Using a Peltier device (for the lid) when you only need to heat is usually not cost efficient.
If you can use a 12V/5V system, you'll do a lot better in terms of available power supplies. Just going below 10 A on the 12 V would let you use many other computer power supplies like ones that you've already found; I found several new for $15 or less. Fan or not, it's *really* hard to beat this kind of price.
I suspect that there are other Peltier options available to you as well. You need a lot of power, obviously: 12V @ 7A = 84 W. But you should also be able to find appropriate Peltier devices that work at a higher current, lower voltage. (Possibly by wiring devices in parallel instead of series.) But probably, you should just see how flexible on this you can be to get better power supply options.
Since you don't need to run very long, there is one other very good alternative available: use a 12 V lead-acid battery, like a (large) motorcycle or car battery. Running at 12 A from 12 V, for 1.5 hours, means that you need at least an 18 A*h battery. This is a more expensive option in the US at least (a new battery like this costs about $100), but it may be a huge advantage in developing countries where car batteries are easier to obtain than steady high-current power. A battery like this can easily be charged from a low-cost 12 V trickle charger or from solar panels. And, if you have several batteries, you can work all day.
Because that would be such an awesome option, it would be neat if you can design your electronics to be *flexible* in terms of whether it requires all 12V, or a combination of 5V and 12 V. If you fabbed your own resistive lid heater, for example, you could have two sections that were wired in parallel or series to work at 5 V or 12 V. (If you use two small Peltier devices on the lid, you could also wire them in series or parallel for the same effect.)
Comment by Tito Jankowski : Jul 05 2010 09:52 AM
Interesting -- what were the power supplies you found? Any external ones with both 12V and 5V?
Using a car battery is an awesome idea, we'll keep the design flexible and find a user out in the field to put it to use!
Answered by toybuilder
Posted Aug 11 2010 07:01 PM
One things to keep in mind is that the reason why PC power supplies are so cheap is their economy of scale. There's not a whole lot of consumer electronics devices out there that draws 12V at that kind of current. That said, try Digikey and Mouser for power supplies that can be sourced easily; or try surplus stores like Marlin P. Jones and Associates (http://www.mpja.com/) or All Electronics (http://allelectronics.com).
Pardon my ignorance, but why do you need a peltier device? It seems like an AC-powered heating coil with a temperature control loop might work just as well. You can find heating coils easily - a $2 soldering iron plus a chunk of iron will probably get the job done?
Answered by DDR2
Posted Oct 10 2010 08:37 PM
An X-box 360 power supply will work.
if you get your hands on one check on the bottom and it will be at least 12a.
there are 3 types and I picked up one for about $17 dollars used and it works great for something I've made. You just have to modify the connector that would normally go to the Xbox console. They also have short circuit protection and a lot of other things.