QRT-PCR:Heated Lid
Desired specs:
- 1/2 inch diameter circle
- can handle up to 3A
- can go from room temp (27C) to 92C in less than 5-ish minutes
OpenPCR first used nichrome wire and silicone tape sandwiched between two plates of aluminum...but it didn't work too well. They said it "smoked and burned out." However, they were designing for 16 samples, not just one (so we won't need to use as much wire). Also, we're not planning on using silicone tape. I still think it's worth trying.
Then they used a peltier device with an aluminum plate + thermal pad.
Another option: using a PCB. Instructions here.
Option 1: Nichrome wire
Researching nichrome wire... looks like NiChrome 80 is the best. Here is the datasheet. Now for calculations:
I = 3A V = 15V n = length of wire (feet) rpf = resistance per foot (ohms/ft) I = V/R = V/(n*rpf)
rpf and n are inversely proportional - and cost goes up with rpf. So we want to minimize that. reasonable values seem to be: rpf = 6.46 n = 0.77 feet which means a 30 gauge wire, d = 0.0100 inches.
Okay, this turns out to be too long. If we're looking to use only one coil of wire, using nichrome is not feasible.
Option 2: Flexible heaters
Mica, polyimide, silicon These are pretty expensive. Is it possible to just attach leads in random places to these foils, I wonder? That would make it cheaper. But as it is, they're more expensive than a TEC.
It may also turn out to be cheap to order bulk custom-made flexible heaters. I will call and get a quote.
Option 3: PCB
Would it be fast enough? who knows.
Windell Oskay says:
"If you have such a specific spec that's not in the off the shelf units, that's a second argument (besides price) for making them yourself.
I don't know about your mechanical design, but you may be more confident in making a heater out of a traditional circuit board-- it can easily handle 110 C, will be rigid, can be custom shaped with mounting holes, and you can dip it in a biocompatible coating if necessary. Final cost will be a few dollars (or lower), per board, exactly meeting your specs in terms of size, shape, voltage input, and power dissipation.
So you need 12 V input, and maybe 4-6 ohm resistance to get 20-30 W. Let's call it 5 ohms, and you'll then need 12/5 = 2.4 A available for the heater. 1 oz copper has resistance of *roughly* 0.05 ohms/inch in a 10 mil trace, so you need about 100 inches of wire. Suppose that you use 10 mil wires with 10 mil spacing, with a simple single-sided back-and-forth pattern. Then one wire plus spacing is 20 mil, and you can fit 50 wires per linear inch. If you have a 1.5" square, then in each linear inch, you get 1.5*50 wires = 75 wires per inch, and in 1.5 inches of that you get 1.5*75 wires = about 112 inches of wire total-- right on target."
resistance per inch for copper ~ 0.05 I = 3A V = 15V n = length of wire (inches) I = V/R = V/(n*rpf) n = V/(I*rpi) = 15/(3*0.05) = 100 inches of wire
That's a lot to fit on a 0.5" diameter circle. Batchpcb.org has a minimum trace width of 8 mils with 8 mils spacing - too big to fit 100 inches. I wonder if other trace materials, with higher resistances per inch, could be used.
