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Silicone heaters? (for OpenPCR)
Asked by Tito Jankowski
Posted Jun 18 2010 05:16 PM
First off, thanks for everyone who has helped to spread the word about the OpenPCR project -- we met our minimum level for donations yesterday in just 10 days!
Josh and I hit a snag building the heated lid for the OpenPCR machine. The heated lid needs to warm the top of the tubes so that the DNA sample at the bottom of the tube doesn't condense. Our prototype uses a piece of nichrome wire, wrapped in silicone tape, sandwiched between two flat pieces of aluminum to distribute the heat evenly. Fine for a prototype, and with extra features like smoking and burning out!
It sounds like silicone heaters would be a good way to go. Our spec is fairly simple -- a 1.5" to 2" square surface that can heat up to 100-110 C, hold that temperature (+/- 1 C) for about 2 hours, and a price $10-$20. We looked into a company called Minco for silicone heaters, who was recommended to us. However, $1,000 order minimum, 7 week lead time, and $250 for a prototype all add up to 'yikes!' for 2 guys in a garage getting by on donated funds.
Any suggestions or even just experience with silicone heaters? Other ideas for off the shelf components, maybe thin film or pre-made nichrome heaters? It might come down to using a $30 peltier for the lid but that seems like a waste. We're looking to make OpenPCR easy to build, so more off the shelf components the better!
Tito and Josh
Answered by Windell_Oskay
Posted Jun 18 2010 11:43 PM
Maybe make a kapton flex PCB to do it? The unit cost is low, it's proven technology, and kapton is biocompatible and handles high temperatures well.
There are also *plenty* of off-the-shelf ones, if you'd prefer to have lower R&D costs but higher per-unit cost (about $12 for that size).
Also, I don't see any red flag about making a minco-style heater *unless* the per piece price will be too high. Of course there will be minimum purchases, tooling costs, and prototyping costs-- but isn't that what kickstarter money is perfect for?
Comment by Tito Jankowski : Jun 20 2010 01:38 PM
We spent yesterday looking at a couple different shops for Kapton heaters. You're right, there are lots of off the shelf units, even with thermistors built in which is encouraging.
However, we weren't able to find units that would meet our spec -- 12 V and around 20-30 W. For instance, we found a Kapton heater from Birk at 28V and 40 W, but since we'll be powering it at 12V we'll only get around 7.3 W.
Do you have a specific shop that you like or might be able to meet this spec?
Answered by Windell_Oskay
Posted Jun 20 2010 03:04 PM
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.
Comment by Tito Jankowski : Jul 01 2010 07:50 AM
Wow thank you for the thoughtful replies. That seems like a really good potential solution and we'll give it a shot!