EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: msrinivasan1995 on June 19, 2017, 01:12:44 pm
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I wanted to use TEC (peltier modules as heating element) to switch it temperature from 90'c to 60'c and again back to 90'c and the loop goes on. My doubt is regarding usage of heat sink and thermal fans. Most of the tutorials which is available use TEC as cooling unit. I want to know how to use the heat sink and fan on the colder side so that i can switch between my desired temperature. I saw that heat sink can be used to transfer heat as well as cold. Wanted to know if its possible.
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What it all boils down to, is that the TEC is a heat pump; it's moving heat from one place to another. The heatsink works by conduction, and so heat is conducted either to or from its fins.
So what you are doing will work, you'll end up with an air conditioner. :-+
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What sort of heat loads/thermal mass are you looking at, and what sort of cycle and transition times? The feasibility of a TEC here depends a lot on those.
It's certainly possible to use a TEC for heating as well as cooling, it's just not often done because a heater is simpler. In your case, a TEC may be a good choice if you need to be able to go from 90 to 60C relatively quickly and the sample is relatively small. However, if you can afford a slower cooling cycle, it's going to be much simpler to use a heating element and a cooling fan. If you need fast cycle times and the sample is big...maybe water cooling, or even a conventional refrigerant-based heat pump?
If you still want to try using a TEC, the design case for heating with a TEC is a bit easier because the power lost in the TEC itself contributes to productive heating rather than being a counter term. So essentially you add the power dissipated in the TEC to your heat flow rather than subtracting it, otherwise the design procedure is basically the same as designing for cooling but with some of the signs flipped.
However, there are a few significant challenges based on your temperature ranges:
- 90C puts you into "high temperature" TEC range. 80C is a typical max operating temp.
- If your sample is swinging between 60 and 90C, your heatsink temp may swing even farther. This means lots of thermal stress, and your mechanical design must account for it
- Because of the above, you probably want to look at something like Laird's PC series (https://www.lairdtech.com/product-categories/thermal-management/thermoelectric-modules/pc-series), which is designed for thermal cycling and has a max operating temperature of 120C.
- 90C at 25C ambient means your delta T is at least 65C, which means coefficient of performance is going to be poor. This is not such a problem for heating (when Qc drosp to zero, the TEC just becomes a fragile and expensive heater, but still a heater!), but is a big problem for cooling. Even the Laird PC TECs are only spec'ed for dTmax of 67C. You'll need a two-stage system of some sort. Possible two TECs or a TEC plus something else. In your case, since even your minimum temp is above any reasonable ambient, you might be able to use some sort of big thermal mass with a heater attached instead of a typical heatsink. By holding the mass at something like, I dunno, 50C you reduce your TEC's delta T and if you give it enough thermal mass the TEC will be able to dump all of the cooling-cycle heat into it without getting into trouble.
You should start by figuring out the thermal capacity of your sample and test chamber. This, combined with combined with your required transition time will allow you to determine the rate of heat flow you need to apply during transitions. Figure out the rate of heat loss from your test chamber, and the amount of heat (if any) generated by whatever's in your test chamber. Add all of these up, and you now have the total heat flow (Qc) your TEC needs to provide for the heating and cooling transitions. Post those numbers here and you may get more useful advice on the feasibility of your project.
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Also note that continuously cycling the temperature of a TEC can significantly reduce it's life, the solder junctions fatigue and crack due to the stress cause by unequal expansion and contraction.
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@ajb :
My actual project is a thermocycler, which need to switch from 95C to 60C and in a loop . I m understanding your constraints but most thermocycler has only small fans as cooling element. I need to check on heat sink
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I wanted to use TEC (peltier modules as heating element) to switch it temperature from 90'c to 60'c ...
Ahh, let me guess. You want to do 2-step PCR (Polymerase Chain Reaction). If so then ...
... yes, Peltier elements are the traditional way of doing this. The tricky part is arranging the elements so that they heat the sample block uniformly. You will still get a temperature gradient across the block, but you can keep it to less than 0.5 deg. C, at your high temperature, if you take care - it depends on the size of your block, which depends on the number of samples you want to process at once.
The elements will eventually wear out. That's standard. You will need a hefty power supply, again depending on the block size.
May I suggest you buy a used Peltier-based thermal cycler, even (or especially) a damaged one, and do a tear-down, to see how it was designed and built. Check your local Universities, blood banks, pathology labs, water quality labs, etc if they have any old cyclers they want to sell or throw out.
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@thermistor-guy: Is there any other way to heat and cold the block instead of traditional Peltier elements.
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Also note that continuously cycling the temperature of a TEC can significantly reduce it's life, the solder junctions fatigue and crack due to the stress cause by unequal expansion and contraction.
They make TECs rated for temperature cycling.
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Is there any other way to heat and cold the block instead of traditional Peltier elements.
In theory, you could heat the block via power resistors, then cool it with a fan. The problem then, for a PCR application, is cooling speed. I'm assuming that you want to do 2-step PCR ...
Too slow a cooling rate adversely affects your PCR results. The DNA strands, which separate ("denature") at your high temperature, then recombine during the slow cooling. They recombine before your probes (oligonucleotides) can attach to the strands' target areas at your low temperature, and before the TAQ polymerase can replicate your targeted sequence. With excessively slow cooling you get less PCR product, poor DNA amplification, and poor DNA quantitation.
Ideally you want to heat and cool between 95 deg. C and 58 deg. C in about 20-30 sec. Faster is better. But it's not just the block; you need to heat and cool your water-based samples (say 25 uL) in plastic tubes, that are sitting in the block, at those rates.
Some block cyclers use a liquid metal, like gallium, flowing within the block, to help transfer heat. But they still use Peltier elements for heating and cooling. The gallium reduces the temperature non-uniformity across the block.
In theory, you could use power resistors to heat the block, but cool the block by pumping a cold fluid through it (like cooling the CPU in a gaming PC). Or you could use dual fluids: heat the block using fluid from a hot reservoir, cool it using fluid from a colder reservoir. I have never tried these approaches. Good luck.
There are other ways of doing PCR, e.g. spinning samples inside a small air convection chamber, but I assume you want to stick to a block-based design.