NEWB HELP: Selecting a pot to control a peltier TEC device 30-100 Celcius (magnetic stirrer project)

[Laughmore]

While easy answers are welcome, if anyone could point me in the right direction to educate myself, bring it!

I'm putting together a small shopping list for a heated magnetic stirrer project. I have a very basic understanding of circuitry (audio experience) and basic soldering and assembly skills (cable and panel repair).

Some basic needs for my application (still need help with the bold):

RPM range: It would be nice to go as low as 100 but as high as 1500 (circuitry on PWM motor control is well documented)

• I've selected this motor (400-3500rpm 3-12V high torque mini dc) and this pot/PWM controller. While these may not be ideal, it's cheap, available, and should work... I think

Small - a single 40mm peltier device should be fine for form factor
Temperature range: room temp (zero) with moderately fine control up to boiling H2O (max)

• Need help pairing a peltier device and a potentiometer(??) to operate between the temps mentioned. I have a handheld digital infrared thermometer for reference.

Inexpensive - Hopefully I can get my 1-4oz boston round glass bottles of vegetable glycerin to 70C for a few hours safely on a cheap switching 12v 6A power supply and a single cheap peltier device. If it works for a few years, that'd be better.

Thanks for taking the time to read this. This is my first time making a circuit with a peltier device - do I need anything else besides a switch, the heating element, a power supply, and the right pot to control the temp how I've described?

 

[cowboybob]

Welcome to ETO, laughmore!

Interesting idea.

If I may critique the concept:

1. Peltiers are flat devices. Right off the bat you'll have contact issues with a round bottle and, as a result, serious thermal transfer inefficiencies.
2. In that same vein, transferring the heat to a glass bottle will also seriously reduce the heat transfer process.
3. You have selected a peltier unit that maxes out at the temps you wish to achieve. even assuming 100% efficiency. Of course, a higher wattage/higher max temp device could help with that.
4. All of the above would result in having to add additional peltiers to achieve the efficiencies needed, thus requiring a higher wattage power supply and bigger heat transfer traps (opposite of a heat sink).
5. The heat traps have to be in direct contact with the peltier (or some less efficient liquid transfer system), thus vastly complicating the magnetic stirring design.

As to your exact question, just google, for instance, "100W potentiometers".

Note that the pot(s) will consume some power (heat up, since they're wire wound)) so figure that in with your estimates of TOTAL power needed to run the peltier(s).

 

[Laughmore]

Thanks for the response. I'm learning that the TEC's I've been shopping for are under spec for this application. I'm leaning towards heating coils or converting a coffee maker.

As for the heat transfer... dammit. I guess that's (at least partially) why labware is flat bottomed. Boston rounds have a concave bottom and little nubs on the contact edge... sigh. I wonder if some soft thermal pad could "solve" that. Staying in boston rounds would help my work (hobby) flow tremendously. I'm mixing flavoring recipes with a liquid nicotine base - I'd prefer the liquids were enclosed while hot as evaporation affects flavor. Flat-bottomed lab enclosures would be vials, yes?

I noticed some lab stirr plates are ceramic. I could make a custom cast plate of the bottles I use to fit whatever hot plate. It may seem petty for me to adhere to the boston rounds but the liquid stores in them and I'd prefer they stay sealed. My operation isn't big enough to justify outfiting with so much new glass.

 

[cowboybob]

You're welcome.

... Flat-bottomed lab enclosures would be vials, yes? ...

I think you're referring to Erlenmeyer flasks.

... I noticed some lab stirr plates are ceramic. I could make a custom cast plate of the bottles I use to fit whatever hot plate. ...

If you were to get a simple lab heating device (or whatever), you might consider using a small metal short walled box, fill it with high thermal conductivity molding sand (https://www.google.com/?gws_rd=ssl#q=high thermal conduction molding sand) and then "snugging" the bottles down into it to improve heat transfer to the bottles. You might improve bottle heating by enclosing the entire rig within a shaped thermal blanket (be careful with that, though).

The stirring process ought to occur naturally as the liquid(s) are heated from the lower portion, naturally migrating (Brownian Motion) to the upper, air-cooled portion of the bottle and then "fall" to the lower section in a continuous eddy.

I have (in the distant past) done this exact activity and it works well, although not as quickly as a magnetic stirrer agitated arrangement.

 

[KeepItSimpleStupid]

You can't pull it off.

Small - a single 40mm peltier device should be fine for form factor
Temperature range: room temp (zero) with moderately fine control up to boiling H2O (max)

delta T max is about 60 C. Even for room temperature, you need a sink BELOW room temperature. I used tap water.

Without custom TEC (i.e. one with a hole in it), dunno how you can stir something unless done from the top. A typical magnetic stirrer is a teflon coated magnet and driven underneath.

 

[Laughmore]

A typical magnetic stirrer is a teflon coated magnet and driven underneath.

Yep that's what I was planning on using. Motor underneath.

What kind of specs should I look for in a peltier device to reach 60-70C easily and for several hours safely? I've heard of 500W TEC devices, which is surely too much, yes? Or maybe all TECs' TMAXs are too low, idunno. Should I give up and work on a heating coil or conventional hot-plate design? (converted coffee maker)

As you can tell I'm fairly uninitiated so any info is welcome.

I want to link an image but no imgur or photobucket?

 

[KeepItSimpleStupid]

The TEC is typically a delta T device and typical numbers 40 to 60 C. Think of it as a solid state heat pump.

When you exceed delta T the device dies (melts). We used them to cool at 25 C using ground water on the other side. It's a really good device to maintain stuff at 25C when doing it that way. We used a fan too, but the temperature had to be hotter than ambient.

A closed loop water/antifreeze works heating/refigeration recirculaitng bath works well in an intermediate range from above freezing. Once you have enough thermal looses resistive heating works. To get 100 C with the same sort of device, I would think the loop would have to be pressurized. Think car radiator. I don't see the problem to be easy.

The peltier devices are typically "slow" to respond.

 

[Laughmore]

I've got a coffee maker that has the right form factor (mini 4 cup), a non-magnetic heating element with space underneath for a rotating magnet bar, and it heats a 30ml boston round to 80C+ fast enough, but that's too hot. If I could trim it down a bit, that would be fine I think.

I'm pretty sure the heating element is powered with AC. 119vAC with my cheap multimeter on the leads to the heating element. DC setting shows 15mv, pretty sure that's not useful. Coffee maker is rated at 650 watts, and my meter is only rated at 400ma so can't test current but supposedly it's more than 5A. The hot plate switches on and off at static intervals to maintain it's temp. What kind of component is designed to "attenuate" current to the heating element? Is that what I need to make the coffee maker operate at slightly cooler temps?

It would also nice to increase the time intervals at which the hot plate turns on, though it seems that would involve more hackery. Or I could k.i.s.s. and put some fire-retardant cloth as a heatsink under the glass, though I'd rather have control with a knob etc.

 

[KeepItSimpleStupid]

Typical stirring hotplates use the bimetalic thermostat. It's usually not much more than on/off control and not precise. The thermal mass helps control a bit.

A light dimer would work, but not "control" temperature. Replace "control" with "varies".

 

[Laughmore]

Thanks for the input. I found this article on varying a popcorn maker's output for roasting coffee beans and it has some good info on modding a dimmer switch to handle more current, should that be necessary.

 

[ronv]

Yes, I think you can get 1000 watt dimmers pretty cheap.
Here is one now:
https://www.amazon.com/Lutron-Elect...61249&sr=8-8&keywords=lutron+1000+watt+dimmer
Might find a better price, Lutron is kind of top of the line.

 

[Laughmore]

sniffing around I see **broken link removed** in lighting supply and one for $20 free shipping to US on ebay as "Dial Router Fan Speed Controller" 15A... I like