Has anyone worked with thermoelectric heaters before? I have one I need to control, and I was just going to make a PWM circuit, but I googled thermoelectric heater control, and these specialized chips came up.
Can you not just PWM these things like a regular resistive heater? I'm doing it, and it's getting hot.... but maybe I'm messing it up?
Thanks!
Den fredag den 2. maj 2014 00.28.35 UTC+2 skrev Fibo:
I guess if you only need to heat PWM will be ok, for cooling PWM is not good at least not with out a filter
remember you need to control current not voltage and if they get too hot some only ~80'C they die
-Lasse
Heaters are supposed to get hot!
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
If you mean a Peltier junction being used as a heater, you have to remember that there's a linear component due to the Peltier effect, where the heat transfer is proportional to current (and can be positive or negative, depen ding on the polarity of the current) and quadratic component due to the res istance of the junction which is proportional to the current squared, and i s always positive.
An unfiltered PWM waveform generates a different amount of heat transfer th an it would if it were well-filtered.
Sloman A.W., Buggs P., Molloy J., and Stewart D. "A microcontroller-based d river to stabilise the temperature of an optical stage to 1mK in the range
4C to 38C, using a Peltier heat pump and a thermistor sensor" Measurement S cience and Technology, 7 1653-64 (1996)includes a section - appendix A - on calculating the temperature difference across a Peltier junction when you know (or can estimate) the thermal resi stances on either side of the junction. E-mail me - snipped-for-privacy@ieee.org - if you want a reprint.
-- Bill Sloman, Sydney
Yeah? I've also known "Heaters" to eject lead back in the older days!
Jamie
Usually when one goes to the trouble to use a TE device it's for cooling.
In that case, you don't want to PWM the thing, because the single biggest cause of inefficiency in a TE cooler is the I^2-R losses from the current going through it. PWM-ing it gives you a much higher RMS current than feeding it a steady current, which means its not going to cool nearly as well.
OTOH, if you want to HEAT with the thing, and if you're not putting more than rated current through it, or heating up the hot side too much, why not?
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
How about a thermal switch like they use for converting chest freezers to beer refrigerators. You can buy the kits on eBay for around 30 bucks. Then just figure how to reverse it for heating.
Klaus Jensen
But using a thermoelectric / peltier, the device can be driven in bipolar mode for better / faster control, less overshoot etc.
Temperature control actuators slow down near the ends of their range. In the thermal-mass approximation, the temperature slew rate is Qdot/Mth, and Qdot goes to zero at the extremes.
So if you're doing temperature control around ambient, heaters are terrible and Peltiers are great.
One additional advantage of Peltiers, seldom remarked, is that heaters get their negative slew exclusively from the heat leak rate, whereas Peltiers cool actively even at above-ambient temperatures. That means you get faster slew for a given amount of heat leak with a Peltier, which in turn reduces the external thermal forcing for equivalent control loop performance. (You do pay a price in mechanical fragility though.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Yeah, that's what I meant - only you articulated it (and obviously understand it) better than me!
Thermal control is fun. There's a lot of room for improvement left, especially with respect to thermal forcing.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Peltiers die very rapidly if you put them on thermostats--the solder joints can't handle the repeated thermal cycling. The soft-solder ones (Melcor/Laird) die much faster than the hard-solder ones (Marlow/Ferrotec/Tellurex).
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Wow, I never thought about that! I've always used TEC's with DC.
I know what you are saying, but that's a bit confusing. Given that the cooling depends on the average (rms) current, In the PWM case there will be more I**2 R heating and so you will have to run it at a higher average current to achieve the same temperature.
Oh I've used TEC both by feeding it a voltage and a current. I've never noticed much difference between the two. But they are always wrapped in a control loop, so that may hide any "sins" in voltage control.
To the OP, you really need to think of the TEC as a heat flow device. It takes heat from one side and moves it to the other. (Stick one side of your TEC to a big hunk of metal and see what happens to the top face, now flip it over.) Cool huh? George H.
Heat flow and heat generation -- the fact that it makes heat is just fine if you need a heater, but a huge issue if you need a cooler. If you don't have a honkin' big heat sink on the cold side you're probably just inventing a really expensive way to implement resistive heating.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
er that there's a linear component due to the Peltier effect, where the hea t transfer is proportional to current (and can be positive or negative, dep ending on the polarity of the current) and quadratic component due to the r esistance of the junction which is proportional to the current squared, and is always positive.
than it would if it were well-filtered.
driver to stabilise the temperature of an optical stage to 1mK in the rang e 4C to 38C, using a Peltier heat pump and a thermistor sensor" Measurement Science and Technology, 7 1653-64 (1996)
ce across a Peltier junction when you know (or can estimate) the thermal re sistances on either side of the junction. E-mail me - snipped-for-privacy@ieee.org
- if you want a reprint.
Yes, that's the kinda information I was looking for, thank you. I copy of that paper would be great, I'm sending you an email, thanks!
er that there's a linear component due to the Peltier effect, where the hea t transfer is proportional to current (and can be positive or negative, dep ending on the polarity of the current) and quadratic component due to the r esistance of the junction which is proportional to the current squared, and is always positive.
than it would if it were well-filtered.
driver to stabilise the temperature of an optical stage to 1mK in the rang e 4C to 38C, using a Peltier heat pump and a thermistor sensor" Measurement Science and Technology, 7 1653-64 (1996)
ce across a Peltier junction when you know (or can estimate) the thermal re sistances on either side of the junction. E-mail me - xxx.org - if you want a reprint.
Yes, that's the kinda information I was looking for, thank you. I copy of that paper would be great, I'm sending you an email, thanks!
ote:
I guess this depends on what you mean by "thermostats".
My 1996 paper
Sloman A.W., Buggs P., Molloy J., and Stewart D. "A microcontroller-based d river to stabilise the temperature of an optical stage to 1mK in the range
4C to 38C, using a Peltier heat pump and a thermistor sensor" Measurement S cience and Technology, 7 1653-64 (1996)describes the thermostat built into the Affinity Sensors IASys biosensor un it.
It didn't do much thermal cycling, since the aim was to keep the biosensor head at a very constant temperature, and I never heard of one where the Pel tier junction failed. We did use a Marlow Peltier junction, and Affinity Se nsors was eventually sold off by Thermo Electron in 2006
so I wouldn't have got to hear about one that failed after 2006.
A popular application for Peltier junctions is in providing the thermal cyc ling required for the
where they would have to work harder. One wouldn't really describe this sor t of gear as a "thermostat" though you'd use much the same hardware.
-- Bill Sloman, Sydney
cause of inefficiency in a TE cooler is the I^2-R losses from the current going through it.
Most people who have worked with Peltiers have horror stores about melting the solder by using unfiltered PWM excitation.
As is spelled out in my 1996 paper, I had trouble making sense of our initial observations, and had to work out my own expression for heat transferred as a function of current (and the thermal resistances around the junction).
The Peltier heat transfer is strictly proportional to current. The voltage drop includes a - small - Seebeck component, as well as the temperature dependent resistance of the junctions.
But don't forget the ohmic heating inside the junction!
-- Bill Sloman, Sydney
Peltiers seems to work best, when about equal power is fed into the Peltier than the heat power that needs to be extracted from the cold side. Thus, the heat sink on the hot side must be able to get away with at least twice the original heat power removed from the cold side.
Using a too small Peltier and you will have to run at very high current (power), much larger than the original power removed from the cold side. Thus several times more power must be transferred from the hot side to the environment. The hot side must be hotter than the environment. For radiative transfer, the power removed is proportional to (Thotplate ^ 4 - Tenvironment ^ 4) in which the temperatures are expressed in Kelvins, and directly proportional to temperature difference for convection and conduction.
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