World's Smallest Refrigerator? Temperature-Controlled TEC Cooler
Objective: Build a small, thermostatically controlled chamber, that can be used in experiments to measure heat production at temperatures below ambient. Production of heat is estimated by measuring the amount of cooling called for by the controller. Such a device is often referred to as a PMC, or poor man's calorimeter. It is not meant to be a commercial device.
Strategy overview: Because of size limitations and the need to go to temperatures below ambient, cooling by Peltier thermoelectric cooling (TEC) was selected. These devices are horribly inefficient , but cheap, simple and effective when minimal cooling below ambient is needed. A heater per se was not required, as it might be in a "real" calorimeter, because all the experiments I intend to perform will generate heat. For development work, I have used 2, 10Ω 10W resistors in parallel to simulate my experimental load.
An internal, thermally-conductive chamber is surrounded by insulation except for a "window" containing the TEC and associated heat sinks, fan, etc.
Electronics to control temperature are nearby. Control is by a simple on-off thermostatic control of the TEC. The circuits include a high-temp cut-out of the load current (for instance if heat production exceeds the cooling capacity), detecting the presence of the load, and a delayed shut-off of the fan so that the fan runs a little while after the TEC shuts down. All functions are indicated with LEDs.
Data collection is performed using data acquisition, details of which are beyond the scope of this write-up. If I had it all to do over, I'd put my data acquisition equipment to better use (eg. PWM control of the TEC). The current PMC design operates completely independently of the DAQ.
Design Parameters: I need to hold the chamber in the range of 10-20°C, and measure heat production of less than ~0.5W. So, this design is not immediately useful for significantly lower or higher temperatures. And, the cooling capacity of the current design limits the amount of heat production that can be used.
Components:
TEC I started this project by buying a couple of TEC 12709 modules, mostly just to play with. They each demand up to 10A at ~12v to run at full capacity. Supplied 5v, the TEC 12709 draws ~4.5A and consumes ~25 watts. It moves less than 1 watt out of the chamber, so it's not terribly efficient.
Power Supply Because of the high current demand of the TEC, I chose an old computer PSU to be the power supply for this project. The 12v supply was not up to that current demand, and I was worried about the TEC getting too hot at high current, so I decided to use the 5v supply for powering the TEC. With the TEC on the 5v supply, I chose to run everything else off the 12v supply. Thinking that I might someday want this all to operate off a car battery, I chose NOT to use the -12v supply, although this would have solved some issues. Only later did I learn what many here already know - that some loading of the 5v side is needed to bring the 12v supply under regulation. So my supply varies from maybe 9 to 16v when the TEC is off. When on, the 5v and 12v supplies both hold very steady. I'm considering putting a light bulb or something on the 5v supply to steady it out.
MOSFETs I chose the N-channel IRF540N because it could safely switch more than the full 10A demanded by the TEC, and because it was easy to find cheaply. The VN10KM was simply a small N-channel MOSFET I had in a parts drawer. It only switches a 60mA fan.
ICs The LM35 thermometer is cheap, widely available and a good fit for this project. I was hoping to use the LM34, since it produces a higher voltage difference between temperatures (10mv per °F, versus °C) AND it produces a voltage from a single supply all the way down to 0°F (instead of 0°C), but it's much harder to find. The LM339 quad comparator was chosen because I could walk into Radio Shack and buy one.
The LM358 dual op-amp was chosen because it can sense down to the negative rail. An early breadboard circuit with another op-amp was failing at lower temperatures because the LM35 voltage, 0.15v at 15°C, was falling too close to the rail and the op-amp could no longer "see" it. Using a negative supply fixed this, but I wanted to stick to a single supply.
Schematics The first post is the cooler and control. The second picture is the constant current (heater) control.
This is an early version of the thermostat that controlled to ±0.6°C or so. That's not bad but I decided I wanted a tighter control.
Here's another variation without the fan circuit, but with the op-amp added back.
Description: All circuitry except the TEC and its LED indicator, which are powered by the 5v supply, are powered off the 12v supply from the PSU. A 7805 regulator is used to provide voltage reference, and is also used to power all ICs. This is an arbitrary choice that limits the upper and lower temperatures that can be used. It would not be difficult to make changes to expand the working temperature range.
Chamber temperature 1-25°C is sensed by IC1 and amplified 18X by the first op-amp. The range is limited on the low end by the LM35 (produces 0v at 0°C) and the use of a single supply for the LM35 and the op-amp (cant sense below the negative rail). The upper end is limited by the 5v upper rail voltage. 25°C gives 0.25v from the LM35, times 18 for the gain is 4.5v, which may be near or above the upper range of the op-amp.
The amplified temperature is compared to a set point established by R9, a multi-turn Bourns precision pot, allowing very nice fine tuning. The comparator employs minimal hysteresis provided by the 10M resistor. I recommend reducing this to 5M or 3.3M. The impact on temperature swing is minimal but switching chatter is reduced. I tried several values and 10M was working OK on the breadboard. It works less well in the finished prototype. Comparator 1 output directs the MOSFET control of the TEC, and energizes an RC tank on comparator 2, turning on the fan and its indicator LED.
When the TEC turns off, the fan is held on a few seconds while C1 drains through R8. For simplicity, comparator 2 uses the same voltage reference as comp-1. So the fan delay is slightly affected by the temperature setting; higher temp equals less delay. Note that the fan turns ON immediately with the TEC. In fact it always comes on a bit before the TEC.
The HEATING part of the circuit is simply a resistive load, such as two 10Ω, 10W resistors in parallel, with a constant current supply. Op-amp 2 compares the voltage on a shunt resistor to a pre-set established by R15 (another Bourns pot). The darlington arrangement under op-amp control thus controls current through the load and shunt resistor. The op-amp connection to the shunt is overwhelmed, and current thereby cut off, when comparator 3 sees too high a voltage output from op-amp 1 (the amplified temperature). Comp-3 compares the output of op-amp 1 to a reference established by R16, a cheap, single-turn pot. So if temperature rises above a coarsely set maximum, the load (and heating) are shut off.
Presence of continuity through the load is detected by comp-4 and indicated with yet another LED. (Can you tell how much I love the pretty little flashing lights?!)
Objective: Build a small, thermostatically controlled chamber, that can be used in experiments to measure heat production at temperatures below ambient. Production of heat is estimated by measuring the amount of cooling called for by the controller. Such a device is often referred to as a PMC, or poor man's calorimeter. It is not meant to be a commercial device.
Strategy overview: Because of size limitations and the need to go to temperatures below ambient, cooling by Peltier thermoelectric cooling (TEC) was selected. These devices are horribly inefficient , but cheap, simple and effective when minimal cooling below ambient is needed. A heater per se was not required, as it might be in a "real" calorimeter, because all the experiments I intend to perform will generate heat. For development work, I have used 2, 10Ω 10W resistors in parallel to simulate my experimental load.
An internal, thermally-conductive chamber is surrounded by insulation except for a "window" containing the TEC and associated heat sinks, fan, etc.
Electronics to control temperature are nearby. Control is by a simple on-off thermostatic control of the TEC. The circuits include a high-temp cut-out of the load current (for instance if heat production exceeds the cooling capacity), detecting the presence of the load, and a delayed shut-off of the fan so that the fan runs a little while after the TEC shuts down. All functions are indicated with LEDs.
Data collection is performed using data acquisition, details of which are beyond the scope of this write-up. If I had it all to do over, I'd put my data acquisition equipment to better use (eg. PWM control of the TEC). The current PMC design operates completely independently of the DAQ.
Design Parameters: I need to hold the chamber in the range of 10-20°C, and measure heat production of less than ~0.5W. So, this design is not immediately useful for significantly lower or higher temperatures. And, the cooling capacity of the current design limits the amount of heat production that can be used.
Components:
TEC I started this project by buying a couple of TEC 12709 modules, mostly just to play with. They each demand up to 10A at ~12v to run at full capacity. Supplied 5v, the TEC 12709 draws ~4.5A and consumes ~25 watts. It moves less than 1 watt out of the chamber, so it's not terribly efficient.
Power Supply Because of the high current demand of the TEC, I chose an old computer PSU to be the power supply for this project. The 12v supply was not up to that current demand, and I was worried about the TEC getting too hot at high current, so I decided to use the 5v supply for powering the TEC. With the TEC on the 5v supply, I chose to run everything else off the 12v supply. Thinking that I might someday want this all to operate off a car battery, I chose NOT to use the -12v supply, although this would have solved some issues. Only later did I learn what many here already know - that some loading of the 5v side is needed to bring the 12v supply under regulation. So my supply varies from maybe 9 to 16v when the TEC is off. When on, the 5v and 12v supplies both hold very steady. I'm considering putting a light bulb or something on the 5v supply to steady it out.
MOSFETs I chose the N-channel IRF540N because it could safely switch more than the full 10A demanded by the TEC, and because it was easy to find cheaply. The VN10KM was simply a small N-channel MOSFET I had in a parts drawer. It only switches a 60mA fan.
ICs The LM35 thermometer is cheap, widely available and a good fit for this project. I was hoping to use the LM34, since it produces a higher voltage difference between temperatures (10mv per °F, versus °C) AND it produces a voltage from a single supply all the way down to 0°F (instead of 0°C), but it's much harder to find. The LM339 quad comparator was chosen because I could walk into Radio Shack and buy one.
The LM358 dual op-amp was chosen because it can sense down to the negative rail. An early breadboard circuit with another op-amp was failing at lower temperatures because the LM35 voltage, 0.15v at 15°C, was falling too close to the rail and the op-amp could no longer "see" it. Using a negative supply fixed this, but I wanted to stick to a single supply.
Schematics The first post is the cooler and control. The second picture is the constant current (heater) control.
This is an early version of the thermostat that controlled to ±0.6°C or so. That's not bad but I decided I wanted a tighter control.
Here's another variation without the fan circuit, but with the op-amp added back.
Description: All circuitry except the TEC and its LED indicator, which are powered by the 5v supply, are powered off the 12v supply from the PSU. A 7805 regulator is used to provide voltage reference, and is also used to power all ICs. This is an arbitrary choice that limits the upper and lower temperatures that can be used. It would not be difficult to make changes to expand the working temperature range.
Chamber temperature 1-25°C is sensed by IC1 and amplified 18X by the first op-amp. The range is limited on the low end by the LM35 (produces 0v at 0°C) and the use of a single supply for the LM35 and the op-amp (cant sense below the negative rail). The upper end is limited by the 5v upper rail voltage. 25°C gives 0.25v from the LM35, times 18 for the gain is 4.5v, which may be near or above the upper range of the op-amp.
The amplified temperature is compared to a set point established by R9, a multi-turn Bourns precision pot, allowing very nice fine tuning. The comparator employs minimal hysteresis provided by the 10M resistor. I recommend reducing this to 5M or 3.3M. The impact on temperature swing is minimal but switching chatter is reduced. I tried several values and 10M was working OK on the breadboard. It works less well in the finished prototype. Comparator 1 output directs the MOSFET control of the TEC, and energizes an RC tank on comparator 2, turning on the fan and its indicator LED.
When the TEC turns off, the fan is held on a few seconds while C1 drains through R8. For simplicity, comparator 2 uses the same voltage reference as comp-1. So the fan delay is slightly affected by the temperature setting; higher temp equals less delay. Note that the fan turns ON immediately with the TEC. In fact it always comes on a bit before the TEC.
The HEATING part of the circuit is simply a resistive load, such as two 10Ω, 10W resistors in parallel, with a constant current supply. Op-amp 2 compares the voltage on a shunt resistor to a pre-set established by R15 (another Bourns pot). The darlington arrangement under op-amp control thus controls current through the load and shunt resistor. The op-amp connection to the shunt is overwhelmed, and current thereby cut off, when comparator 3 sees too high a voltage output from op-amp 1 (the amplified temperature). Comp-3 compares the output of op-amp 1 to a reference established by R16, a cheap, single-turn pot. So if temperature rises above a coarsely set maximum, the load (and heating) are shut off.
Presence of continuity through the load is detected by comp-4 and indicated with yet another LED. (Can you tell how much I love the pretty little flashing lights?!)
