This project, a thermoelectric temperature circuit from 1990, is interesting because of the special property of the TEC element which is that it can either heat up or cool down.
The great thing about TEC devices is that they can cool a device and yet do not have any moving parts at all so they are ideal for electronics that must be cooled yet must may be buried underground or underwater and that must also have high reliability e.g. telecoms equipment.
A thermoelectric cooler temperature control circuit provides a periodic control signal having a substantially constant peak-to-peak magnitude and an average value dependent on a sensed temperature to be regulated. The control signal is coupled to control the heating or cooling of a thermoelectric cooler.
In a preferred embodiment, complementary positive and negative going control signal pulse streams are provided. A switch selectively couples one of the complementary pulse streams to cool the thermolectric cooler or the other stream to heat the cooler. The control signal pulse streams can be pulse width modulated signals.
The present design relates to thermoelectric coolers, and more particularly to a control circuit for regulating the temperature of a laser or other device coupled to a thermoelectric cooler.
Temperature control of electronic devices is an important design consideration. Solid-state components such as transistors generate heat in operation. Excessive amounts of heat can damage the component or other sensitive elements of a circuit in close proximity.
With advances in optical technologies, such as optical fiber communication, electrical signal processing is being replaced in many applications by optical signal processing. For example, cable television signals previously transmitted over wire (i.e., coaxial cable) are now capable of being carried on optical fiber networks. Optical transmission technologies provide greater bandwidth, better signal-to-noise ratio, and potentially lower cost.
In order to transmit a signal over an optical fiber, lasers are used to provide a coherent light source. The lasers generate a substantial amount of heat, and must be cooled to prevent failure. Pump lasers in particular require a great deal of cooling for reliable operation.
In cable television applications, lasers are provided along the communication network for signal amplification. Signal distribution amplifiers are mounted on utility poles and must withstand severe environmental conditions. The internal temperature of the amplifier housing can range, for example, from -20° C.
to 90° C. The distribution amplifiers must be designed to operate over this entire temperature range. In order to maintain the temperature of the lasers used in the amplifiers within their design specifications, cooling and heating devices are required.
One device that can be used to regulate the temperature of a laser is a thermoelectric cooler ("TEC"). These devices are small heat pumps that obey the laws of thermodynamics as do conventional mechanical heat pumps such as refrigerators. TECs, however, are solid-state devices that operate as Peltier coolers.
A single-stage thermoelectric cooler is composed of a matrix of thermoelectric couples, connected electrically in series and thermally in parallel. When current is conducted through the thermoelectric couples in one direction, a first plate of the TEC cools down and a complementary second parallel plate heats up.
When current is conducted in the other direction, the first plate heats up and the second plate cools down.
TEC cooling is proportional to the applied current. The power dissipated by Joule heating in the TEC is proportional to the square of the current. Thus, an increase in current above a certain value will result in less net cooling because the Joule heating is increasing at a faster rate than the Peltier cooling. It is therefore important to carefully control the current that is applied to the TEC.
Known control circuits for regulating the temperature of a thermoelectric cooler use a linear driver to input current to the cooler. Because of their design, the operation of linear drivers can be very inefficient, generating additional heat that must be dissipated into a heat sink. Since they operate in a linear fashion, these drivers are always drawing current and must dissipate power constantly.
Such operation defeats the cooling function of the TEC, and is particularly damaging in a cable television fiber distribution amplifier that encloses the circuitry in a heat retaining weather-tight box.
It would be advantageous to provide a temperature control circuit for a thermoelectric cooler that operates in a nonlinear fashion. Both heating and cooling of a device to be protected should be provided in a reliable and efficient manner. It would be further advantageous to provide such a circuit that operates using the same voltages already present in the circuitry for the device being protected. The present design provides a control circuit and method for driving a thermoelectric cooler having these advantages.
In accordance with the present design, a temperature control circuit is provided for a thermoelectric cooler. Temperature sensor means provide an output signal related to a sensed temperature. Means responsive to the output signal generate a periodic control signal having a substantially constant peak-to-peak magnitude and an average value dependent on the sensed temperature. The control signal is coupled to regulate the temperature of a thermoelectric cooler.
In a preferred embodiment, complementary positive and negative going control signal pulse streams are provided. Switch means selectively couple one of the complementary pulse streams to cool the thermoelectric cooler or the other pulse stream to heat the cooler. In order to provide the pulse streams with an average value dependent on the sensed temperature, the pulse streams can be pulse width modulated. Such modulation imparts a variable duty cycle to the signals that depends on the magnitude of the temperature sensor output signal.
The temperature sensor means can be designed to provide a first polarity output signal when the sensed temperature is on one side of a threshold (e.g., above the threshold temperature) and a second polarity when the sensed temperature is on the other side of the threshold (e.g., below the threshold temperature).
The different polarities can be used to signal the circuitry to either heat or cool the thermoelectric cooler. Means for obtaining an absolute value of the output signal are provided for use in generating the control signal.
In a more specific embodiment, apparatus is provided for regulating the temperature of a laser. A thermoelectric cooler is thermally coupled to the laser. Temperature sensor means, thermally coupled to the laser, provide an output signal related to the laser temperature. Means responsive to the output signal generate a periodic control signal having a substantially constant peak-to-peak magnitude and an average value dependent on the laser temperature. A control signal is coupled to the cooler for regulating the laser temperature.
A method is provided for driving a thermoelectric cooler. A temperature to be controlled by the cooler is sensed, and a periodic control signal is generated having a substantially constant peak-to-peak magnitude and an average value dependent on the sensed temperature. The control signal is coupled to regulate the temperature of the cooler.
In a preferred embodiment, two complementary control signal pulse streams are provided, one having a positive polarity and the other having a negative polarity. One of the complementary pulse streams is coupled to the cooler to provide a cooling effect when the temperature to be controlled is above a threshold.
The other pulse stream is coupled to the cooler to provide a heating effect when the temperature to be controlled is below the threshold.
FIG. 1 illustrates a prior art thermoelectric cooler control circuit. A conventional differential bridge amplifier uses a differential DC amplifier 14 to amplify the output of a transducer bridge generally designated 10. One element of the bridge is a thermistor 12 that has a resistance which varies with temperature.
Nominally, the four arms of bridge 10 have equal resistances. For example, thermistor 12 can have a resistance of 10K ohms at 25° C., and each of the remaining three branches of the bridge can comprise a 10K ohm resistor. As the temperature rises or falls from 25° C., the fractional change of the thermistor resistance is input to amplifier 14 and amplified.
The amplified difference signal output from amplifier 14 is integrated by an active filter 16. The resultant signal controls a pair of transistors 18, 20 each configured as an emitter follower. These transistors determine which polarity and how much current is applied to a thermoelectric cooler 22.
Since transistors 18, 20 operate in a linear fashion, they can dissipate a substantial amount of power when turned on. Thus, for example, where the thermoelectric cooler requires a current of two amps and there is a three volt drop across transistor 18 or 20, a total of six watts will have to be dissipated by the transistor.
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