II-VI Inc 33.18 (+0.91%)

• Last Trade 33.18
• Trade Time: 3/11/2010
• Change: +0.30,(+0.91%)
• Prev Close: 32.88
• Volume: 144513
• Market Cap: 1.020B
• 52wk Range: 15.15 - 34.29
• EPS (ttm): 0.776
• P/E (ttm): 42.37

• Technical Support Guide
• Support FAQ
• TEC Selection
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• Do you accept credit cards?

  Yes, we now accept VISA, Master Card, American Express, Discover & JCB.

• How do thermoelectric coolers (TECs) work?

  Thermoelectric coolers are solid state heat pumps that operate on the Peltier effect, the theory that there is a heating or cooling effect when electric current passes through two conductors. A voltage applied to the free ends of two dissimilar materials creates a temperature difference. With this temperature difference, Peltier cooling will cause heat to move from one end to the other. A typical thermoelectric cooler will consist of an array of p- and n- type semiconductor elements that act as the two dissimilar conductors. The array of elements is soldered between two ceramic plates, electrically in series and thermally in parallel. As a dc current passes through one or more pairs of elements from n- to p-, there is a decrease in temperature at the junction ("cold side") resulting in the absorption of heat from the environment. The heat is carried through the cooler by electron transport and released on the opposite ("hot") side as the electrons move from a high to low energy state. The heat pumping capacity of a cooler is proportional to the current and the number of pairs of n- and p- type elements (or couples).

  FIGURE 1 - Diagram of a typical thermoelectric cooler

  

  N and P type semiconductors (usually Bismuth Telluride) are the preferred materials used to achieve the Peltier effect because they can be easily optimized for pumping heat and due to the ability to control the type of charge carrier within the conductor.

  FIGURE 2

  

  Figure 2 illustrates an "N-type" semiconductor element utilized to facilitate the Peltier effect. In this example, negatively charged electrons are repelled by the negative pole and attracted to the positive pole of the DC power source. This forces electron flow in a clockwise direction through the "N-type" material. Heat is absorbed at the bottom junction and actively transferred to the top junction by the electrons as they flow through the semiconductor element. This is known as electron transport.

• What do the thermoelectric parameters Imax, Vmax, dTmax and Qmax mean?

  As current flows through a material, heat is generated. Thermoelectric material is no different. There is a point where the heat generated internally offsets the TECs ability to pump heat. Each TEC has a limit on how much heat that it can pump. This limit is referred to as Qmax. The current associated with Qmax is referred to as Imax. The corresponding voltage across the coolers is referred to as Vmax. If a TEC is completely insulated and isolated from the environment and running at Imax, it will produce its maximum temperature difference, dTmax. At this point it will also be pumping no heat whatsoever. As heat is applied to the cold side of the TEC, the temperature differential is suppressed. Effectively, one trades temperature differential for heat pumping. As such, if the temperature differential is 0, the corresponding heat load is Qmax. The coefficient of performance (COP) is defined as the amount of heat pumping one gets for each unit of electrical power supplied.

• What simple test can I use to verify that my TEC is operating correctly?

  Measuring electrical resistance is a good way to check the health of a TEC. Marlow recommends making an AC resistance measurement using a digital impedance meter or LCR. Using a standard ohm meter with a dc input signal will not yield accurate readings due to the fact that the dc voltage applied across the TEC will generate a temperature change and variations in resistance readings. Nominal AC resistance specifications for Marlow coolers are available upon request.

• What does a thermoelectric system offer over a compressor?

  Thermoelectric coolers are solid state heat pumps which have no moving parts and do not require the use of harmful CFCs. Because they have no moving parts they are inherently reliable and require little or no maintenance. They are ideal for cooling devices that may be sensitive to mechanical vibration. Their compact size makes them well suited for applications that are space or weight limited such as portable or airborne equipment. The ability to use TECs to heat as well as cool is great for applications requiring temperature stabilization of a device over a wide ambient temperature range such as laser diodes.

• What is the best way to power and control a TEC?

  Thermoelectric coolers require smooth dc current for optimum operation. A ripple factor of less than 10% will result in less than 1% degradation in delta T. Voltage or current limiting should be used in order to ensure that Imax for the TEC is not exceeded. A bipolar power supply is required for those applications requiring both heating and cooling. Pulse width modulation may be used at frequencies above 1 kHz. Linear proportional, PI or PID control can also be used. Marlow does not recommend ON/OFF control. While this is the simplest control technique, temperature cycling within the TEC as power is cycled from full ON to full OFF may result in premature failure.