Thermoelectric cooler

Thermoelectric cooler is a device that converts from electricity to temperature difference. This device consists of p-type and n-type materials. When voltage is applied, current flows and result the difference temperature between its surfaces . Hot and cold surfaces can be reversed by reversing the direction of current.

Thermoelectric

Thermoelectric cooler is one of the methods to produce low temperature. Comparing with conventional method (vapor compression), thermoelectric cooler has advantage such as: thermoelectric can be a cooler or heater by change the direction of electrical current, no vibration, quite, reliable, and no refrigerant (environmental friendly)

However, disadvantage of thermoelectric cooler performance is very low. Coefficient of performance of Thermoelectric cooler is only about 0.3 (vapor compression COP : 3-4).

Due to its advantages, thermoelectric cooler can solve the problem that conventional method cannot be applied to it, such as electronics cooling.

The application of thermoelectric has been introduced in many aspects. We can find thermoelectric cooler system at small refrigerator or even thermoelectric air conditioner. In electronics, thermoelectric cooler also is one of the solutions to maintain chip temperature below allowable maximum temperature. Thermoelectric can be attached to the chip and cool the chip directly.

Performance of thermoelectric will decrease if heat dissipation from hot side is not released properly.  Heat sink can be attached to the hot side of thermoelectric cooler to support heat transfer from hot side to the ambient. Moreover, thermoelectric cooler can produce very low temperature by making cascade thermoelectric cooler. It means hot side thermoelectric cooler is attached by another thermoelectric cooler.

Heat Sink in Electronics cooling

Electronics technology has been developing rapidly since the first transistor was invented. Semiconductor technology is the most significant part for electronics technology development. At the first time one transistor with very small size could replace cathode tube. Now, thousand or even million transistors can be operated in one single chip. However, electronics have maximum temperature to be operated properly. A survey showed that the most cause of electronic failure is temperature. Thus electronics cooling cannot be separated in electronics technology.

Many types of cooling system for electronics have been introduced. The most common electronics cooling system is heat sink. Heat sink system can be divided into two parts, forced convection and natural convection.

Natural convection is convection heat transfer without any force applied to heat sink. This convection occurs because of buoyancy force naturally. Fluid (e.g air) has lower density if its temperature is high, this causes air moves up.  In electronics, heat dissipated from chip causes increasing temperature surrounding then air density becomes low, thus air moves up. Since natural convection only uses buoyancy force, usually natural convection heat sink is attached vertically.

Forced convection need additional force to move air flows on heat sink. Fan is usually used to support air flow on heat sink. Heat transfer rate at this heat sink is bigger than natural convection. Many electronic equipments use this type.

Heat sink is attached to heat source to enhance heat transfer rate. When heat dissipated from electronic device cannot be overcome by heat sink, it needs additional system or even different cooling system. Heat pipes or thermoelectric cooler may be an option for additional system in heat sink. 

How to measure pulsed current on electrical circuit??

In some cases, pulsed current generator is needed, such as of LED performance test. In that case, LED need to be injected by pulsed current. Actually, there are some company that produce and sell pulse current generator but it is usually very expensive. To solve this problem we can use pulsed voltage generator instead of it which are cheaper than pulsed current generator.

By using pulsed voltage generator, we may be confused how to measure the current. Pulse current is difficult to measure because of two reasons. First, it is pulsed, so we cannot measure the current by ampere meter or clamp ampere since those are for constant current. Second, it is current which cannot be measured by oscilloscope. So, how to measure it??

To measure pulsed current on electrical circuit, we need a known resistant (such as resistor) to be attached serial on the circuit. Then, activate the circuit and measure voltage at the resistor by oscilloscope. By this method, voltage can be measured and resistance is already known. As known that currents at serial connection have same value, Then by simple equation that current value equals voltage divided by resistance (I=V/R)current can be calculated easily.

Also we can control the current by controlling voltage of pulse generator. If we can’t control the voltage we may use potentiometer. These have the same principle, (I=V/R). When we control voltage of pulse generator it means we change V. And the other hand if we use potentiometer, it means we change the resistance(R).

In using pulsed generator, be careful to set the current (it means set up voltage and selection of the resistor). If current exceed generator limit (see the specification of generator), it may cause damage to it. If it happens, fuse inside generator will be cut. But if fuse is already broken, damage may occur in generator component. 

Refrigeration Cycle

Refrigeration cycle produces hot part and cold part (compare to ambient temperature). It needs at least four components, they are compressor, condenser, expansion device, and evaporator. Work fluid (refrigerant) goes around from one component to other component again and again, so we call it cycle. Refrigerant experiences all process in components

Figure 1. Refrigeration cycle schema

Figure 1 show schema of refrigeration cycle which consist of four main components and connected pipes (showed by line), and also the flow direction of refrigerant is showed at that figure. At the figure, there are two types of line, red and blue. The red one shows that the pressure is high and the blue shows the pressure of refrigerant is low.  As known, pressure is proportion by temperature, then the color of line also show the temperature condition of refrigerant.

1 to 2, Compression process. Output from evaporator, refrigerant is gas phase. Then compressor sucks it from evaporator and discharge it to condenser.  Before through the compressor the refrigerant pressure  is low and after through compressor refrigerant pressure is high. This process also makes the refrigerant temperature increase.

2 to 3, Condensation process. Output from compressor, refrigerant phase is still gas. As its name condenser is a component to make refrigerant phase change  from gas into liquid. This process needs to release heat. Usually condenser is put outside, because the temperature of ambient outside is lower than temperature of refrigerant in condenser, then heat transfer can occur from refrigerant to outdoor air temperature.

3 to 4, Expansion Process. Output from condenser, refrigerant phase  is already liquid and has  high pressure and temperature. Expansion device makes pressure of refrigerant drop into low pressure, this process also is followed by drop temperature. Most of refrigerant phase after through expansion valve is still liquid and a few gas.

4 to 1, Evaporation Process. As its name, the function of this component is to make refrigerant phase changes from liquid into gas. This process need heat, and heat is got from the objects which want to be cooled. The temperature of refrigerant in evaporator is lower than the objects, then heat transfer will occur from objects to refrigerant, then heat is used for evaporate refrigerant. After the refrigerant is already in gas phase, refrigerant  will experience the process from number   1 again. It happen repeat and repeat again.