This website displays advertisements, if you click on Used only to respond to your message, it will not be given to anyone else. If you send an email your email address and any personal information will be This website does not collect personal information. If the steps above seem too complex you can try attaching a moderately large heat sink and hope for the best.Ĭautiously monitor the transistor temperature with your finger, if it becomes painfully hot switch off Then the transistor will be hotter than the heat sink and the calculation becomes more difficult.įor typical mica sheets you should subtract 2☌/W from the thermal resistance (Rth) value calculated in step 4 above. However, you may have to put a mica sheet or similar between them to provide electrical insulation, This is a reasonable assumption if they are firmly bolted or clipped together. All the above assumes the transistor is at the same temperature as the heat sink.So the transistor temperature will rise to 25 + 30 = 55☌ (safely less than the 100☌ maximum). (remember lower value means better heat sinking) for example 5☌/W would be a sensible choice to allow a safety margin.Ī 5☌/W heat sink dissipating 6W will have a temperature difference of 5 × 6 = 30☌ Choose a heat sink with a thermal resistance which is less than the value calculated above.Work out the maximum thermal resistance (Rth) for the heat sink using:.If the heat sink is going to be outside the case Tair = 25☌ is reasonable,īut inside it will be higher (perhaps 40☌) allowing for everything to warm up in operation. Estimate the maximum ambient (surrounding air) temperature (Tair). ![]() These power transistors will almostĬertainly need a heat sink to prevent them overheating. On most of the time and V CE may be about half the supply voltage. However, power transistors used in circuits such as an audio amplifier or a motor speed controller will be partly Switch because when 'full on' V CE is almost zero. The heat is not a problem if I C is small or if the transistor is used as a Power is determined by the collector current I C and the voltage V CE across the transistor: P = I C × V CE Usually the base current I B is too small to contribute much heat, so the thermal The rate of producing waste heat is called the thermal power, P. The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for transistors passing large currents. They may have a different case style so take care when placing on the circuit board. Possible substitutes - transistors with similar electrical properties which will be suitable There may be separate tables for different categories. Transistors operating as amplifiers, the power is roughly I C × V CE.įor transistors operating as switches the maximum collector current (I C max.) is more important.Ĭategory - typical use for the transistor, a good starting point when looking for a substitute. Heat sink will be required to achieve the maximum rating. maximum total power which can be developed in the transistor, note that a Transistor to transistor this detail is only really of interest to experts. Since the gain is roughly constant for various currents but it varies from Sometimes minimum and maximum values are given. Which is usually in the middle of the transistor's range, for example means the gain is at least 100 at 20mA. ![]() The gain is often quoted at a particular collector current I C Note that current gain is just a number so it has no units. ![]() The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! H FE - the current gain (strictly the DC current gain). maximum voltage across the collector-emitter junction, ignore this for low voltage circuits. Structure - type of transistor, NPN or PNP, a substitute must be the same type. ![]() These are some of the terms you are likely to see: They contain a great deal of useful information but can be difficult to understand if you are not familiar with the To make a final choice you may need to consult tables of technical data provided in catalogues, books and on-line. To make selection easier most suppliers group their transistors in categoriesĭetermined either by their typical use or maximum power rating. The most important properties to look for are the maximum collector current I CĪnd the current gain h FE. Most projects will specify a particular transistor but you can usually substitute an equivalent transistorįrom the wide range available.
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