How to Calculate the Heat in a Zener Diode
Most diodes are intended to conduct current in only one direction, from anode to cathode. Zener diodes, however, are designed to experience "breakdown" at a relatively low voltage. This means that a zener diode will conduct current in the reverse direction when the applied reverse voltage is larger than the zener voltage of that particular diode. A zener diode is used to maintain a stable voltage in a circuit. If a zener diode is conducting current, it is also dissipating power, causing its temperature to rise. Zener diodes are heat-sensitive, so it is important to calculate the heat they generate.
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Instructions
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Find the zener voltage of your zener diode. This information is available in the data sheet. The zener voltage indicates the voltage drop across the zener diode when it is conducting current.
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Determine the maximum amount of steady current that will flow through your zener diode. The most effective method is to calculate this maximum value using circuit analysis. If you have access to the finished circuit, you can also measure the current by placing the leads of an ammeter in series with the diode. This measurement, though, may not give you the maximum current, which would only occur under worst-case conditions.
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Multiply the zener voltage by the maximum current. This gives you the maximum power, in watts, that will be dissipated by your diode. Almost all of this power is converted to heat.
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Look in the diode's data sheet to find the "junction-to-ambient thermal resistance," sometimes abbreviated as the Greek symbol theta with a JA subscript. This number will have a unit of degrees Celsius per watt.
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Multiply the junction-to-ambient thermal resistance by the maximum power. The result of this multiplication is a number in degrees Celsius that represents the difference between the temperature of the diode and the ambient temperature. Add this number to your ambient temperature to determine the operating temperature of your zener diode.
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