Description
The 18V 0.5W Zener diode is a standard component used for voltage regulation and transient overvoltage protection in electronic circuits. It is most commonly identified by the industry-standard part number 1N5248 or the BZX79-C18 series.
Technical Specifications
| Parameter | Specification |
| Nominal Zener Voltage ($V_Z$) | 18V |
| Power Dissipation ($P_D$) | 0.5W (500mW) |
| Package Type | DO-35 (Small Glass Axial) |
| Tolerance | $\pm 5\%$ |
| Operating Temperature | -65°C to +200°C |
Operating Principle
This component is designed to operate in the reverse breakdown region. When the voltage across the diode reaches 18V, it begins to conduct, shunting excess current to ground and “clamping” the output at 18V.
To function as a regulator, you must install the diode in reverse bias and include a series current-limiting resistor ($R_s$). This is critical: without this resistor, the diode would draw unregulated current, exceed its 500mW power limit, and likely fail as a short circuit.
Application & Safety Guidelines
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Thermal Management: To keep this 500mW device safe, ensure the current ($I_Z$) flowing through it does not exceed the limit. Using the formula $I_{Zmax} = P_D / V_Z$, your maximum current should be kept below ~27mA ($0.5W / 18V$).
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Procurement: You requested 2 pieces. These are widely available; search for “1N5248” to find this exact specification.
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Troubleshooting: If the voltage on your 18V rail is zero or lower than expected, check the Zener diode. A failed Zener diode typically measures as a dead short (near 0 ohms) in both directions on a multimeter.
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