Description
The BZX79-C18 is a standard 18V Zener diode, frequently used in circuits requiring voltage regulation, clipping, or transient protection where an 18V reference or clamp is needed.
Technical Specifications
| Parameter | Specification |
| Nominal Zener Voltage ($V_Z$) | 18V |
| Power Dissipation ($P_D$) | 500mW (0.5W) |
| Package Type | DO-35 (Glass Axial Lead) |
| Tolerance | $\pm 5\%$ |
| Zener Impedance ($Z_Z$) | 45 $\Omega$ (at $I_Z = 5mA$) |
Operational Overview
The BZX79-C18 is designed to operate in the reverse breakdown region. Once the applied voltage reaches 18V, the diode begins to conduct, effectively clamping the voltage at that level by diverting excess current to ground.
To implement this as a regulator, the diode must be connected in reverse bias (with the cathode/banded side connected to the positive supply) and placed in series with a current-limiting resistor ($R_s$). The resistor is essential to ensure that the current through the Zener does not exceed its 500mW power limit, which would cause the device to overheat and fail.
Application & Safety Guidelines
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Thermal Management: Using the power formula ($P_D = V_Z \times I_Z$), the maximum current ($I_Z$) allowed through this device is:
$$I_{Zmax} = \frac{0.5W}{18V} \approx 27.7mA$$For reliable operation, you should design your circuit so that the current is significantly below this limit, ideally targeting the 5mA to 15mA range for stable regulation.
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Troubleshooting: To test the component with a multimeter, check for a forward voltage drop of approximately 0.6V–0.7V in one direction. In reverse bias (at low voltage), it should appear as an open circuit. If you measure near 0 ohms in both directions, the diode has failed due to an overcurrent or thermal event.
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