Microchip Technology 1N978A

Overview

1N978A from Microsemi is a silicon 500 mW axial-lead Zener diode in DO-35 (DO-204AH) package, rated for 47 V nominal Zener voltage with ±10% tolerance (A suffix), 2.7 mA Zener test current, 105 Ω dynamic impedance at I_ZT, and maximum 7.9 mA DC Zener current. It operates across –65°C to +175°C and regulates voltage in low-power reference and protection circuits.

For engineers reviewing the 1N978A datasheet, 1N978A pinout, 1N978A application, or 1N978A equivalent, this part serves as a standard-voltage, through-hole Zener regulator where stable 47 V clamping, low leakage (<5 μA @ 35.8 V), and JEDEC-registered performance are required in industrial power supplies, sensor biasing, and overvoltage protection.

Technical Context

The 1N978A functions as a reverse-biased voltage reference device, maintaining regulation via controlled avalanche breakdown at its cathode-banded terminal. Its Zener voltage exhibits +0.095%/°C temperature coefficient and 1500 Ω maximum impedance at knee current (I_ZK = 0.25 mA), confirming stable operation under varying thermal conditions.

Designed for axial-leaded mounting on FR4 PCBs with 1 oz copper, it delivers 400 mW maximum dissipation at 75°C lead temperature and supports surge currents up to 40 A (1/120 sec pulse). Polarity is marked by cathode band; operation requires banded end positive relative to anode.

Key Specifications

Pinout & Package

DO-35 (DO-204AH) hermetically sealed glass axial-leaded package with cathode indicated by colored band; terminals are solderable per MIL-STD-750 Method 2026.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar Zener regulation applications.

Compared with 1N978B and MMBZ5256B, the 1N978A provides the lowest-cost through-hole solution for non-critical 47 V clamping, balancing tolerance, thermal robustness, and JEDEC-standardized form factor-ideal for legacy designs and manual prototyping.

Availability

1N978A is available at Aetrix Electronics and suitable for industrial power supplies, sensor biasing circuits, and overvoltage protection systems requiring stable component supply, long-lifecycle support, and JEDEC-compliant Zener performance.

Supply support for 1N978A includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Microsemi (now part of Microchip Technology) is a U.S.-based semiconductor company specializing in high-reliability analog and mixed-signal components for aerospace, defense, and industrial markets.

The 1N957B–1N992B series was engineered as a JEDEC-standardized family of 500 mW glass Zeners for rugged, temperature-stable voltage regulation in mission-critical and high-temperature environments.

FAQ

What is the Zener voltage tolerance for 1N978A?

The 1N978A has a nominal Zener voltage of 47 V with ±10% tolerance, as indicated by the "A" suffix per Microsemi's documentation. This means the actual measured V_Z falls between 42.3 V and 51.7 V at 2.7 mA test current and 25°C ambient. The tolerance is wider than B-suffix variants (±5%) to accommodate cost-sensitive applications where absolute precision is secondary to reliability and thermal stability.

What is the maximum power dissipation for 1N978A?

The 1N978A is rated for 500 mW steady-state power at lead temperature <50°C (3/8″ from body), but its usable DC power is limited to 400 mW at 75°C lead temperature per I_ZM calculation. When mounted on FR4 with 1 oz copper, derating begins above 25°C ambient, reaching zero dissipation at 175°C per the published power derating curve in Figure 1 of the datasheet.

How is polarity identified on the 1N978A?

The 1N978A uses a cathode band - a visible colored ring near one end of the DO-35 glass body - to indicate the cathode terminal. For proper Zener regulation, the banded end must be held at a higher potential than the unbanded (anode) end. Forward voltage is guaranteed ≤1.3 V at 200 mA, confirming standard silicon diode conduction in forward bias.

Is 1N978A RoHS compliant?

The base 1N978A is not inherently RoHS compliant; RoHS compliance requires the "e3" suffix (e.g., 1N978Ae3), which denotes annealed matte-tin plating instead of traditional tin-lead. Microsemi's documentation confirms that "e3" devices meet JEDEC J-STD-609A Class 1 requirements, while unsuffixed or "A"-only variants contain lead per exemption 7a.

What is the temperature coefficient of 1N978A?

The 1N978A has a positive temperature coefficient of +0.095 %/°C, meaning its Zener voltage increases by approximately 44.65 mV per °C rise in junction temperature. This value is confirmed in the Electrical Characteristics table on page 2 of the Microsemi datasheet and applies across the –65°C to +175°C operating range, enabling predictable thermal drift compensation in reference designs.