BZX79-B4V7 — 4.7V ±2% 500mW Zener Diode (DO-35) from Nexperia

What Are the Technical Specifications of BZX79-B4V7?

Key numbers that matter: the 5mA test current is the whole story — per the NXP BZX79 datasheet (M3D176), 4.7V is only specified at that operating point. The 500mW rating and ±2% B-grade tolerance make it the tightest through-hole option in the low-voltage range.

When Should You Use (and NOT Use) the BZX79-B4V7?

✅ Use BZX79-B4V7 when:

• Through-hole repair or prototype work. The axial DO-35 body solders into a plated hole, survives rework, and reads clearly on a bench multimeter — still the repair-bench default.
• Temperatures above 150°C. The glass-sealed package is rated to 200°C, where plastic SOT-23 zeners stop.
• 500mW-class clamping. Double the continuous power of a BZX84-class SMD zener, plus 40W of non-repetitive peak for surge clipping.
• Low-cost ±2% reference. For an op-amp bias rail or a comparator threshold at 5mA, the B-grade holds tight enough without a reference IC.
• Board-level surge protection. A 4.7V rail clamp with a series resistor — the classic undervoltage clamp that keeps logic rails honest.

❌ Don't use BZX79-B4V7 when:

• You need precision regulation. Below 6V the zener knee is soft — voltage drifts with current and temperature. A TL431 programmable reference regulates to 1% regardless of current.
• Your board is SMD-only. No through-hole parts in the line means BZX84 (SOT-23) or BZT52H (SOD-123F) for the same voltage.
• You're regulating real current. A zener shunts current; it doesn't pass it. For a 100mA rail, use an LDO — the zener is the reference, not the regulator.
• You need a sharp clamp above 75V. The series tops out at 75V, and above ~6V the knee physics flips to avalanche anyway — pick the voltage to match.

What Are the Alternatives to BZX79-B4V7?

Which one for a repair bench? Through-hole and 200°C → BZX79; SMD assembly → BZX84; precise regulation → TL431.

On a repair bench, the BZX79-B4V7 is what the schematics call for.

What Are the Typical Applications of BZX79-B4V7?

Low-voltage rail clamping: a 4.7V zener across a 3.3V or 5V rail clips over-voltage events above its knee. With a series resistor to limit current, it's the standard cheap guard for logic rails in adapter-fed boards — the 40W non-repetitive peak rating absorbs the surge a capacitor bank throws at power-up.

Op-amp bias and comparator thresholds: at 5mA, the B-grade ±2% makes a stable enough reference for a bias rail or a comparator trip point. The 0.2mV/°C coefficient is small enough that the reference moves less than the circuit it feeds.

Repair-bench replacement: the schematic says "4.7V zener" and the through-hole pad is waiting — BZX79-B4V7 drops in where decades of power supplies and audio gear used BZX79, BZX55, or 1N5230B-class parts. Glass-sealed means it also survives the heat of old chassis designs.

Industrial temperature sensing and I/O protection: the 200°C rating and hermetic glass seal make it the choice for control boards that sit near motors, ovens, or engine bays — places where plastic SMD zeners fail early and the through-hole part keeps working.

Why Buy BZX79-B4V7 from ICMASS?

B-grade tolerance verified at the test current. We pull V_Z at the 5mA datasheet point on incoming lots. The failure mode that matters in this part — a C-grade die relabeled as B-grade — shows up immediately on that measurement.

Cross-reference support for the whole zener family. Not sure whether your board wants BZX79, BZX84, or BZT52H? Send the package, the power budget, and the temperature — we'll tell you which family the design actually needs, and keep the through-hole vs SMD decision straight.

BOM consolidation for power-supply repair kits. The same bench restock usually carries rectifiers (UF4007, 1N5819), a TL431, and BZX79 zeners — one shipment from Shenzhen covers the whole discrete section.

Same-day dispatch, 5–10 days worldwide. Orders before 15:00 CST ship same day via DHL or FedEx. For volume orders, we source directly from the Nexperia production line.

Frequently Asked Questions About BZX79-B4V7

Q1: Why does my BZX79-B4V7 measure the wrong voltage?

A: Because it isn't getting 5mA. The 4.7V rating is guaranteed at the 5mA test current — the dynamic impedance is 80Ω, so the voltage moves with the current. An EDAboard thread measured far over 4.7V after driving a zener with 91mA; a Raspberry Pi forum thread found a 3V zener clamping near 1V behind a 100k resistor. Feed it 5mA and the number lands.

Q2: What's the difference between BZX79-B4V7 and BZX79-C4V7?

A: The tolerance grade: B = ±2%, C = ±5%. Same 4.7V nominal, same 500mW DO-35 package, same test current. The C grade is the budget option; the B grade is what you pick when a comparator threshold or reference rail can't absorb a 5% swing.

Q3: Why is the knee "soft" at 4.7V?

A: Below roughly 6V, zener breakdown is quantum tunneling, not avalanche. The IV curve bends gradually instead of snapping, so the clamp voltage drifts with current. EEVblog threads on low-voltage zeners make this point repeatedly — the rated voltage is only true at the test current, and "soft knee" is a feature of this voltage band, not a fault in the part.

Q4: Can I use it instead of a BZX84-C4V7?

A: Electrically yes, mechanically no. Same 4.7V family, but BZX79 is through-hole DO-35 with 500mW and 200°C; BZX84 is SOT-23 with 250–300mW and 150°C. If your board is SMD-only, the BZX84 or BZT52H is the drop-in — if you have a hole to fill, the BZX79 is the stronger part.

Q5: I need tight regulation — is this the right part?

A: For precision, use a TL431. Forum threads consistently steer precision-regulator questions to the TL431 programmable reference: 1% regulation, set by two resistors, 100mA drive, and no knee physics to fight. The BZX79 is a reference-grade zener, not a precision regulator — at 5mA it's stable, but it won't beat an IC at its own game.

Q6: My zener blew on power-up. Why?

A: Surge current, not steady-state power. The 40W non-repetitive peak reverse rating covers transients, but a discharged capacitor bank can exceed even that for microseconds. A Hackaday repair log documented a 4.7V BZX79 that let a surge through on re-power — the fix is a series resistor sized to cap the peak current. Ten volts of headroom through 220Ω keeps the surge at the diode's rating.

Q7: What's the operating temperature range?

A: −65°C to +200°C. The glass-sealed DO-35 body is hermetically sealed and runs hot — that's the through-hole advantage. Plastic SMD zeners in the same family stop at 150°C, so for ovens, engine bays, or chassis near heat sources, the BZX79 is the one that keeps its spec.

Q8: How low can the operating current go?

A: It works below 5mA, but the voltage drifts down. A Raspberry Pi forum test showed a 3V-class zener clamping near 1V behind a 100k resistor (≈1mA). The knee is soft and the impedance is 80Ω, so under-driven means under-voltage. For stable operation, budget the 5mA — it's the price of a predictable clamp.

Q9: How much does temperature shift the voltage?

A: About 0.2mV/°C — roughly 53mV across the full −65 to +200°C range. 4.7V sits near the zero-temperature-coefficient point of the zener family, so it drifts less than high-voltage parts. A comparator threshold or bias rail moves less than 30mV over normal board temperatures — usually fine without compensation.

Q10: How do I spot a relabeled BZX79?

A: Measure V_Z at exactly 5mA. A C-grade die (5% tolerance) relabeled as B-grade (2%) fails the ±2% window at the test current — the measurement catches it immediately. On low-voltage zeners the profit on relabeling is thin, but the check costs one minute on a bench supply and a DMM.