1SMA5942BT3G — 1.5W 51V Zener Voltage Regulator (SMA)

What Are the Technical Specifications of the 1SMA5942BT3G?

Key numbers that matter. The 29mA I_ZM is the whole story: at 51V, 1.5W buys a quarter of the 12V grade's current. Thermals are identical across the series — 1.5W at a 75°C lead temperature, derating 20mW per degree above; a minimum footprint manages 0.5W at 25°C ambient.

High-voltage zeners have soft knees — 1100Ω says so. Near the knee, one milliamp of current change swings V_Z by roughly a volt. Above the knee it stiffens to 70Ω — 70mV per milliamp. And at 38.8V reverse the part leaks 0.5µA max, so anything below that line stays quiet.

When Should You Use (and NOT Use) the 1SMA5942BT3G?

What Are the Alternatives to the 1SMA5942BT3G?

The trap in one line: SMAZ5942B-E3 is a 500mW part with the same "5942B" in its name. It's a fine zener — for a different job. When a cross-reference list offers it as a "1SMA5942B equivalent", the pinout matches and the thermal budget doesn't.

Need more headroom? The 1SMB5942B doubles the power to 3W in an SMB body. Need tighter voltage? Zeners don't get tighter — that's what the TL431 is for.

Same "5942B" on the label, very different wattage:

What Are the Typical Applications of the 1SMA5942BT3G?

48V undervoltage detection. Telecom and industrial plants run 48V nominal, and equipment must shut down cleanly when the bus sags. A divider, this zener, and a comparator make a hardware trip point — no firmware, no hang risk, and the 48.45V minimum sits right on the "healthy rail" boundary.

36V battery overvoltage clamping. Charger overshoot and regenerative spikes ride above a 36V pack's normal 42V peak. The zener's 48.45V minimum stays clear of normal operation and clamps only the real excursions.

HV-bus bias supplies. Gate drivers and floating measurement front ends on 120V+ rails need a local supply. A series resistor plus the zener derives 51V from the bus with two parts, and the resistor keeps the zener inside its 29mA limit.

Pre-regulation ahead of LDOs. Two-stage dropping protects a low-voltage regulator from a hostile HV bus: the zener caps the LDO input, and the LDO does the precision work the zener can't.

High-voltage test-point protection. With a series resistor ahead, the zener clamps transients before a measurement pin. The cost is a fraction of a protection IC — at the price of the 0.5µA leakage you must account for on high-impedance sources.

Why Buy the 1SMA5942BT3G from ICMASS?

V_Z lands where the datasheet says: 48.45–53.55V at 7.3mA. The B-suffix bin guarantees the window, and the reels arrive with date and lot codes intact so your 48V detection thresholds stay where you calculated them.

Quiet below 38.8V. The 0.5µA maximum leakage point sits well under the operating voltage of 36V systems, so the part draws nothing measurable in normal operation — and only 0.5µA max even at the edge.

One order covers the whole power section. This zener rides with the rectifiers, MOSFETs, and switcher controllers on a single PO from our Shenzhen stock — full 5,000-unit reels or cut tape, dispatched same day.

Traceable factory reels. Date and lot codes intact, straight from the manufacturer's packaging. For automotive programs, the SZ1SMA5942BT3G variant carries AEC-Q101 qualification and PPAP capability.

Frequently Asked Questions About the 1SMA5942BT3G

Q1: Can I clamp a 48V telecom bus with this 51V zener?

A: No — the bus floats above the zener. Telecom 48V plants float at 49.9–56.4V depending on cell count, per Lucent LINEAGE 2000 manuals and IEEE 802.3 documents. The zener's 53.55V maximum sits inside that range, so it conducts at float and burns up. Use it to sense and flag, or pick a clamp voltage above the float ceiling.

Q2: Why does a 1.5W zener only handle 29mA?

A: Because 1.5W divided by 51V is 29mA. I_ZM is the power rating divided by the zener voltage, so high-voltage grades carry far less current than low-voltage ones for the same watt. The 12V grade in the same series manages 125mA.

Q3: What does the 1100Ω knee impedance mean in practice?

A: Near the knee, 1mA of current change swings V_Z by about a volt. High-voltage zeners have soft knees, so a "51V clamp" running at 1mA sits noticeably below 51V. Above the knee it stiffens to 70Ω — 70mV per milliamp. If your circuit runs the zener near its test current of 7.3mA, you get the datasheet window; far below it, you get less voltage.

Q4: How much current can it handle continuously?

A: 29mA absolute maximum, far less on a real board. The 1.5W rating needs a 1-square-inch pad and a 75°C lead temperature, derating 20mW/°C above; a minimum footprint manages only 0.5W at 25°C ambient — 10mA at 51V. Design for 8mA on a small footprint, 25mA with real copper; beyond that, step to the 1SMB5942B (3W).

Q5: Does a zener always need a series resistor?

A: Always. Without one, the supply fights the diode directly and the part fails. Size R_S = (V_IN − V_Z) / (I_Z + I_LOAD), then check worst-case zener dissipation at minimum load and maximum line voltage.

Q6: My 48V undervoltage detector trips at 47V. Is the part bad?

A: Probably not — check the zener current and junction temperature. The 48.45V minimum holds only at 7.3mA and 25°C. Run the zener at a milliamp or so and it sits below spec; the datasheet curves also show V_Z moving with temperature. This "detector trips early" report recurs on EE StackExchange, and it almost always traces back to operating point, not the part.

Q7: What's the forward voltage drop?

A: 1.5V max at 200mA per the datasheet. In practice a forward-biased zener drops about 0.7V at low current like any silicon diode, rising toward the maximum at higher current. For polarity protection a Schottky is far better, at 0.2–0.3V.

Q8: Should I use this 51V part or the 1SMA5927BT3G (12V)?

A: Match the zener to the bus voltage, then check the current. The 12V part suits 12V and 24V systems; the 51V part suits 36V and 48V systems. Remember the trade: the higher the voltage grade, the fewer milliamps the same 1.5W rating buys.

Q9: 51V zener or a TVS for surge protection?

A: TVS for energy, zener for thresholds. A TVS absorbs joules of surge current for microseconds; a zener holds a predictable voltage for detection and light clamping. Telecom cross and lightning-induced transients are TVS work — this part clamps overshoot, not strikes.

Q10: Can I parallel two zeners for more current?

A: Don't — zeners don't share. A few hundred millivolts of V_Z mismatch sends most of the current through one device while the other idles. It's a classic electronics-forum question, and the answer never changes: use one larger part (1SMB5942B) or move to a proper regulator.