FAQ
TL;DR: To ID an unreadable Zener, forward-test it (≈0.5–0.75 V), then reverse-bias with a series resistor; a 1 kΩ limit gives ≈1 mA one volt above the knee—“begin with a voltage around 1 or 2 volts.” [Elektroda, Steve Lawson, post #21662546]
Quick Facts
- Forward diode-test shows ≈0.5–0.75 V; reverse clamp level is the Zener voltage. [Elektroda, Frank Bushnell, post #21662545]
- Using 1 kΩ limits test current to ≈1 mA at ~1 V above breakdown; ramp slowly from 1–2 V. [Elektroda, Steve Lawson, post #21662546]
- For unknown parts, 1.5–2 kΩ keeps ≤30 mA up to 50 V; use a ≥2 W resistor. [Elektroda, Farhan Ali, post #21662547]
- SMD Zeners often lack printed voltages; many show only a code or nothing. [Elektroda, Steve Lawson, post #21662549]
- Power handling is estimated by package size; compare with known diodes of similar size. [Elektroda, Frank Bushnell, post #21662545]
Who is this FAQ for, and what problem does it solve?
For repairers, hobbyists, and test engineers who find Zener markings unreadable. It shows safe ways to measure breakdown voltage, estimate current and power ratings, and confirm polarity using common bench gear. You’ll avoid overcurrent damage and misidentification while extracting the key specs you need. [Elektroda, Frank Bushnell, post #21662545]
How do I measure an unknown Zener voltage safely?
1) Put a resistor in series to limit current (~1 mA). 2) Reverse-bias the Zener and place a voltmeter across it. 3) Slowly raise supply voltage until the diode clamps; that stable value is the Zener voltage. “Begin with 1–2 V and increase slowly.” [Elektroda, Steve Lawson, post #21662546]
What resistor value should I start with for a gentle test?
Use 1 kΩ. It limits current to around 1 mA when your supply sits about 1 V above the Zener knee. This low-current method protects small-signal Zeners and gives a clear clamp reading without heating. Ramp the supply slowly from 1–2 V upward. [Elektroda, Steve Lawson, post #21662546]
How can I push-test an unknown Zener when I don’t know its rating?
Pick 1.5–2 kΩ and size it for a couple of watts. That keeps current at or below about 30 mA even if you raise the supply to 50 V. Watch the diode voltage for the clamp and stop increasing once it stabilizes. [Elektroda, Farhan Ali, post #21662547]
How do I know it’s really a Zener and not just a regular diode?
Do a forward diode test first; expect about 0.5–0.75 V like a normal silicon diode. Then reverse-bias with a series resistor. A true Zener will reach a stable clamp voltage in reverse; a regular diode will not. [Elektroda, Frank Bushnell, post #21662545]
What does the “Zener knee” mean?
It’s the reverse-bias point where current starts flowing significantly and voltage stabilizes near the rated breakdown. Below the knee, voltage tracks the supply; above it, the diode clamps near its Zener value. “The Zener will not allow reasonable current until avalanche voltage.” [Elektroda, Steve Lawson, post #21662546]
Which way round do I connect the diode during the test?
Reverse-bias it: place the Zener’s cathode toward the positive side of the battery or supply. Put the series resistor in line, and measure across the diode. This orientation reveals the reverse breakdown voltage safely. [Elektroda, Farhan Ali, post #21662547]
What if the measured reverse voltage sticks near ~1 V and won’t rise?
Your polarity is likely wrong. Reverse either the diode or the power supply connections and measure again. If the voltage now climbs and then clamps, you’ve corrected the biasing error. This edge case is common during quick tests. [Elektroda, Farhan Ali, post #21662547]
How can I estimate the Zener’s power rating without markings?
Compare its package and size to known parts. Larger glass or epoxy bodies usually handle more power. Match to a similar physical diode whose datasheet lists wattage (e.g., 0.25 W, 0.5 W, 1 W). This gives a practical ceiling for test current. [Elektroda, Frank Bushnell, post #21662545]
Are voltage and current ratings printed on Zeners?
Often no. Many through-hole parts show only part numbers, and many SMD Zeners lack any legible marking. Expect to look up a code or measure directly when codes are absent or ambiguous. [Elektroda, Steve Lawson, post #21662549]
Can I just look up the ratings if I find a part number?
Yes. Enter the printed part number into a search engine and read the datasheet for Zener voltage, test current, and power rating. This is the fastest path when markings are present and readable. [Elektroda, David Figueroa, post #21662550]
Can I use a battery instead of a bench power supply?
Yes. A DC battery works if you include a suitable series resistor. Increase the applied voltage by changing battery count or using a variable supply, and watch for the clamp point across the diode. [Elektroda, Farhan Ali, post #21662547]
Why is current limiting critical in Zener tests?
Above breakdown, Zeners conduct heavily. Without a series resistor, current can surge and overheat the part. Limiting to around 1 mA for initial tests protects unknown devices while you identify their breakdown voltage. [Elektroda, Steve Lawson, post #21662546]
What forward voltage should I see on the multimeter’s diode test?
Expect approximately 0.5–0.75 V in forward bias for a silicon Zener. This confirms diode junction behavior before you attempt the reverse-bias breakdown measurement. [Elektroda, Frank Bushnell, post #21662545]
