How to Read a Diode Datasheet — VRRM·IF(AV)·IFSM·VF·IR·trr, Comparing Rectifier·Fast·Schottky·Zener·TVS Values

Image description: A customer holds envelopes of 1N4007 and UF4007 and asks, “Both are 1000V 1A, so either one works, right?” A female clerk in a red apron points her finger at the trr line in the datasheet and answers, “Their speed is different!”

When replacing a diode with a different part number, it is easy to think "1000V 1A is all the same", but a datasheet has many more values than that, and each value comes with a condition. This article lays out 7 representative Vishay diode datasheets side by side (general-purpose rectifier 1N4007 · ultrafast UF4007 · Schottky 1N5819 · SS14 · small-signal 1N4148 · Zener 1N4742A · TVS SMAJ15A) and explains which values to compare under which conditions.

All values are taken from Vishay's official datasheets and application notes (Fundamentals of Rectifiers · Physical Explanation), and text in English quotation marks is the original wording. Click a part number to find stock with eGuro Part Search.

1. Where to start — three values

A Vishay application note lists the main values for choosing a rectifier diode as follows: “The major parameters for the selection of the appropriate rectifier are the maximum reverse voltage (VRRM), the average forward current (IF(AV)) and for switching application the reverse recovery characteristic (trr) too.” Also, “TA = 25°C unless otherwise noted” at the top of a datasheet table means that every value not otherwise noted is based on an ambient temperature of 25℃. Real boards are hotter than that.

2. Voltage — VRRM · VRMS · VDC

VRRM is “Repetitive peak reverse voltage, including all repeated reverse transient voltages” — the maximum reverse voltage applied repeatedly. When rectifying AC directly, the peak value (about 311V for 220V) is applied, not the RMS value. So for AC input, choose a part number whose VRMS column in the datasheet is higher than the AC voltage (for 220V, 1N4004 or higher with VRMS 280V). 1N4001–1N4007 differ only in voltage values and are otherwise identical, so a higher-numbered part can replace a lower-numbered one, but not the other way around. The Schottky 1N5819 has a VRRM of 40V, much lower than rectifier diodes.

3. Current IF(AV) — same 1.0A, different conditions

Image description: A customer holds two datasheets side by side and says, “They're both 1.0A!” A female clerk points to the condition column in the table and answers, “The condition temperatures are different!”

All three part numbers say "1.0A", but the temperature at which it is 1.0A differs. When the temperature rises above that, the current that can flow decreases, and this decrease is shown in the datasheet's Forward Current Derating Curve graph. The application note defines IF(AV) as “Average forward output current, at a specified current waveform … a specified reverse voltage and a specified mounting condition (e.g. lead-length = 10mm or PCB mounted with certain pads and distance)” — meaning the value is fixed for a given waveform · reverse voltage · and even mounting method.

4. Surge current IFSM — check the pulse condition

IFSM is how much momentary current the diode can withstand, such as the inrush current that flows once in a large burst as capacitors charge at power-on. The application note states “It is not an operating value.” — it is not a value you can pass continuously. Rectifier diodes are rated on an 8.3ms half-sine basis, while 1N4148 is rated on a 1µs basis, so you cannot compare the numbers alone.

5. Forward voltage VF — heat and loss

VF is the voltage across the diode when current flows, and VF × current = heat. At 1A, the 1N4007 dissipates up to 1.1W as heat and the Schottky 1N5819 up to 0.6W. This low VF is the main reason to use a Schottky. Always check at how many A VF was measured; most are “Pulse test: 300µs pulse width, 1 % duty cycle” — values measured with short pulses (before the chip heats up).

6. Leakage current IR — Schottky is sensitive to temperature

IR is the current that leaks while the diode is blocking in the reverse direction (all values at rated reverse voltage). Schottky leakage is hundreds of times larger than a general-purpose rectifier and grows tenfold more as temperature rises. Be sure to check this value when using a Schottky where standby current matters, such as battery devices, or in hot locations. The Schottky 1N5819 also has a low maximum junction temperature of 125℃ (section 9).

7. Recovery time trr — rectifier · fast · ultrafast

When a diode switches from on to reverse, it does not turn off immediately; current flows backward for a moment. That time is trr. For 50 · 60Hz mains rectification a slow part like 1N4007 is fine, but at tens of kHz or more, as in switching power supplies (SMPS) · DC-DC converters, a slow diode heats up. So UF4007 can usually replace 1N4007, but 1N4007 cannot replace UF4007. trr also has a condition; for UF4007 it is “IF = 0.5A, IR = 1.0A, Irr = 0.25A”.

8. Capacitance CJ · CD

CJ (junction capacitance) is “Capacitance due to a pn junction of a diode which decreases with increasing reverse voltage.” — the higher the reverse voltage, the smaller it gets. For 1N4148 it is given as CD (total diode capacitance, including the package). For high-frequency signals · switching, smaller capacitance is better, and Schottky diodes tend to have large capacitance.

9. Thermal resistance · maximum temperature

RθJA 50℃/W means that when 1W is dissipated as heat, the inside of the diode becomes 50℃ hotter than its surroundings. This value also comes with conditions like “9.5mm leads · PCB mounted · copper pad size”, so if you draw small pads on the board it gets hotter.

10. Values specific to Zener · TVS

For a Zener, check at which current (IZT) VZ was measured; for a TVS, check that the protected circuit voltage ≤ VWM and that VC is lower than the voltage the protected part can withstand. For SMAJ, the band is on the cathode side if unidirectional, and “no marking on bidirectional types” — bidirectional types have no polarity band. Voltages by Zener part number are covered separately in the Zener voltage code article.

11. 6 types compared at a glance

All are maximum values from Vishay datasheets (capacitance is typical). “-” means a value that does not apply to that type, and “Not in table” means the datasheet has no such item.

12. Checklist for substitution

• VRRM — equal or higher. For AC rectification, at least the peak value (RMS × 1.41)
• IF(AV) — not just the number, but the condition temperature (TA · TL), lead length · pads on the same basis
• trr — for switching circuits, the same type (ultrafast · Schottky) or better. 1N4007 → UF4007 usually works, but not the reverse
• VF · IR — switching to a Schottky lowers VF, but leakage increases greatly and the maximum temperature may be lower
• IFSM — whether the pulse conditions (8.3ms half-sine · 1µs · 10ms) are the same
• Package · heat — RθJA condition (pad size) and leaded vs. SMD differences
• Zener · TVS — for Zener, VZ and IZT · tolerance; for TVS, VWM · VC · unidirectional/bidirectional (CA)

Sources: Vishay 1N4001 (88503, 2020-04-29) · UF4001 (88755, 2020-04-06) · 1N5817 (88525, 2020-07-01) · SS12 (88746, 2020-04-23) · 1N4148 (81857, Rev. 1.6) · 1N4728A (85816, Rev. 2.7) · SMAJ5.0A (88390, 2024-01-09) datasheets, application notes Fundamentals of Rectifiers (88867) · Physical Explanation (84064). Values and conditions may differ by company even for the same part number, so check the datasheet of the company you actually use.