1N4007 vs 1N4148 vs 1N5819: three diodes, three different jobs

The 1N4148 is the easy one: a small glass DO-35 body with a black band. The 1N4007 and the 1N5819 both come in the black plastic DO-41 body and look the same, so the printed part number is the only reliable check. On all three the band marks the cathode, the end the current flows out of.

Diotec's sheet adds one more trap. Its 1N4007-13, a 1300 V version, is marked plain "1N4007" on the body; the full type is on the packing label only. That works in your favor (it blocks more), but the print doesn't always give the whole part number.

• Where the two makers disagree, both figures are shown. Design to the lower one unless you know whose part is in the bag.
• The 1N4148's forward voltage is specified at 10 mA, not 1 A, because it isn't meant to carry 1 A.
• The 1N5817 and 1N5818 are the same Schottky at 20 V and 30 V, with lower drop (0.45 V and 0.55 V at 1 A on the Vishay sheet). The 1N4001 to 1N4006 are the same rectifier at 50 V to 800 V.

The 1N4007 belongs wherever the reverse voltage is high: rectifying mains or a transformer secondary, and protection across anything that can kick back hundreds of volts. Its leakage is tiny (5 µA at 25 °C at the full 1000 V). The weak point is speed. A typical 1500 ns recovery is harmless at 50 or 60 Hz, but in a switching converter running at tens or hundreds of kilohertz it takes a real slice of every cycle.

The 1N4148 is for signals and small currents: logic OR gates, clamping an input to a supply rail, a flyback diode on a small relay, a reverse-polarity guard on a sensor that draws a few milliamps. It switches in 4 ns and its capacitance is 4 pF or less, so it barely loads a fast signal. It is not a power diode: 150 mA average on the Vishay sheet, and a 1 µs surge rating of 2 A.

The 1N5819 is the low-loss choice at low voltages: polarity protection on a 5 V or 12 V input, the catch diode in a small buck converter, OR-ing two supplies. Vishay lists exactly those uses. It costs you two things: the 40 V limit, and leakage of up to 1 mA at 25 °C and 10 mA at 100 °C at full rated voltage, about 200 times the 1N4007's at 25 °C. In a battery circuit that has to sit for months, that leakage can matter.

1. Reverse-polarity protection on a 12 V, 1 A input

A series diode in the positive lead blocks current if the supply is plugged in backwards. The load sees the supply minus the diode's drop, and the diode turns that drop into heat.

• 1N4007: up to 1.1 V × 1 A = 1.1 W. At 50 °C/W that is a rise of about 55 °C, so a 25 °C room puts the junction near 80 °C. The load gets about 10.9 V.
• 1N5819: up to 0.6 V × 1 A = 0.6 W, a rise of about 30 °C, junction near 55 °C. The load gets about 11.4 V.
• 1N4148: no. 1 A is far past its 150 mA average rating.

Both rectifiers work at 12 V, since the reverse voltage is only the 12 V supply. The 1N5819 runs cooler and leaves more voltage for the load. Both thermal figures assume 9.5 mm leads soldered to a PCB, as in the Vishay test; short leads on thin copper will run hotter. Keep some margin under the 1 A rating, because Vishay rates the 1N5819 at 1 A with the leads held at 90 °C, and Diotec rates its 1N4007 at 1 A only up to 75 °C ambient (0.8 A at 100 °C).

2. Rectifying 230 V or 120 V mains

The diode has to block the peak of the AC, not the RMS figure printed on the socket.

• 230 V RMS has a peak of 230 × 1.414 = 325 V. In a bridge rectifier each diode blocks about that peak, so the 1N4004 (400 V) is the lowest that fits. At the top of the allowed mains range (253 V, +10 %) the peak is 358 V, which leaves the 1N4004 only 42 V of room. The 1N4007 (1000 V) leaves room for mains spikes too.
• 120 V RMS peaks at 120 × 1.414 = 170 V (178 V at 126 V). The 1N4003 (200 V) is the minimum on paper; the 1N4007 is the usual pick because it costs about the same and covers both.
• A half-wave rectifier feeding a reservoir capacitor sees close to twice the peak in reverse: about 651 V on 230 V mains, so only the 1N4006 (800 V) and 1N4007 cover it. On 120 V it is about 339 V, so 1N4004 or higher.

The 1N5819 (40 V) and 1N4148 (100 V peak) can't be used here at all.

3. Why not a 1N4007 in a switching converter?

Take a small buck converter switching at 100 kHz. One cycle lasts 1 ÷ 100,000 = 10 µs. Diotec gives the 1N4007 a typical reverse recovery of 1500 ns, which is 1.5 µs, or 15% of every cycle. During recovery the diode conducts backwards for a moment, and that current becomes heat in the diode and the switch. At 50 Hz mains the same 1.5 µs is 0.01% of a 20 ms cycle, which is why it doesn't matter there.

The 1N5819 is the part Vishay lists for "DC/DC converters" and "freewheeling", and the 1N4148 recovers in 4 ns but can't carry an amp. So for a 1 A converter under 40 V, the 1N5819 is the pick from these three.

• Read the print before fitting a DO-41 part. A 1N5819 put in a mains rectifier position will fail; a 1N4007 in a converter will run hot.
• The band is the cathode on all three. In a polarity-protection circuit the band faces the load.
• For a relay or motor flyback diode, the diode only has to block the supply voltage and carry the coil current for a moment. A 1N4148 covers small relay coils within its ratings; for larger coils and motors use a 1N4007 or 1N5819. The flyback diode lesson works through the numbers.
• Don't parallel two 1N5819s to get 2 A without a check. Each diode's share of the current depends on its forward drop, and the datasheets give only maximum drops, so the split is not guaranteed.
• Remember the 1N5819's leakage in battery and sample-and-hold circuits. A 1N4148 leaks at most 25 nA at 20 V.

Set the meter to diode test, red probe on the end without the band (anode), black on the band (cathode). A silicon diode like the 1N4007 or 1N4148 usually reads somewhere around 0.5 to 0.7 V at the meter's small test current; a Schottky like the 1N5819 reads lower. Reversed, all three should read open (OL). A reading near zero both ways means the diode is shorted. These are typical bench readings, not datasheet figures: the sheets give the forward voltage at 10 mA (1N4148) or 1 A (the other two). The diode testing lesson goes further.

Keep a strip of each. We'd reach for the 1N4007 for anything touching mains or high voltage, the 1N5819 for low-voltage power paths where saving half a volt matters, and the 1N4148 for signals, logic and small relays. When a design calls for one, don't substitute across the families without checking reverse voltage first, then current, then speed.

The diode lesson covers the junction from the start, and the half-wave and full-wave rectifier lessons work through the reverse voltages in example 2. For transistors picked the same way, see BC547 vs 2N2222 vs 2N3904.