2N7000 MOSFET: pinout, ratings, a 5 V relay switch, BS170 and 2N7002

Read the print on the flat face. 2N7000 is the JEDEC number, made by Vishay, Diotec and others. The BS170 is a different registration in the same TO-92 body with the same 60 V and threshold range, but more current and the drain and source swapped. The 2N7002 is the SOT-23 surface-mount part, marked 72 by Vishay and K72 by Diodes Incorporated, not 2N7002.

The two 2N7000 makers differ on one number. Vishay guarantees the 2N7000 at 5.3 Ω or less at 4.5 V on the gate, Diotec at 6 Ω or less. A part from an unknown maker should be designed to the higher figure, so the example below uses 6 Ω.

The BS170 is D-G-S: drain on pin 1, source on pin 3. Put one into a 2N7000 footprint and the drain and source trade places. The gate stays in the middle, so it still switches, but backwards: the body diode then conducts whenever the load supply is on, and the load never turns off. Vishay rates its BS170 at 500 mA and 830 mW, more than the 2N7000, but gives its on-resistance only at 10 V on the gate (5 Ω max), with no limit at 4.5 or 5 V.

The 2N7002 has pin 1 gate, pin 2 source and pin 3 drain, with pin 3 alone on one side. It shares the 60 V rating but carries less current: 115 mA in Vishay's sheet, 170 mA on minimum pads and 210 mA on a square inch of copper in Diodes Incorporated's. Its threshold window is tighter, 1.0 to 2.5 V, and its on-resistance is given at 5 V on the gate (7.5 Ω max at 50 mA) rather than 4.5 V.

All three are 60 V parts with ±20 V on the gate. The current and power ratings depend on the package and, for the 2N7002, on how much copper sits under it. The 2N7000's power rating is 400 mW from Vishay and 350 mW from Diotec, whose figure assumes the leads are held at room temperature 2 mm from the body. Design to the lower one.

The threshold, 0.8 to 3 V, is the gate voltage at which the 2N7000 passes just 1 mA. What a design needs is the next row: at 4.5 V on the gate the sheet promises at least 75 mA of drain current and at most 5.3 Ω (Vishay) or 6 Ω (Diotec) at 75 mA. A 5 V pin clears that. Nothing in either sheet covers 3.3 V, and a part with a 3.0 V threshold is only just starting to conduct there.

• Small relays, buzzers, opto LEDs and short LED strings, up to about 100 mA, from a 5 V logic pin.
• Loads on a higher rail than the logic: a 12 V or 24 V relay coil, with the source at 0 V.
• Open-drain outputs and simple level shifting, where its low gate capacitance (22 pF typical) keeps edges fast.
• Pulling a line low under logic control: a reset, an enable pin, a status LED.

It is the wrong part above about 150 mA, for motors and solenoids that draw amps, or where only a 3.3 V pin is available. For those, see our guide to choosing a MOSFET for Arduino and ESP32.

A 12 V relay with a 400 Ω coil draws 12 V ÷ 400 Ω = 30 mA with an ideal switch. The 2N7000 switches its low side: source to 0 V, coil between +12 V and the drain, a diode across the coil. The pin drives the gate through 100 Ω, with 100 kΩ to 0 V on the pin side so the relay stays off while the pin floats at reset.

The diode across the coil, cathode to +12 V, is not optional; see the flyback diode and driving a relay.

• Check the print before fitting: a BS170 in a 2N7000 footprint is reversed and the load stays on.
• Keep the 100 kΩ pull-down. With the gate floating, the part can sit half on, and a stray touch can switch it.
• Measure drain to source with the load on. Around 0.1 to 0.2 V at 30 mA is right; a volt or more means the gate voltage is too low or the part is damaged.
• Handle it as static-sensitive. The gate oxide is rated ±20 V, and a charged finger is far above that.
• From a 3.3 V board, use a part rated at 2.5 V on the gate, or a 2N7002 only after checking that the maker gives a figure at your drive voltage.

The BS170 carries more current but fits only turned round, and it has no rating below 10 V on the gate. The 2N7002 is the SMD equivalent, with less current.

1. Short all three leads for a moment to empty the gate. Set the meter to diode test.
2. Red probe on the source, black on the drain: the body diode reads like a silicon diode. Reversed, it reads open.
3. Touch red to the gate with black on the source to charge it. Now red on the drain, black on the source: a good part conducts. Short gate to source and it opens again.
4. A meter's diode-test voltage can sit near the 3 V threshold limit, so a good part may only partly turn on in step 3.

The transistor testing lesson and reading datasheets go further.

Use the 2N7000 for loads up to about 75 mA from a 5 V pin, with a 100 Ω gate resistor, a 100 kΩ pull-down and a diode across any coil. Design to the 6 Ω worst case. Keep a few BS170s only if you remember to turn them round, and from a 3.3 V board choose a part whose data sheet gives an on-resistance at 2.5 V.

The 2N7000 and BS170 are stocked by most Indian electronics shops and online sellers. Check the print on each one, since the two look alike, and meter a part before it goes on a board. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.