Which way round does a 1N4001 diode go?

Current flows from the plain end (anode) to the striped end (cathode). For flyback protection across a motor, the stripe points toward the + supply.

What is a flyback diode for?

When current through a motor or coil is switched off, the coil produces a voltage spike that can destroy the transistor. A diode across the coil gives that current a safe loop to fade away in.

What is the voltage drop across a 1N4001?

About 0.7 V at small currents, rising to roughly 1.1 V at its full 1 A. Include it in Ohm’s-law sums like an LED’s forward voltage.

part · Level 4 · Motion · passive

What a 1N4001 diode does

Diode (1N4001) — A one-way street for current. Protects circuits from motor kickback.

What it is

A diode is a one-way street for current. Current flows in through the anode and out through the cathode; try to push it the other way and the diode blocks. The 1N4001 is the workhorse rectifier diode: a small black cylinder with a silver stripe at the cathode end.

An LED is a diode too, one that happens to glow. The 1N4001 is the plain version: no light, but it can carry 1 A continuously and hold back up to 50 V in reverse — enough for anything in your box.

Its main job here is flyback protection: sitting backwards across a motor so that when the motor is switched off, the energy stored in its coil has somewhere safe to go instead of into your transistor.

How it works

Forward. Like any silicon diode it needs about 0.7 V across it before it conducts, and once conducting it holds close to that — around 0.7 V at 10 mA, rising toward 1.1 V at a full amp. It behaves like the LED's 2.0 V in the resistor maths: a red LED through a 1N4001 and 220 Ω on 4.5 V gets (4.5 − 0.7 − 2.0) ÷ 220 ≈ 8 mA instead of 11.

Reverse. Below 50 V it leaks only microamps — call it an open circuit. Above the rating it breaks down, which at 4.5 V will never happen from the battery.

Flyback. A motor coil carrying 300 mA has a magnetic field storing energy. Open the switch and the coil tries to keep its 300 mA flowing; with nowhere to go, the voltage across the coil shoots up (V = L × ΔI ÷ Δt — a fast cut makes a big spike) until something breaks down, often the transistor. Put the diode across the motor, stripe toward +: in normal running it is reverse-biased and does nothing; at switch-off the coil's spike forward-biases it and the current circulates through the diode at about 0.7 V until it dies away. The transistor never sees more than 4.5 + 0.7 ≈ 5.2 V.

Reverse-polarity guard. In series with the battery's red wire it lets current through the right way and blocks it if someone connects the pack backwards — at the cost of 0.7 V.

The numbers

Forward voltage	≈ 0.7 V — up to ~1.1 V at 1 A
Max continuous current	1 A
Max reverse voltage	50 V — 1N4002–1N4007 go higher, same package
Stripe	cathode (−) — current flows toward the stripe
Flyback orientation	stripe toward + — across the motor, backwards to the supply

Mistakes everyone makes

Stripe toward − across the motor. That is forward across the supply — a dead short through the diode, hot diode, flat battery. Fix: stripe faces the + side.
Used as an LED substitute. It does not light; it just drops 0.7 V. Fix: it is a valve, not a lamp.
Forgetting its 0.7 V in a series calculation. Your LED is dimmer than planned. Fix: subtract 0.7 V along with the LED's 2.0 V.
Assuming it protects against everything. Across a motor it protects the switch; it does not limit stall current. Fix: size the transistor for stall.

Questions people ask

Which way round does a 1N4001 diode go?: Current flows from the plain end (anode) to the striped end (cathode). For flyback protection across a motor, the stripe points toward the + supply.
What is a flyback diode for?: When current through a motor or coil is switched off, the coil produces a voltage spike that can destroy the transistor. A diode across the coil gives that current a safe loop to fade away in.
What is the voltage drop across a 1N4001?: About 0.7 V at small currents, rising to roughly 1.1 V at its full 1 A. Include it in Ohm’s-law sums like an LED’s forward voltage.

Projects that use this part

🌀 First Spin — Your first motor. A switch, a coil, and a diode that catches the voltage kick when the current stops. L4 · 15 min
💨 Handheld Fan — Press the button, the fan spins. A tiny finger current commands a hundred-milliamp motor through a transistor. L4 · 20 min
🌞 Sunrise Fan — Sunrise Alarm with the LED swapped for a motor. The sun rises, the fan starts — an automatic wake-you-up desk breeze. L4 · 25 min
🆘 Panic Fan — Two buttons, either one fires the fan. Panic Doorbell wearing a motor — an OR gate that spins. L4 · 20 min
🎉 Party Spinner — One button, three mouths: a spinning motor, a red LED, and a buzzer all fire together. One transistor drives them all. L4 · 25 min
🚗 Twin Wheels — Two motors from one button — a Level 9 robot drivetrain, five levels early. L4 · 25 min
🎚️ Dial-a-Spin — Dial-a-Brightness with a motor instead of an LED. Twist the knob and watch the fan speed slide from full spin to a slow crawl. L4 · 20 min
🕸️ Trip Wire Fan — Trip Wire Alarm's brain wearing a motor. The wire is intact — the fan is silent. Snap it — the fan spins as the alarm. L4 · 25 min

verified against: onsemi — 1N4001–1N4007 datasheet · SparkFun — Diodes · Kuphaldt — Lessons in Electric Circuits, Introduction to Diodes and Rectifiers