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Relay driver circuit

@electrace/relay-driver@1.0.0 · CC-BY-4.0
vcoil 5–24 Vicoil_max 0.2 Avctrl 3–5.5 V
The actual schematic inside this block — every part is explained below.

Relay Driver

A GPIO can't drive a relay coil — 30–150 mA is 10× what a pin can source, and the coil bites back when you turn it off. One transistor and one diode fix both problems.

  • Q1 — an NPN switch on the coil's low side. The GPIO supplies a few milliamps of base current; the transistor sinks the full coil current. Wire the coil between vcc and coil.
  • R1 (1 k) — sets the base current: (3.3 V − 0.7 V) / 1 k ≈ 2.6 mA. The rule of thumb is base ≈ collector ÷ 10 for solid saturation — 2.6 mA comfortably saturates a 100 mA coil load.
  • D1 — the famous flyback diode, and the reason this block exists. A coil is an inductor: its current cannot stop instantly. Open Q1 without D1 and the coil voltage flies up — hundreds of volts — until something breaks down, usually Q1. D1 sits reversed across the coil doing nothing… until turn-off, when it gives that current a quiet loop to decay through.

Exposes: ctrl (GPIO), vcc (coil supply — also the flyback return), coil (the coil's low side), gnd.

⚠ The relay's contact side is a separate world — this block drives the coil only. Switching mains on the contacts is its own safety problem (creepage, isolation, fusing) that no driver circuit solves for you.

Watch it explained

Exposed nets

●ctrlin · signal
●vccin · power · 5–24 V
●coilout · signal
●gndin · gnd

Inside this block

Q1
npn-gp
the switch — saturates to pull the coil's low side to ground; a garden-variety NPN handles a signal relay easily
R1
1k
sets the base current (~3 mA at 3.3 V) — enough to saturate Q1 at 10× less than the coil current
D1
flyback
the flyback path — catches the coil's inductive kick at turn-off so it never reaches Q1

Limits & gotchas

⚠flyback.note 0 — D1 is NOT optional. A relay coil is an inductor; cutting its current without a flyback path generates hundreds of volts at the transistor's collector. The diode gives the coil current somewhere to go while it dies down.
⚠icoil.max 0.2A — Sized for typical 5–24 V signal relays (30–150 mA coils). A contactor-class coil needs a bigger transistor — check the coil's rated current, not the contact rating.
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✦This page is generated from the block's source — the same content powers the editor's explanations and live checks.