Why Use a Relay?
A relay is an electrically-controlled switch. A small signal current triggers it, which then switches a much larger current on or off. So why not just wire a regular switch directly to the load?
When controlling high-current devices, a direct switch must be rated for that full current — making it bulkier, pricier, and requiring thick wiring throughout. If the switch is far from the device, running heavy-gauge wire across the whole distance becomes a serious headache.
With a relay, the control wiring can be thin and lightweight because it only carries a small signal. The heavy current flows through a short, direct path right at the load. This is the core advantage.
Products Tested
12V / 40A C-Contact Relay — ¥899
5-terminal C-contact type (A-contact + B-contact). Comes with wiring harness and connectors. Available on Amazon, listed for over 3,000 days — a reliable budget pick at roughly half the price of branded equivalents.
12V / 120A High-Capacity Relay — ~¥1,300
Near the top of the range in terms of amperage. Simple 4-terminal A-contact design. Physically larger and easy to disassemble for inspection. Strong value for the power it delivers.
Basic Operation
Terminals 85 and 86 are the control coil inputs — apply power here to activate the relay. Either polarity works; place your small switch on whichever wire you prefer. The relay draws about 2W when active, so almost any signal source can drive it.
Terminals 30 and 87 are the A-contact load terminals, which close when the relay energizes. Critically, the control circuit and load circuit are completely electrically independent — no shared ground or common voltage required.
C-Contact Terminal Reference (40A)
| Terminals | Contact Type | Relay OFF | Relay ON |
|---|---|---|---|
| 85, 86 | Control coil | ~2W draw when energized | |
| 30 → 87 | A-contact (NO) | Open | Closed ✅ |
| 30 → 87A | B-contact (NC) | Closed ✅ | Open |
Contact Resistance & Heat
Measured with a milliohm meter. Heat dissipation calculated using P = I²R:
| Relay | Contact | Resistance | Heat at Rated Current | Verdict |
|---|---|---|---|---|
| 120A | A-contact | 0.63 mΩ | ~9W @ 120A | ✅ Fine |
| 40A | A-contact | ~7 mΩ | ~11W @ 40A | ⚠️ Monitor continuous use |
| 40A | B-contact | ~12 mΩ | ~19W @ 40A | ❌ Too hot at full load |
Operating Voltage
These relays require roughly 8V minimum to pull the contacts in reliably. Below that, the electromagnet cannot overcome the return spring and switching becomes erratic.
Operate close to the rated 12V. Too low causes unreliable behavior; too high accelerates mechanical wear. Automotive charging voltage (13.5–14.4V) is perfectly acceptable.
Inside the Relay
Disassembling the 120A unit reveals a straightforward mechanism: energizing the coil creates an electromagnet that pulls a contact bridge closed. A return spring pushes it open when the coil de-energizes. The load circuit and control circuit share no electrical connection whatsoever — clearly visible inside.
Failure Mode: Welded Contacts
If a massive overcurrent flows through the contacts, they can weld together permanently. Once welded, the relay stays closed regardless of the control signal — making a short circuit impossible to break by normal switching means.
Summary
- 1Use relays when your switch or signal wire is far from the high-current load — it saves on heavy wiring and cost.
- 2The 120A relay suits pure high-power switching; the 40A C-contact adds B-contact logic for more complex circuits.
- 3Calculate heat dissipation at your actual load current — the contact resistance matters significantly at high amperage.
- 4Always fuse the load side when pulling power directly from a battery.
- 5MOSFETs can do the same job for even less money — worth considering for solid-state or low-noise applications.
