Relay Coil Current
The current the coil draws continuously to keep the relay pulled in, and the actual load the control output has to carry. It is typically 120–200 mA on 12 V automotive relays and 60–100 mA on 24 V types. The number of coils that can be paralleled is set by the output current limit.
Coil current is simply coil voltage divided by coil resistance. A relay with an 80 Ω winding draws 150 mA at 12 V; if four relays are driven in parallel on the same line, 600 mA is demanded from the output. If the transistor or semiconductor output of the control board is below that, the output heats up, the voltage collapses and the relays pull in erratically.
- Calculation: total current = single coil current × number of relays. This value should typically not exceed 70% of the output's rated current.
- Rise with temperature: the resistance of the copper coil rises as it heats, so the current falls somewhat; but the pull-in voltage also rises, so at low supply voltages the relay may fail to pull in.
- Measurement: you cannot insert an ammeter in series without disconnecting the coil terminals, and a clamp meter's resolution is inadequate at this level; the most practical way is to measure the coil resistance with an ohmmeter and divide the voltage by it.
Connecting a contactor coil directly to driver and control outputs is for this reason one of the most common installation mistakes. Driving high coil-current devices through a small interposing auxiliary relay protects the output.
Context in which this term is used: Installation, redundancy and field continuity