Ignition-Switched Supply
This is the supply arrangement that energises the system only while the ignition is on. Because the load is completely disconnected when the ignition is switched off, the problem of battery drain while parked disappears. It is the alternative to a permanent supply; which method is chosen depends on the device's standby current and the usage scenario.
Both approaches are workable, and their differences are clear:
- Permanent supply: power comes directly from the battery and the system is ready even with the ignition off. Over long parking periods the standby current can consume the battery charge.
- Ignition supply: the system is completely dead when the vehicle is not running. It is safe for the battery, but you cannot move the implement while the ignition is off.
In a correct implementation the ignition signal does not carry the load current directly. The motor current is drawn through a relay, and the ignition line only triggers the relay coil. The ignition circuit is not sized to carry a load of the order of 10 A, and connecting it directly causes faults on the vehicle side.
The second benefit of the relay arrangement is this: when the ignition is switched off, a motor that is in motion is also cut, so the system's need for a separate master switch is reduced. On simple kits that draw no standby current, such as the LFT20, a permanent supply is also acceptable; the decision is made according to the installation location and the tractor's parking habits.
The context in which this term is used: Converting a tractor hydraulic lever to electronic control