The TF1170 series self-switching combination relay are designed to switch two relays when the vehicle alternator is charging the battery. The normal operating environment is a 12-volt DC circuit within a motor vehicle.

Application.
Typically these relays are used in an towbar auxiliary electrical circuit to switch the split-charging and fridge circuits on and off. They do not need any dedicated signal feed to the coils of the relays. When the voltage in the line from the battery reaches the appropriate level as the alternator begins to charge the battery, the trigger unit switches the relays on. When the alternator ceases to charge and the voltage drops, it switches the relays off.

Accommodating different cars and different conditions.
In order to work effectively, the unit has to accommodate a number of variables.  Some variables occur between one    car and another and some occur within individual cars depending on circumstances.  Therefore the trigger unit has,       at the outset, to be preset very precisely to operate within that narrow window of conditions which can be identified as common to all or, at least, the great majority of cars in most circumstances.

1. Variations between one car and another.
The performance of battery and alternator will vary between one car and another. Factors that affect performance include good or poor battery condition, alternator condition and slipping alternator belts.

2. Variations within each particular car.
The performance of the battery and alternator of any individual car will also be affected by circumstances. For instance, not only do winter conditions make batteries and alternators work harder by virtue of the fact that heaters, wipers, lamps etc. are used more but in cold weather the voltage drop observed in a battery when even a fairly small load is applied to it is much greater than it would be in warm conditions. Other factors that affect performance and battery condition include night driving, frequent short journeys and additional loads created by in-car accessories.

4. Accommodating normal voltage drops.
To succeed in doing its job in the context of all these variables the device has to be preset very precisely as described above. The unit is also intelligent enough to deal with a number of other variables that would otherwise prevent it working efficiently. It is also equipped with a time delay that allows it to ignore transitory voltage changes.

Eliminating variables which would prevent efficient operation In a device that depends for its operation on measuring voltage levels, it is obvious that installation should be carried out in such a way as to avoid accidental voltage drops.