Why have hysteresis on voltage sensitive relays?

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I set my VS12 turns on at 13V, but it turns off at 12.7V, why the difference, why can’t they be set to the same voltage?

Any voltage sensitive relay (this also applies to the Redarc SBI series of Smart Battery Isolators) needs some “hysteresis”, (difference between turn off & turn on) otherwise it will cycle on/off/on or chatter when the sensed voltage is around the set point. This can be caused in two ways:

• For example, if a VS was set to turn on and off at 12V (no hysteresis). The battery voltage is around 12V.

The current being drawn by the load causes a voltage drop on the cable between the battery and the VS. This voltage drop causes the VS “see” less than 12V and it switches off. When it switches off there is no longer a current draw so there no longer the voltage drop so the VS sees more than 12V and switches on again… this cycle is repeated possibly many times.

The current being drawn by the load causes the battery voltage to drop. This voltage drop causes the VS to switch off. When it switches off there is no longer a current draw so the battery voltage recovers. The VS sees more than 12V and switches on again…this cycle is repeated possibly many times.

• For example, if a VS was set to turn on and off at 13.6V (no hysteresis). The battery voltage is around 13.5V (alternator charging).

While the battery holds above 13.7V, the VS is on OK, but the load on the battery increases, Eg because the headlights or wipers or heater are turned on, dropping the battery voltage to 13.55V. The VS would turn off.
As above but the indicator is turned on. The VS would turn on/off with the indicator.

It can be seen that having the turn on and turn off voltages the same is impractical, some allowance must be made for voltage fluctuations without unnecessary triggering of the sensor.

Experience has shown that (for a 12V system), 0.7V is a good compromise setting for hysteresis. Even this can have unplanned chattering if, for example, the load being switched by the VS is very high or if battery capacity is too low or if the wiring resistance (between the battery & the VS) is too high.

NOTE: Redarc Voltage Sensors and SBI* Smart Battery Isolators can be custom made with different settings, even with no hysteresis if this is specifically required, provided the above is understood.

Updated: Nov 2, 2009 12:50