Relay buzzes because voltage increases to 12 V over time rather than immediately

Question

I'm trying to fit an internal light to a campervan that switches on when the current internal lights that are are factory fitted come on.

I'm trying to use a relay for this so that when the factory fitted internal lights come on the relay closes and switches on the extra light which is powered from a different battery.

This seems to work, but the problem I have is that the voltage for the internal lights seems to slowly increase from about 7 V to 12 V when they are turned on, which I guess is so they fade in and out. This causes the relay to vibrate as the voltage increases to 12 V.

Is there a simple way to stop this from happening, so that the relay only switches on when the voltage reaches about 11 V?

EDIT - Just to clarify, this is how i've got it setup.

EDIT - Would using this time delay relay on amazon be an easy fix? Am I correct in thinking I could just use this to put a delay of about 1s on it, so that the relay closes after the voltage has reached 12 V?

I'm not sure what it means by a high level pulse signal sent to x1, can someone explain please?

https://www.amazon.co.uk/Delay-Timer-Relay-Self-locking-Switch/dp/B07SR4BX95/

Answer 1

The circuit would depend on whether a high side or a low side dimmer has been used.

1. High side dimmer

2. Low side dimmer

Answer 2

My first solution is to simply put a diode across the relay coil as shown by the red arrow in Figure 1. The diode allows the coil current to continue to flow in a controlled fashion. Without it the coil current must flow through the lighting system. This should work because the dimmer cannot sink the current that is required at the threshold of switching. This causes the voltage at the lower end of the coil to rise thus reducing the current, disengaging the armature causing chatter. As the armature moves toward the coil's magnetic pole, less current is required to hold it in so the relay should not chatter. There is built in hysteresis. The diode allows the coil current to continue in a short low voltage path thus maintaining the current and thus the engaging force.

This simple solution should work. If not the solution in figure 2 will work. It requires two sets of contacts (DPST). Choose \$R\$ to raise the pull-in voltage by about 5% to 10%. When the relay engages, the contact will short out the resistor thus raising the current preventing chatter. On dropout, the contact opens speeding up the current reduction.

It should not matter if the dimmer is high-side or low-side.

^{simulate this circuit – Schematic created using CircuitLab}

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