I'm working on a little car project, not really relevant to my question, but for the curious, the stereo was stolen from the prior owner, and since I dislike pretty much all head unit implementations that play MP3s, I decided to use an old Android phone as a head unit.
When fishing around for a source of ignition-switched power, I found an odd looking chunk of plastic/rubber, and when I looked it up in the vehicle circuit diagram, it turned out to be a "Stop Lamp Switch Suppressor" capacitor. But the way it's wired makes my head scratch, because it doesn't seem to make much sense. Here is how it is wired:
(I didn't break the rubber molding around the cap to see its value. Yet. But it's rather beefy.)
Based on the description in the vehicle service manual, I think that the intent is for this capacitor to suppress the voltage drop in the rest of the circuit caused by the sudden activation of multiple high-wattage brake lamps. We're talking maybe 100 watts between the three brake lamps, or about 8 amps at 12 volts (very roughly), so I can see why this would be useful.
But from what I can tell, the current implementation merely AC-couples the battery/charging circuit to the stop lamps. I don't see anything interesting happening on the DC side of things; I don't even know that the cap will charge up, because the voltage drop across the brake pedal switch is negligible compared to the voltage drop across the brake lights (I've confirmed that the switch drives the brake lights directly; there is no relay), and with the switch open there isn't any voltage drop at all.
If the intent is to suppress noise caused by the sudden load going to the brake lights, shouldn't the bottom of the cap be connected to ground? That way the positive side of the cap will charge to 12v, and when the brake pedal is pressed, the cap will discharge through the brake lights, hopefully negating any voltage drop caused by the sudden load.
The vehicle is an older Land Rover, and the electrical system is a minor nightmare. Extremely noisy even by vehicle standards, at least a dozen relay-isolated subcircuits running via tiny wires and dodgy connectors causing a number of different voltage levels (with the battery at 12.6v, various circuits are anywhere from 10.8 to 11.6), an over-complicated ground distribution running over similarly tiny wires and dodgy connectors causing several different ground levels as well (I have yet to find a true ground in the cabin, everything is slightly above the battery negative terminal due to excessive resistance on the return path)... It's bad enough that I gave up trying to eliminate ground loops on the audio circuit and resorted to using a transformer-based ground loop eliminator; the theoretical decrease in quality is much less than the actual noise that was present with a direct connection.
But I digress. Any ideas on what that capacitor might be trying to accomplish? Interestingly, it seems to be doing something. When removed from the circuit, there is an audible pop out of the OEM subwoofer whenever the brakes are applied (and I can see the voltage drop with a multimeter and in the headlights dimming very slightly).
I am not thinking of removing it, but I am thinking of adding an actual suppressor cap of perhaps 1/2 size, with the positive end wired the same way and the other end grounded. This could only help, and can't hurt anything, right?
(Don't worry, I won't hold anyone to their word, just looking for brainstorming ideas. In fact, assume for the purpose of this discussion that this vehicle will never be driven on public roads, and that I'm doing this because I have a strange fetish for functioning brake lights when driving on my private off-road course, which we'll assume I own.)
When fishing around for a source of ignition-switched power, I found an odd looking chunk of plastic/rubber, and when I looked it up in the vehicle circuit diagram, it turned out to be a "Stop Lamp Switch Suppressor" capacitor. But the way it's wired makes my head scratch, because it doesn't seem to make much sense. Here is how it is wired:
(I didn't break the rubber molding around the cap to see its value. Yet. But it's rather beefy.)
Based on the description in the vehicle service manual, I think that the intent is for this capacitor to suppress the voltage drop in the rest of the circuit caused by the sudden activation of multiple high-wattage brake lamps. We're talking maybe 100 watts between the three brake lamps, or about 8 amps at 12 volts (very roughly), so I can see why this would be useful.
But from what I can tell, the current implementation merely AC-couples the battery/charging circuit to the stop lamps. I don't see anything interesting happening on the DC side of things; I don't even know that the cap will charge up, because the voltage drop across the brake pedal switch is negligible compared to the voltage drop across the brake lights (I've confirmed that the switch drives the brake lights directly; there is no relay), and with the switch open there isn't any voltage drop at all.
If the intent is to suppress noise caused by the sudden load going to the brake lights, shouldn't the bottom of the cap be connected to ground? That way the positive side of the cap will charge to 12v, and when the brake pedal is pressed, the cap will discharge through the brake lights, hopefully negating any voltage drop caused by the sudden load.
The vehicle is an older Land Rover, and the electrical system is a minor nightmare. Extremely noisy even by vehicle standards, at least a dozen relay-isolated subcircuits running via tiny wires and dodgy connectors causing a number of different voltage levels (with the battery at 12.6v, various circuits are anywhere from 10.8 to 11.6), an over-complicated ground distribution running over similarly tiny wires and dodgy connectors causing several different ground levels as well (I have yet to find a true ground in the cabin, everything is slightly above the battery negative terminal due to excessive resistance on the return path)... It's bad enough that I gave up trying to eliminate ground loops on the audio circuit and resorted to using a transformer-based ground loop eliminator; the theoretical decrease in quality is much less than the actual noise that was present with a direct connection.
But I digress. Any ideas on what that capacitor might be trying to accomplish? Interestingly, it seems to be doing something. When removed from the circuit, there is an audible pop out of the OEM subwoofer whenever the brakes are applied (and I can see the voltage drop with a multimeter and in the headlights dimming very slightly).
I am not thinking of removing it, but I am thinking of adding an actual suppressor cap of perhaps 1/2 size, with the positive end wired the same way and the other end grounded. This could only help, and can't hurt anything, right?
(Don't worry, I won't hold anyone to their word, just looking for brainstorming ideas. In fact, assume for the purpose of this discussion that this vehicle will never be driven on public roads, and that I'm doing this because I have a strange fetish for functioning brake lights when driving on my private off-road course, which we'll assume I own.)