Question about strange capacitor wiring

Discussion in 'General Electronics Chat' started by rallyemax, Jun 6, 2013.

  1. rallyemax

    Thread Starter New Member

    Oct 28, 2010
    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.)
  2. rallyemax

    Thread Starter New Member

    Oct 28, 2010
    I just thought of something. The cap would suppress noise on releasing the brake pedal, and if the load were inductive, this would probably smooth out the resulting spike. Of course, the load is resistive, but who knows how much inductance is in the wiring. And the wire from the brake pedal switch also goes to the ABS module, I suppose that could be an inductive load if it drives some sort of motor/pump (but only when engaged).
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
    The capacitor may be 'beefy' in physical terms but it will not be a very large capacitance in electrical terms.

    Its purpose is to absorb the transients due to 'contact bounce' that occurs with any mechanical switch.

    That is the switch does not switch cleanly on or off but allows multiple transients on the power line as the contacts close or open. These trnasients generate interference unless absorbed by the capacitor.

    Does this help ?

    Incidentally this is nonsemse.

    There is no AC coupling in a landrover or most other vehicles.
    Last edited: Jun 7, 2013
  4. gerty

    AAC Fanatic!

    Aug 30, 2007
    Another thing it'll do is keep the arcing on the brake light switch contacts to a minimum.
  5. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    It is there to keep the stereo form going POP when you take your foot off the brake.
  6. rallyemax

    Thread Starter New Member

    Oct 28, 2010
    Thanks for the answers. I realized soon after posting that I need to draw out the circuit with pencil and paper so I can figure out what the current is doing. I had been thinking about it all wrong based on just seeing it, and even after duplicating it in a circuit drawing application.

    For instance, I missed the fact that the negative end of the cap IS actually grounded when the switch is open -- it's grounded through the brake lights, but grounded nonetheless. So we'll have a potential difference across the cap of Vbatt when the switch is open. When the switch closes, the positive end discharges through the switch and to the lights, at the same time what used to be the negative end of the cap will charge.

    It would seem it's a snubber. I imagine that I will find a resistor in series with the cap once I break it apart.

    That was my point. I was confused about the function of the cap, and when thinking of it as a simple series connection +12v------||-----(lights)----GND, it would be acting as an AC coupler, which makes no sense in a DC system (ignoring for the time being the fact that a car electrical system has quite a bit of AC noise superimposed on the system).

    Exactly. The way it's wired, I believe it does work both when the switch is closed and when it's opened. When the switch closes, the 'upper' part of the cap discharges and smooths out the voltage dip that would otherwise result from the large inrush current to the lights. While it's closed, the 'bottom' part of the cap charges. When the switch opens again, the 'bottom' part discharges through the lamps -- I suppose that if there is any inductive load at all downstream of the cap (and since it goes into the ABS system, there may well be inductance there), this will prevent a voltage spike and arcing at the switch.

    Sorry for the confusion, folks. <face slap> on not seeing the circuit correctly, despite drawing it out on my screen. Sometimes there is no replacement for pen and paper.
  7. sheldons

    Well-Known Member

    Oct 26, 2011
    its purely there to stop arcing at the switch contacts due to switch bounce-similar type of circuitry is used across relay contacts etc
  8. rallyemax

    Thread Starter New Member

    Oct 28, 2010
    Not quite. By also preventing voltage in the system from dipping when the brake light switch is closed due to the inrush current to the lights, it acts in a filtering capacity. Most of the components in the car are quite resilient to both negative and positive voltage spikes (as they must be; vehicle electrical systems are notoriously chaotic), but sudden voltage spikes are audible through the audio system.

    Sure enough, when the cap in removed, pressing and depressing the pedal causes audible pops through the OEM subwoofers. This is true even when there are no audio inputs connected, and in fact even when the head unit preamp is disconnected from the system entirely. The sub amplifier is generating the pops solely from spikes on the +12v rail. It's an OEM sub amp, so perhaps a better amplifier would have internal filtering for this kind of thing, but then again the capacitor provides adequate filtering when it is in place.

    I've opened up the cap assembly and it's just a crude film cap, with no resistor. My DMM doesn't have a capacitance mode, but comparing the discharge time against a few known capacitors, it's quite low, in the range of 0.5 - 1.0 uf.