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Series, Parallel or Series/Parallel Circuit? LED's won't turn on, not enough V?

Discussion in 'LEDs and Optoelectronics' started by Josh Zacheis, Feb 1, 2018.

  1. Josh Zacheis

    Josh Zacheis

    10
    1
    Jan 25, 2015
    I'm trying to use LED bulbs in my overhead console map lights in my truck, however they don't work/turn on whereas incandescent bulbs do. I believed this to be because the bulbs were wired in series and therefore didn't get enough voltage to turn on but looking at the wiring it seems to that they are wired in parallel but I'm not sure because there are three wires at the harness yet a bulb only requires a positive and negative to function.

    I need help identifying if this is a parallel or series circuit. If series circuit I'm going to try to make it parallel so that I can use LED bulbs.

    I believe Fused B(+) is shorthand for direct positive battery "B(+)" connection via a fuse, "Fused". Ground is easy enough which begs the question, what is "Courtesy Lamp Driver" connection?

    Circuit.png
     
  2. Kiwi

    Kiwi

    313
    71
    Jan 28, 2013
    Going by your drawing I assume you have festoon(sausage) bulbs.

    LED bulbs are polarity sensitive.
    One metal end cap will be positive and the other negative. Won't go if you have them the wrong way around.
    First thing to try is swap them around.
    Bulbs shown in drawing are in parallel.

    Courtesy Lamp Driver is usually the door switches, but in your case may be a module that grounds this terminal when a door is open?
    May also have a timer to keep the lights on for a few seconds after the doors are closed.

    LH and RH switches probably have three positions; On, Off, Door.
     
  3. Josh Zacheis

    Josh Zacheis

    10
    1
    Jan 25, 2015
    Thanks Kiwi. For sure, they are sausage bulbs. I of course made sure they were in correctly, and the switches are just on/off.

    Probably should have mentioned that if I use one incandescent and the other LED, both work, but as soon as I try to use two LED's neither work. Tested this scenario with both bulbs, both working when used in conjunction with an incandescent. On another note, I wired these bulbs in series to the vehicle battery, they didn't come on which makes me think that there is a series circuit somewhere.

    I attached the circuit diagram for this system direct from the service repair manual. Since I really stink at electronics maybe this will help identify the issue.
     

    Attached Files:

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    From the PDf I take it the lamps are not in series. This wouldn't make sense anyway as each lamp seems to have its own switch. The original lamps should read 12V on their metal connectors.
    What make and type of bulbs do you use (datasheet)?
     
  5. Kiwi

    Kiwi

    313
    71
    Jan 28, 2013
    Do the Dome Light and the Glove Box light work?
     
  6. Josh Zacheis

    Josh Zacheis

    10
    1
    Jan 25, 2015
    Yes, looking at the harness for the bulbs they do indeed look parallel, however the since the LED's aren't turning on if two are used makes me think somewhere is a series causing it to have too much of a voltage drop to run the bulbs.

    Not sure about the datasheet but I've tried three types, Philips Vision 38mm and 42mm bulbs and Sylvania 42mm bulb.

    I've measured the voltage at several places, first is at the supply side, getting ~12.7v, when the bulb harness is plugged in, I get the same voltage at both driver and passenger bulb terminals.

    Dome light works no problem with a Philips Vision 38mm LED bulb. Vehicle isn't equipped with a glovebox light.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    Improbable. A series circuit would usually drop even more voltage at the higher current of the original incandescent lamps.
    Although there are circuts that require a minimum amount of current to flow I think it is unlikely that this is the case here.

    • Did you measure the voltage across the LEDs at the time they are supposed to be on?
    • Are there any other lights that do not come on when the LEDs are installed in the map lamp fixture but that do come on with incandescent lamps installed?
    • Another check you could do: Instal an incandescent lamp into one of the sockets, this makes the setup work per your description. Then connect the LED lamp in parallel to th eincandescent one using some wire for the eletrical connection. Does the LED lamp light now? This would indicate that, contrary to my above assumption, a certain minimum current is required.
     
  8. Josh Zacheis

    Josh Zacheis

    10
    1
    Jan 25, 2015
    OK so I did some more testing. Inc=Incandescent; N/A=No Bulb; LED = LED Bulb. On/Off indicates the state of the switch as either on or off. Asterisk * indicates if bulb is lit. Here is what I did:

    Test with Incandescent bulb on Drivers Side.
    Code:
    Driver Side                           Passenger Side                           
    Inc-On 12.1v*                         N/A-Off 4.0v
    Inc-On 12.0v*                         N/A-On 12.0v
    Inc-Off 0.0v                          N/A-Off 0.0v
    Inc-Off 0.0v                          N/A-On 1.3v
    
    Inc-On 12.1v*                         LED-Off 4.0v
    Inc-On 12.0v*                         LED-On 12.0v*
    Inc-Off 0.0v                          LED-Off 0.0v
    Inc-Off 0.0v                          LED-On 1.3v
    
    The last test above shows that the LED bulb turns off if the incandescent bulb is turned off.

    For this round I moved the incandescent bulb to the passenger side and repeated the tests.
    Code:
    N/A-Off 4.0v                          Inc-On 12.0v*
    N/A-On 12.1v                          Inc-On 12.0v*
    N/A-Off 0.0v                          Inc-Off 0.0v
    N/A-On 7.7v                           Inc-Off 0.0v
    
    LED-Off 4.0v                          Inc-On 11.9v*
    LED-On 12.1v*                         Inc-On 11.9v*
    LED-Off 0.0v                          Inc-Off 0.0v                         
    LED-On 7.8v                           Inc-Off 0.0v
    
    Last round of testing with two LED bulbs.
    Code:
    LED-On 4.5v                           LED-Off 1.1v
    LED-On 4.5v                           LED-On 4.5v
    LED-Off 1.1v                          LED-Off 1.2v                         
    LED-Off 0.2v                          LED-On 4.5v
    
    None of the LED's turned on. An interesting but confusing find was I did notice that if I bridged a positive and negative terminal, didn't matter which positive or negative, with the incandecent bulb, that all three lights would illuminate (provided the switches were on). Bridging positive/negative of the same terminal or positive of one and negative of the other, would yield the same results. I thought maybe I found a fix, just bridge them with a wire permanently, so I attempted a test to measure the voltage when the switches were off to see if it was viable fix but the LED would flicker and go out.


    I haven't noticed any other lights that do not come on. The only other interior light is the dome (with LED bulb), which functions correctly regardless of other factors.

    For the second suggestion, I thought that the wiring harness illustrated in my first post shows it's parallel. If it is, then the LED does turn on, so long as the incandescent switch is switched to on.
     
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