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Need fundamental help understanding this circuit

Discussion in 'General Electronics Discussion' started by oodly, Jan 17, 2016.

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  1. oodly

    oodly

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    Sep 1, 2014
    Hi. In my home alarm panel, the door sensor circuits are each landed on a pair of screw terminals. The terminals read 13.5vdc with nothing attached. Each door sensor circuit is a loop through a normally closed contact, with a 2200 ohm resistor on one end. Most of the pairs of terminals for the door sensors read ~5vdc when the door is closed, and ~12.5vdc when the door is opened.
    Except for one of the sets of terminals. It reads around 13 volts with the door closed and very slightly higher, like 13.4v with the door open. I confirmed the resistor value, even jumpered a new resistor in its place. Why wouldn't 2200 ohms draw down the voltage in this one circuit the same as it does in the other ones?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    perhaps there is a bad connection somewhere,

    try wiring a 2k2 resistor right at the panel rather than at the end of a cable run.
     
  3. oodly

    oodly

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    Sep 1, 2014
    I tried that... wiring the resistor directly from one terminal to the other. The results didn't change. I'm stumped.
     
  4. ChosunOne

    ChosunOne

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    Jun 20, 2010
    Hi, oodly, welcome to the forum. I just tonight stumbled across this thread by accident. I don't check the forum every day, but when I do, I look for questions pertaining to alarm systems in the thread titles. Which is to say that I don't usually look at the first post of every one of the dozens of threads started on this forum almost every day.
    So for future reference, it would speed the process if you would mention "home alarm" in your thread title. A lot of guys know a LOT more electronics than I do, but I'm an alarm technician with some decades of experience and usually alarm system troubleshooting has more to do with particulars of how various alarm systems work than arcane electronic knowledge.

    To start with, it always helps to name the make and model of your alarm control panel. For example, Honeywell/Ademco Vista-20P panel, GE Concord, ITI SX-V, etc, etc. Even if the problem doesn't hinge on the type of panel (which this probably doesn't), it lets a pro know how to tell you to test or try certain things. Different panels work different ways.

    Also: Don't be secretive about the zone you're having a problem with. It takes no more time to type "Zone 1" than to type "one of my zones". Not all zones are alike in some panels, believe it or not, Please name which number zone you're having trouble with.

    Now you didn't mention having a problem with your system, but I'm guessing that that's why you were metering the zones in the first place. It sounds like a pretty straightforward problem. The 2.2K End-of-Line Resistor (EOLR) is wired in series with the magnetic switch in/on (which is it?) the door frame. The mag switch is a NO switch that is held closed by a magnet on/in the door when the door is closed and brings the magnet in proximity to the switch in the frame.

    So when the door is closed, the switch is closed, completing the zone circuit (aka zone loop), and the panel terminals "see" 2.2K resistance, which typically pulls the voltage down from about 12.5VDC to about 5.5VDC. (The exact readings vary from one model to another, and sometimes on different zones, and are a red herring to chase unless you have some kind of arcane problem.)

    When the door opens, it opens the circuit and the voltage goes back up to ~12.5VDC.

    From what you've described, the mystery zone ('cause I don't know the number yet) loop/circuit is reading open or high-resistance (In alarm jargon, we call it a "high-resistance open") with the door closed, when it should be seeing only 2.2KOhms.

    The most common causes for this symptom are, in order of statistical likelihood, are:
    (1) Door magnet is missing, too far away from mag switch/contact, or misaligned. (most often)
    (2) Loose, corroded or severed connection(s) at mag switch or control panel terminals. Sometimes it's as simple as switch lead wires being twisted together and not soldered. Sometimes a crimp-on connection was crimped improperly and took years to go bad.
    (3) Mag switch in/on door is bad. (much less often).
    (4) Bad or corroded splice or severed wire the walls between panel and switch. (Very rare)

    In some instances, the door switch is a plunger-type, where a button is pushed into the switch by a closing door; sometimes on the hinge side and sometimes a roller/ball -type plunger on the latch side. If you have one of these switches, this would be the time to describe it. They tend to be high-maintenance.
    -----------------------------------------------------------------------

    A final word, in case you decide to order a new switch for your door; In the alarm industry, nomenclature for sensor switches is different than standard electronic nomenclature. "D/w" (door/window) switches are nearly always referred to as "Normally Closed" when they are in their "standby" state, i.e., with the door or window closed, which means with the paired magnet in proximity. What an alarm tech calls NC and NO in door/window mag switches is opposite what an electronics tech would call them. So be aware of that if you order new ones.
     
    Last edited: Jan 24, 2016
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I liked CO's post but it should be worth mentioning that mag switches can be purchased that contain both N/O - Common - N/C contacts. In general electronics terminology this is equivalent to a Single Pole Double Throw (SPDT) switch. If you purchase one of these you can't go wrong regarding door open or door closed logic. Rephrased... normally closed circuit vs. normally open circuit. These mag switches will handle both scenarios. They can be identified by having 3 screw terminals. They're only slightly more expensive.

    Chris
     
  6. ChosunOne

    ChosunOne

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    Jun 20, 2010
    Some days I'm a little slower than other days.

    Just reviewed this thread and realized that I made an assumption I had reason to make: The simple zone loop circuit I described is standard in the USA, but in Europe (including the UK) and some other parts of the world, a "Double End-of-Line Resistors" (DEOL) zone loop is commonly used, and the troubleshooting, while not overly complicated, takes a bit more attention to detail. Knowing the control panel model becomes even more critical since I'm likely not familiar with it and may need to browse the manual.

    Oodly, if you're not in the USA, this would be the time to mention it.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Yes, membership should make that mandatory with the flag icon displayed as nearly all of us have. It's a nice feature for those of us that don't have the entire world's national flags committed to memory.

    Chris
     
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