set up a slow discharging capacitor on a 6.8v loop

Discussion in 'General Electronics Chat' started by skyesmechanical, May 22, 2012.

  1. skyesmechanical

    Thread Starter New Member

    May 22, 2012
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    Hi I am new to this forum and to electronics so please bear with me.

    Here is what is happening:
    I have a wire loop that leaves my security panel. In the middle of the loop is a NC (normally closed) pressure switch (opens when PSI goes below 25 PSI) connected to one of my AC units at my church. For some reason the pressure is dropping momentary and causing the sensor to open, thus setting off the alarm and calling the police (not good because they started charging for false alarm calls). Yes I am looking into the reason for the pressure drop to begin with.

    This is what I would like to do:
    I was wondering if it is possible to install a small capacitor and a resistor in the loop so that if the pressure drops, thus cutting the power, the capacitor will start to discharge slowly keeping power going through the loop and not let the alarm trigger. If power is lost for a long enough time say 5 to 10 seconds the capacitor will have emptied and then the alarm will be triggered. This would prevent these momentary drops in pressure that we are seeing with our HVAC gauges.

    My security panel outputs 6.8v DC and no amps that my digital non-TRMS meter could read through the loops/zones. The loop already has 3.3k (gold red orange orange) resistor in it, this has to be in the loop for the circuit board in the security panel to know if it has a proper signal.

    Thanks for your help
     
  2. #12

    Expert

    Nov 30, 2010
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    Many problems associated with this. If the current is actually very small, say, a thousandth of an amp, 6400 microfarads would hold the voltage loss to 10% across 10 seconds. However, this is probably too much capacitance and the right size would have to be found experimentally. The next however is: the charging time of the capacitor would probably show as a fault condition every time you start up the alarm.

    If you were really knowledgable about the inner workings, you could tell the alarm to ignore faults less than about 50 seconds in duration with the capacitor installed, or 15 seconds without the capacitor.

    Much better to fix the cause than the symptom.

    If the low pressure switch opens every time the air conditioner starts, it is likely you are short on Freon. The compressor scavenges most of the Freon from the low pressure side before a good flow is established in the evaporator coil. If this is "normal" for your air conditioner, you will just have to find something else to measure. For instance, if this is a theft protection, and if you have extra wires in the information cable to the condensor, you can use the extra wires by just shorting them together at the condensor. If a thief cuts the wire to steal the condensor, he will activate the alarm himself.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If the capacitor is installed in parallel with the switch, it will normally be discharged. If the switch is already closed when the alarm is started up, then no fault condition should occur.
    If the switch opens and remains open long enough for the cap to accumulate significant charge, then the switch could be damaged when it closes and discharges the cap. A small resistor in series with the cap would limit the current, and probably would not incur enough voltage drop to trip the alarm instantly, but this depends on the loop's short circuit current.
     
  4. #12

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    Nov 30, 2010
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    Perhaps about 10 ohms in series with only a few microfarads?
     
  5. Ron H

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    Apr 14, 2005
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    I guess that would be a good place to start. It depends on loop current, the width of the dropout, and how much voltage rise the dropout sensor can tolerate.
    In my last house, I had installed my own alarm system. It was set up to allow me a short amount of time (90 seconds, 3 minutes, something like that) to open the garage door, exit, and close the garage door. That worked fine, but when the time was up, the alarm yelled, "Fire! Fire!". Then it continued to be armed as planned. It was due to a short dropout in a signal that was supposed to be constant. I never tried to fix it. Moving finally solved the "problem".:D
     
  6. skyesmechanical

    Thread Starter New Member

    May 22, 2012
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    As far as I can tell (I've been an HVAC tech for 5 years) the ac system is fine, with one possibility that I have it narrowed down to a possible plugged up dryer. It will be operating at a normal level and then the low pressure will drop to 30 psi for an instant. I watched the system for about an hour and saw this happen several times (didn't notice it on start-up). I just would like to verify this is my problem first before I replace the dryer.

    --The security panel outputs voltage continuously, it only triggers an alarm if the system is active.

    --The security system I install my church doesn't have a delay option on the zones once they are active. You can delay start them but that's it.

    --I completely agree about fixing the AC system, a faulty system could cost us a lot.

    --I my thought was to use it in series.

    --As far as size and resistance that is what I don't have a clue on so I'm asking the experts. :)

    I knew when I set up the low pressure switches that these types of problems with the AC system would set off false alarms but after 4 thefts and about $6000.00 in deductibles and other expenses I thought better have extra sensors than not enough. If our insurance hadn't picked up the rest it would have cost us another $15,000.00
     
  7. Ron H

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    Apr 14, 2005
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    As I see it, the cap and the resistor would be in series, with that combination in parallel with the switch that is dropping out. We can't give you hard values unless we know the parameters I mentioned in my previous post.
    Excess capacitance has no penalties other than size, cost, and possibly switch damage if the series resistor is too small (in ohms). Thinking back on the system that I had in my last house, 100 ohms would have probably been tolerated, and would almost certainly protect your switch. I would probably start with 100uF and 100 ohms, test it, and if the alarm still trips when the pressure drops momentarily, go up to 1000uF, etc.
     
  8. #12

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    With this much information I think about a sticking expansion valve.
    (The parallel-to-the-switch solution seems better than my idea.)
    HVAC pressure switches are rated for a fairly high current if I remember correctly. Sometimes as much as an amp or two, not in the milliamp range.
     
  9. Ron H

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    Yeah, I started thinking about that too. I doubt that you need much series resistance. Ten ohms should be more than adequate.
     
  10. BSomer

    Member

    Dec 28, 2011
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    Here is a really simple solution. Take it out of the loop until you get the leak fixed. That is basically all you are trying to do with the capacitor anyway. This is possible if, as you say, the security system is a loop and not an addressable system.

    I assume you have the HVAC unit in the alarm system to help prevent copper theft otherwise it would have been more sensible to put it and other similar equipment on a monitoring loop that just gives a trouble alarm instead of a full blown alarm that calls in the police.
     
  11. #12

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    I still like the idea about using 2 wires in the control cable as a loop that will open if a thief cuts the wires to steal the condensor. I intentionally installed a 6 wire cable at my house so I could do a high pressure sensor, the contactor signal, and an alarm loop.
     
  12. skyesmechanical

    Thread Starter New Member

    May 22, 2012
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    This unit does not have a TXV valve.

    After the third police call i did take it out of the system so its not monitored anymore. We did do like you said about the wire loop and more. The theifs know about HVAC equipment and knew how to disassemble the units without cutting a single wire. So I have wire in the fence around the units if they cut into the fence again to get into the area it will trip the alarm (they could climb over the 6' fence, shhhhh dont tell them). And i wrapped each unit in a separate wire loop so they cant disassemble the units (like they were doing) without cutting the wire loop. This zone is for the line sets.

    I was thinking in series because i have 9 units at my church on 3 zones/loops this way i would only have to have three of these setup instead of 9.

    I will have to get back to you with the specs you wanted, it will be atleast tonight before i can even get back to the church.
     
    Last edited: May 23, 2012
  13. #12

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    Hmmm...no TXV...back to the filter as prime suspect.

    The thiefs have enough time to disassemble? Serious problem!

    There are cages you can buy to protect condensor units but the bad guys obviously have the equipment to slice them up, too.
     
  14. Ron H

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    This reminds me of the guy whose firewood was being stolen. He decided to store a half stick of dynamite in a hole in one of the pieces of firewood. This solved his problem.
    I'm not suggesting dynamite, but perhaps some sort of booby trap could be set. Nothing un-christian (or whatever religion you adhere to), of course.:rolleyes:
     
  15. #12

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    Perhaps a skunk that sleeps in the condensor at night?
     
  16. skyesmechanical

    Thread Starter New Member

    May 22, 2012
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    I would love to do something like that but my southern baptist preacher would not let me (or city codes). Still laughing.
     
  17. Ron H

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    Yeah, and the long version of the story as I heard it was a lot funnier.
     
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