Smoke detectors false trips

Discussion in 'General Electronics Chat' started by MikeML, Feb 7, 2014.

  1. MikeML

    Thread Starter AAC Fanatic!

    Oct 2, 2009
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    I have a 2yr old house with 120Vac powered smoke detectors. They are cross wired so that when one goes off; they all go off.

    We have had several unexplained false alarms, usually between about 1am and 4am. Could the power company be doing some switching or other line work that could be causing this?

    The smoke detectors are on a separate breaker, so it would be easy to install a line-filter (common mode choke out of a SMPS?). Would it help?

    Any other ideas?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Could there be some glitch or momentary interruption in the power causing the false alarm? If so, that would be difficult to suppress, other than with a UPS.
     
  3. inwo

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    Nov 7, 2013
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    I thought it was batteries when ours did that, but since vacuuming (cleaning) them, they have been silent.

    If it's always the same time.:confused:
     
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Check the dates on the detectors. When they get old and the Americium slug becomes depleted, you start getting false alarms. Also, dust, vapor deposition from cooking or smoking and spider webs blocking an optical path are possible culprits.

    You should be able to temporarily unlink and operate the detectors individually to find the offending unit.
     
  5. MikeML

    Thread Starter AAC Fanatic!

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    Standard AC-powered alarm clocks (without back-up batteries) are not perturubed. If it is a glitch, it is more likely a voltage spike.

    I'm thinking an isolation transformer with an LC-filter and an MOV on the smoke-detector branch circuit.
     
  6. MikeML

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    The original batch were less than one year old; new out of the box when installed in 2012. The were the Firex brand. After numerous false alarms, the commercial electrician that wired the house and installed them originally came back and replace all five with new ones made by a different mfgr. That reduced the frequency of alarms, but hasn't eliminated the problem.

    There was no dust or cobwebs in the first batch. They had been is place less than 6 months...
     
  7. inwo

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    False alarms seem common.

    When wiring my house 10 years ago, I asked a more experienced residential electrician for advice.

    The ones he supplied were BRK. Only a half dozen problems since that time.

    Most were batteries (I think). Since cleaning I haven't had a beep in a year or more.

    On another note.
    I'm not a believer in annual battery changes. They are ac powered and beep when low. I'm on about my 3rd set.
     
  8. alfacliff

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    Dec 13, 2013
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    my alarms go off when the batteries go down or when my wife makes pancakes.
     
  9. gerty

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    Aug 30, 2007
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    I've seen issues with people that used wood heat and had a humidifier causing false alarms. Dust is another issue, but yours don't seem to be old enough for that.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Although AC powered, do they have back-up battery power?
    Can you reverse-engineer an alarm to see how its power supply could be improved (e.g. by adding a cap)?
     
  11. inwo

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    Nov 7, 2013
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    Pretty standard design. 120vac, all on the same circuit, with a third wire linking them. 9v battery in each for backup.

    I don't know how the link works. Ac relay powered I would think. To keep low voltage signal off the common.

    Oh wait, that wouldn't work on standby power. Duh!
     
  12. davebee

    Well-Known Member

    Oct 22, 2008
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    Is there any chance that the detectors are detecting an actual problem?

    I've had detectors go off before my nose detected that any problem existed, where it turned out that something really was burning.

    It wouldn't cost that much to buy a separate battery powered detector to use for a second opinion.

    On a related issue, when I had a contractor do some kitchen work, the local building codes made him install smoke detectors throughout the house to bring them up to code. The codes said to mount a detector right over the bedroom door, which he did.

    But that location was also right outside a bathroom door, where the shower steam would regularly set the detector off, to our great annoyance. It turned out that the detector instructions themselves said NOT to mount it right outside a bathroom door.
     
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  13. GopherT

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    Nov 23, 2012
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    Do you have teenagers or house guests who might be smoking?

    Simplest answer is usually the right answer. Look for a real problem. I doubt two sets of detectors from two different manufacturers would lead to the same problem. Especially when no other electrical problems exist in the house (popped GFIs, garage door opener failures, TV or radio interference, ...?). Look for the smoke.

    Another possibility is rapid temperature change - do set the thermostat at 60 overnight? Does it happen in the summer? Do you have a humidifier? Look for condensation - just in case there is poor insulation above and the smoke detector is cold when the humid air hits it.
     
  14. #12

    Expert

    Nov 30, 2010
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    Just a block diagram of the basic structure of a smoke alarm.
    As long as the radiation exists, Q2 deletes the base drive to Q1.
     
  15. MikeML

    Thread Starter AAC Fanatic!

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    They go off during REM sleep time. No one is cooking, showering or smoking. We live in very dry AZ; air is clean.

    The Electrician trouble-shooter told me that almost every new house they wired in the past couple of years has the same problem. The trips are happening in the wee hours of the morning.

    I still lean toward something coming in on the AC power.
     
  16. davebee

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    Oct 22, 2008
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    Hmmm... Everyone in the neighborhood is affected around the same time... How far are you from Area 51? Secret military tests?
     
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  17. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Just to be clear, the decay of the Americium isotope slug begins at the time of it's manufacture. All shelf time prior to installation counts against you as much as time after installation does.
     
  18. GopherT

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    Nov 23, 2012
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    The decay starts immediately as this non-natural element is made from plutonium decay.


    Americium 241 (the isotope used in smoke detectors) has a half life of 432 years. The decay is first-order, just like a discharging capacitor.

    I don't think the percent of the decay will be noticed in the first 20 years.
     
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  19. ifixit

    Distinguished Member

    Nov 20, 2008
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    It is possible that "new house" off-gassing (even after 2 years) is tripping the ionization type smoke detectors. You could switch to the photo-detector type and see if that helps. The gas builds up at the ceiling at night when the air is still.

    What type do you have? Use ionization type near kitchens to reduce the likely hood of false alarms do to burned toast etc. Use the photo-detector type near the bedrooms where smouldering type fires are more likely. Either will work in either situation regardless. It more about reducing false alarms and preventing the homeowner from riping the unit from the ceiling and throwing it out the window.:eek:

    No kind of power supply problem should ever put a smoke detector into full alarm mode. It could, however, sound a short chirp to signal that the power has glitched, gone off, or come back on.
     
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  20. #12

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    Nov 30, 2010
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    I wish you'd tell my smoke detectors! I have 6 of them in my house and every batch of 6 has 2 go into false alarms within 5 years.:mad:

    I must have tried 3 brands because every time I install a 6 pack of smoke detectors I have to change the plugs in the ceiling boxes.

    It's always a case of annoying. I swear I'd quit replacing them if the stakes weren't so high.
     
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