The door-bell at the residence is (nominally, was) a simple buzzer between live/neutral that sounded Bzzzzzzzz using a simple electromagnetic hammer powered by the 250VAC. The problem is - some creature went and kept the button pressed too long burning out the buzzer (and some wiring too).

Things to do now -

  • Gut the destroyed wiring til lthe nearest junction (I can do this)
  • Replace buzzer (No problem)
  • Try to put some kind of a delay so the buzzer does not keep receiving supply.

For the last my plan is (admittedly simplistic - feel free to shoot me down) to put an AC capacitor from the ceiling fan (1.5MFD or thereabouts) in series between the line to the buzzer. I'm hoping the buzzer will receive supply until the capacitor is charged up after which it won't until the capacitor discharges on it's own (which may be a while).

What, if any, are the risks inherent in this scheme? Should I put the capacitor in parallel to the buzzer?

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    A thermal fuse in series (possibly incorporated into the transformer) that will stop the buzzer when it is activated too long is the standard measure. – ratchet freak Feb 8 '17 at 12:23
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    A capacitor in an AC circuit will never really "charge up". It will be constantly charging & discharging at 100 or 120Hz (depending on whether your mains line frequency is 50 or 60Hz). In this case it will act more like a resistor. – brhans Feb 8 '17 at 13:22
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    A capacitor will not do what you want. A time delay relay can be set up to turn the power off after the bell fires for a specific time but this would be more expensive and harder to install than a digital chime that wont burn up it just keeps repeating until the button is no longer pushed. – Ed Beal Feb 8 '17 at 14:21
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    The output of the transformer that powers my door chime is 20 VAC. Some antique door buzzers originally worked on DC and some on either. What is the power to this one? A doorbell button can stick. Is that what happened or was this pranks by bratty kids? Where is this installation? – Jim Stewart Feb 8 '17 at 21:07
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    @Everyone You can get self-resetting PTC thermal fuses rated at 240 V. To increase the reset time, you can add some thermal insulation, for example heat-shrink tubing. – Andrew Morton Feb 12 '17 at 21:25

Don't do anything like this. Mains voltage is not a toy and it's not for tinkering. Everything that touches mains voltage should be UL/TUV/etc.* listed as mains equipment and installed according to its labeling and instructions. Bare components from the electronics supply are not equipment: they lack labeling and instructions for use in mains wiring, and cannot be so used.

It's very likely the device failed not because of over-use, but because of defectiveness. Running a buzzer til it's burnt out won't damage wires unless the buzzer melts down and shorts out. You may have dodged a bullet as far as your house not burning down.

I would go to a low voltage DC doorbell, because low voltage DC is treated much more lightly by the electrical codes.

Either get a (very, very small) solar panel, battery and charge controller, or kitbash a consumer product that already combines those, such as one of those solar path lights. Or get a low-voltage "wall-wart" transformer. Have that power a low-voltage doorbell via the button obviously. Ordinary use won't flatten the battery, but extraordinary use would.

* CE is not a testing lab.

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Just because I can :-), here's my solution, based directly off the method used in electromechanical pinball games for a time-delay circuit.

Your doorbell is probably 12 or 24 Volt AC-powered, so go to this hobbyshop or equivalent and get a blinker bulb. This bulb turns itself off when the filament heats it up - typically 2-5 seconds -- so just wire it in series with your buzzer coil. Note: pins use 6.3V so if that's better suited to your needs, try MarcoSpecialties

BTW, do NOT get blinker LED bulbs, as those, so far as I know, do not break the circuit.

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  • Afraid not Carl :( the doorbell receives the entire line voltage - 250VAC - when the button is pressed. – Everyone Feb 12 '17 at 15:51
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    Everyone I believe your wrong I have never seen a doorbell at any line level they all are usually 12,18,24 volt try looking for 120volt good luck unless you include a remote doorbell that has a plug in receiving unit. Most homes have a small transformer in the attice usually close to the attic access or where the doorbell is not the doorbell switch at door. – David Moritz Jul 2 '17 at 5:08
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    240V....I...am...never...ringing...a...doorbell...in...Europe... – Harper - Reinstate Monica Apr 13 '18 at 20:53

There are a variety of ways to make a "one shot" to protect a solenoid.

Or, you can take this incident as a chance to rethink the entire doorbell setup. The typical solution uses energy 24/7, for a use case of just a few seconds per day (assuming no local kids or vandals are mashing the button down for hours).

For an energy efficient design see: http://www.olino.org/us/articles/2009/02/03/an-energy-saving-doorbell "An energy saving doorbell" by Dick Kleijer (Olino Feb 2006).

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  • Don't like the link, busts every electrical code and barks up the wrong tree. This is too easy: (very small) solar panel and (very small) bank of NiCd batteries (because they're so docile) and done. Cheaper than what he's doing. If you really, really want to backlight the buttons, ok bigger panel/batteries... I'd add an LV motion sensor and only illuminate the buttons when approached. – Harper - Reinstate Monica Apr 13 '18 at 21:11

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