So recently my doorbell button got stuck and it burnt out both the electromagnetic coil in the chime and the transformer. As I'm replacing everything I was thinking that I could add fuse to prevent a rogue button from causing so much damage again.

The transformer is 16VAC 10W and I measured the voltage as just over 20VAC. The chime consists of an electromagnetic coil that drives a pin hitting a chime (separate coils for the front and back buttons). It claims it's suitable for a 16VAC 10W or 16VAC 15W transformer.

Circuit diagram

Let's say that x amps run through the circuit when the button is depressed. Normally the button is depressed for very short periods of time (let's say <10 seconds for an enthusiastic doorbell ringer). I've read that a fuse can withstand more than its rated current for a period of time depending of how high the current is over the rated current... can I undersize a fuse such that the blow time for an x amp load would be 10–15 seconds? Would sizing the fuse be so finicky as to be not worth it?

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    Instead of a fuse, I wonder whether a suitable PTC thermistor could be found that would have an appropriate thermal time constant. – Dave Tweed May 30 '14 at 14:03
  • @user43744 This is a nicely written question. However, it deals with house wiring, and as such, it's off-topic on EE.SE . There is a stack dedicated to home improvement, which is where I'm migrating this question. – Nick Alexeev May 30 '14 at 16:55
  • Are you sure you're fixing a problem that actually exists? If you mind the switch (maybe check it once a year?) and replace it if it shows the least bit of wear you shouldn't have this problem again. Also how old was the old switch and chime and transformers? – Freiheit May 31 '14 at 1:30
  • Also does your new chime continue to ring or does it get stuck on and buzz if your switch gets stuck on? In other words will it fail or just keep ringing if the switch sticks? – Freiheit May 31 '14 at 1:33
  • @Freiheit it gets stuck on with the solenoid holding the pin against the chime and buzzes. Eventually this causes the solenoid to overheat. The chime and transformer were many years old but in good condition. The button had been recently replaced and was only a couple months old (I guess it was defective). – user43744 May 31 '14 at 17:51

Wouldn't a slow-blow fuse work? I don't think you'd want to under size it. Just understand what voltage/current ratings you need and find a slow-blow fuse that can handle 10-15 seconds of that.

Something like this. Just figure out where you need to be on the Average Time Current Curves.

  • 10W at 16–20V would put me at 0.5–0.625A... I think the 0315 from the 396 series would work. I don't have a multimeter that can measure >0.2A but I'm assuming that if the electromagnetic coil is energized holding the pin against the chime the resistance would be very low. – user43744 May 30 '14 at 15:02
  • Can't you find the coil resistance by removing the chime from the circuit and using your multimeter? That will give you the minimum resistance (maximum power) that your chime will use. – horta May 30 '14 at 15:08
  • 7.5 Ohms, so 2.6A? It's been a while since I've done this. That's quite a bit higher than I expected. – user43744 May 30 '14 at 15:56
  • That's what ohms law would state. If you're driving it from a transformer though, it probably drops to around (or slightly below) 16 volts under load. That would make it around 2.13 Amps. It's not terribly surprising that the designers under-spec'd the transformer and coil as they never expected it to be held down for long periods of time. You could try out one of the fuses shown in the link and see how long it would take for the fuse to blow. – horta May 30 '14 at 17:48

A good old bimetallic strip positioned close to the bell can open the circuit to prevent excessive over-heating.

enter image description here

As the bell solenoid gets a little warm, the bimetal elements warp mechanically and open circuit the bell current. After it has cooled down a bit the contacts close: -

enter image description here

  • But I'd probably have to build it from scratch, right? – user43744 May 30 '14 at 16:36
  • Try ebay ebay.co.uk/sch/… BUT read the spec to make sure it is suitable and is normally closed at low temperatures and opens at the correct temperature. – Andy aka May 30 '14 at 16:51
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    Self-resetting thermal breaker is another name for these, commonly used in automotive circuits where you have similar problems, something sticks, but it's the prolonged current, not overcurrent that is the issue. The intermittent circuit operation is the failure indicator, a constantly ringing doorbell would alert you that the button was stuck. – Fiasco Labs May 30 '14 at 17:44
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    @FiascoLabs Actually older vehicles used something like this to control their turn signals. – Brad Gilbert Jun 1 '14 at 16:30
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    Yep, Thermal Resetting Breakers and Bimetalic Turn/Emergency Signal Flashers. One neat thing was you could tell by the frequency of the flash if all bulbs were lit (burned filament slow or no flash). Adding a trailer made them work like they had heart arrhythmia. – Fiasco Labs Jun 1 '14 at 19:37

Having had doorbell problems myself, I feel your pain.

I would recommend against this kind of hack. Consider what would happen if an overly enthusiastic Girl Guide decided to hold down your doorbell button.

From my personal experience, I found no problems holding down the doorbell continuously. This leads me to suspect that one of your components failed in some other way that violated the part's ratings. Perhaps some coils short circuited. Hopefully new parts rated to the power you specified will last longer.

Edit: As a tip, taping down your doorbell while inside testing connections is particularly helpful.

  • As the button was stuck and I didn't catch it I think it was held open continuously (possibly a couple days). :( If it was held continuously wouldn't the fuse just blow? I'd rather replace a fuse than the chime and transformer next time. – user43744 May 30 '14 at 14:18
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    Perhaps my answer wasn't clear. My belief is that under normal operation (which includes holding the button), the components should be able to withstand the continuous current. Otherwise it seems unfathomable to me that houses around the world are operating a system that can be so easily destroyed. You have two possible failure modes. 1. Doorbell systems can't withstand long term current. 2. You were subject to bad luck/age and your components just randomly failed creating condition 1. I find #2 easier to believe. – lm317 May 30 '14 at 15:26
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    I think that in the case of a door chime that operates with a solenoid pushing a pin mechanically against a chime this is something they are subject to. Take any solenoid and fix the load and run current through it for a prolonged period of time and it will overheat. Any electric fan is vulnerable to this failure mode as well. – user43744 May 30 '14 at 16:24

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