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My house is a 117 year old masterpiece of evolving building codes and improvisation. One of the things I expect to do at some point in the future is get a doorbell camera, though it's not on the immediate horizon. When we moved in, the doorbell was a wireless, battery powered sort that I find to be moderately annoying. Considering those two things, I bought a Honeywell AT140A1000, as it had good reviews regarding smart doorbells specifically.

Anyway, I finally got around to installing it yesterday, and here's how I had it set up.

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Aside from the transformer being 24VAC and the chime calling for 16, I don't think I broke any rules. Indeed, it worked for an hour or two. Eventually, though, I heard a pop over by the panel, where I have the transformer mounted. It wasn't loud, so I figured I had blown a fuse, even though at that moment I wasn't even touching any wires/connections/outlets/switches. I went to investigate, and all fuses were in order. But it stunk a little. Later reading indicated that transformers can stink a little when you first power them up.

Sometime after, I went to check the doorbell again and saw it no longer cause the chime to fire. So I tested everything, and sure enough, the transformer's secondary was at 0 volts. Not even the 0.065 volts I get from other "broken" circuits in the house, but flat zero. the transformer was also extremely hot. Like, I wouldn't want to handle it for more than a couple of seconds kind of hot.

I still had the original transformer (I wonder how old it is) so I hooked it up again as the sacrificial lamb juuuust in case. This one also got extremely hot, but appeared to not even last long enough for me to get up from the basement to press the doorbell.

So before I grab another Honeywell... help? I am an absolute novice when it comes to higher voltage AC (12v DC is my playground, usually) but this ought to be so simple even I can figure it out. And yet... =)

Thanks in advance for your help!

EDIT: Well, I just learned that you need a certain (larger) amount of resistance on the primary side of this kind of step-down, and a different (smaller) amount of resistance on the secondary side. That, and if there's a lack of continuity on either side, the thing is toast. Or, if there is continuity between the two, it is also dead. Turns out that both transformers I blew up yesterday have no continuity on the primary side, but both are still beeping on the secondary side. So... I wonder if that points even harder toward the breaker box feeding too much power into the transformer. I get a 126V read from the leads coming out of the breaker box. Is that enough to explode everything? I dunno.

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    "My house is a 117 year old masterpiece of evolving building codes and improvisation." Congratulations on one of the best lines ever written on Home Improvement!
    – FreeMan
    Apr 16 at 16:32
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    @manassehkatz you should make that an answer. For the transformer to smoke it was wired wrong or overloaded in some cases the chime is a larger load than the transformer can handle but normally the current draw is momentary so the transformer actually can ring a chime over 2x it’s rated kva, however the new smart doorbell will be a continual load and they should be at .8 of the transformer rating. (Verification of the required kva would be a good idea I would expect this to be more of a problem than the rated voltage in a doorbell circuit).
    – Ed Beal
    Apr 16 at 16:47
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    When you say that you "went to check the doorbell again and saw it no longer cause the chime to fire"... are you calling the switch/button the "doorbell", or (per the diagram) do you actually have both a chime and bell on the circuit? Please revise your diagram to show the switch... Apr 16 at 16:48
  • Thank you @FreeMan =) I like to see the world through an author's glasses, though I've never written anything. Haha
    – pdm
    Apr 16 at 17:12
  • @JimmyFix-it Sorry for the poor naming - the "doorbell" is in fact the button/switch in the circuit.
    – pdm
    Apr 16 at 17:13
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Since both old and new have problems, it is probably not a "bad" transformer, but rather something else that lead to the transformers failing.

Two things to check:

  1. Mains voltage - Should be 120V (anywhere 110V to 125V is perfectly fine). If it is in the 200V - 240V range then you have this wired up to a 240V circuit instead of 120V and everything double and transformers release the magic smoke.
  2. Short circuit - A short would cause a real problem. Check continuity on the doorbell switch to make sure it is not welded closed; check the chime (may get a bit trickier) to make sure it is not acting as a short (or near-short) circuit. A few hundred watts would be enough to fry the transformer but would not trip a 15A circuit. Keep in mind that while an electronic chime will keep ringing if the circuit is closed (until something burns up), an old style bell will give one ding (or dong) for each press of the doorbell switch. If the switch stays closed, you get one ding and then nothing...until something overheats.

Keep in mind that power (Watts) = Volts x Amps. Volts is easy - check (edits and comments indicate you did that) that the voltage is ~ 120V. In a typical residential situation, voltage will be pretty constant if the system as a whole (utility feed, breaker panel, etc.) is installed and functioning correctly. So that leaves current = Amps. That will depend on what is connected on the circuit. In the case of a transformer, the load on the secondary determines the amount of current flowing through the primary. (Don't ask me how that works - it still kind of baffles me - "physics"). So if you have a short on the secondary, the primary will try to provide as much power as it can, up to the value determined by Ohm's law - nothing is a 100% short circuit. On a DC measurement, each coil should show continuity (i.e., very low resistance). The rest of the circuit (in this case, on the secondary) is the key - if, for example, you replace the chime with a piece of wire then you cause a short circuit every time you press the doorbell. If you replace the chime with a 1 Ohm resistor, then Ohm's law says that at 24 Volts you will have 24 Amps = 576 Watts, far more than the 40 Watt (technically VA, doesn't matter Watts vs. VA for this right now). On the other hand, that 576 Watts divided by 120 Volts (nominal) is less than 5 Amps, so no breaker is tripped - but the transformer is fried.

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  • Thanks very much! I did check the mains voltage (126V, usually) and verified the doorbell switch as only completing the circuit when the button is pressed. I am not entirely sure how to check the chime, because there is continuity between the doorbell post and the transformer post. It seems like it's supposed to be there? So the switch is the only break in the circuit? But I agree a short seems most likely, just not sure where with such a simple setup.
    – pdm
    Apr 16 at 17:16
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    The ultimate problem was that I had put a staple through some of the two-conductor wire. Just one, out of the couple dozen I put in. Haha. I found it by testing the continuity at one end of all three pairs of wires, twisting the conductors at that end together, then testing the other end. I quickly found my error after I got a closed circuit when I shouldn't have. =)
    – pdm
    Apr 27 at 21:46

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