Why is it impossible to connect these two doorbells in series or parallel?

Question

I have:

• one 220 V AC -> 10 V AC transformer

  

• one push-button switch (at the door)

• two bells for two different places of the house; they should ring together when the (single) door push-button switch is pressed. Note: I don't want to replace the bells because I like their sound.
  Photo of a single bell:

  

Question: Why do the 2 following solutions fail?

Initially, there were 2 transformers connected like this, and it worked (unfortunately, the 2nd transformer is now broken and unusable):

(Schematics thanks to @Transistor)

Edit:

• with "case B" above, we have, at the output of the transformer:

  • 10.3 V AC when no bell rings
  • 7.7 V if either one of the two bell rings
  • 6.0 V if both bells are connected. Then only one bell rings.

• close-up photo of the other bell (it seems that it is the same model) (you can click to zoom):

  

• when the bell is not powered, we have 15 Ohm between the two wires. When the coil armature touches the right metal part, the two wires don't seem neither shorted nor broken circuit: we still have 15 Ohm. NB: I have only been able to push the right metal by hand, and I have not been able to pull out / make the armature go out.

Answer 1

The first solution fails because to have two bells in series you would need twice the voltage of a single bell. The transformer is not going to supply enough voltage. Even if you had enough voltage it would probably not work because these bells work by interrupting the circuit. When the button is pressed the coil is energized and attracts the armature, which hits against the bell, at the same time there is a contact that opens breaking the circuit, this removes power from the coil, the armature is released and falls back to its original position, the contacts are closed again, and the circuit is completed starting another cycle of the bell ringing. Otherwise the clapper would just stick to the bell as long as the button was pushed. With two in series you'll have both trying to break the circuit and it's going to not work as intended.

The second solution probably fails because two bells in parallel would need twice the current of a single bell and one transformer may not be able to supply enough current.

Taking a closer look at the picture it appears as these might not actually be breaking the circuit, they might be shorting the coil which would interfere with their operation in parallel, so a higher current transformer might not solve the problem.

You can see how clapper doorbells typically work here.

Update: From your tests and additional photos it appears that this does not work like a typical electric bell. Every one I've ever seen, and every diagram I find online shows them having a set of contacts that interrupts the power to the coil to make the bell cycle. I can't see that on yours. The only thing I can think of is that they rely on the AC current's changing polarity and the dampening of the spring to make the bell ring repeatedly.

If this is the case we're back to case 1 being not enough voltage for two in series, case 2 being not enough current for 2 in parallel.

Answer 2

Circuit "Case B" is correct. If one bell is not ringing, it's because the transformer is not powerful enough to power both bells, the voltage sags, and only one bell is able to ring at a lower voltage. Either that, or there is an error in your wiring.

Answer 3

Wiring the bells in series will divide the available voltage by 2 so each bell receives only 5 V, insufficient to operate.

Wiring the bells in parallel overloads the class 2 (Canada) transformer which pulls the transformer voltage too low, insufficient to operate.

The original wiring with two transformers is the correct (only?) way to make both bells work with the same key-press.

You will have to replace the defective transformer with a new one.

Be certain to follow local electrical codes. Check with an electrician if you are uncertain.