Detecting 12v AC at doorbell button

[Harry Bloomfield]

Also, the bulb I tried was an indicator bulb, so probably 21W. I think (from my A-level physics) it would therefore draw more than 1A with a 12v supply. So I won't be surprised if the fuse has blown!

Trying to draw 21watts would completely swamp that transformer. 50/50 whether the fuse would blow, the resistance of the windings would likely be enough to limit the current and avoid the fuse being blown.

You may or may not see a spark shorting the wires, depending on where in the sinewave you short them.

For testing if the transformer has an output, assuming the push is illuminated, you could use the lamp in the bell push. Or if you are careful not to press it whilst testing - connect the push itself.

 

[333rocky333]

It's ac, no +ve and -ve

Good point forgot that
Thinking about it not sure really how i could have described the difference between the 2 pairs of wires though

 

[SiliconS]

Well I took the covers off the transformer and proved with the input side that the multimeter is working after all.
I put a new fuse into the output side of the transformer and now have 19v across the terminals (from a 12v transformer!). Happy days.
Thanks everybody who helped.