Detecting 12v AC at doorbell button

[SiliconS]

I want to use the existing doorbell button wiring to power a camera doorbell, which needs 12v AC or more. The existing doorbell transformer is 12v, 1A. So far so good.

There are four cores in the thin wire leading to the front door button: red, yellow, blue and black.
The red and yellow are wired to one screw terminal. The blue and black are wired to the other.

I've tried to check the voltage available at the button using a multimeter across the terminal screws. No voltage detected in either AC or DC modes, whether I push the doorbell button or not. A non-contact voltage detector pen thing does beep and flash near the wires though.

Any suggestions why my multimeter can't measure the voltage?

(I can't check the voltage at the chime because it was boxed in behind plasterboard last year as I thought we didn't need it.)

 

[AndyPRK]

you certainly won't detect a voltage with the button pressed.

I guess you aren't getting enough current thru the bell.
Anyway, connect the video bell and see if it works.

 

[RandomGrinch]

I may be very wrong, but:
1. Those wires are badly corroded and need stripping back. You might not get any voltage reading even if there were any!
2. It looks like the 4 core cable could have been there to supply an illuminated bell push, but has been configured to be just two wires. This leads to:
3. The bell push could (unusually) be configured as a zero volt switch, with no useful voltage appearing at that end. The bell transformer would directly feed the chime box. And that may be where you need to tap the power from.

Firstly I would strip the cable back to clean copper and retest the voltage.

I hope that makes sense!

 

[oldbutnotdead]

As @AndyPRK , good odds 1 side oftransformer is connected direct to chime, the other side goes to switch then to other side of chime. You don't have a ground to reference (except through whatever resistance the chime is offering) therefore your meter won't show any voltage.
You need to find the other end of that 4 core...

 

[333rocky333]

Does the existing chime work when you press the button, if it does then you should get a voltage across the button, but not when pressed.
That is how a luminated bell push stays lit but goes off when button pressed.
Your meter would be in series with the chime load.
It is quite common to find 4 core cable used like that as it increases the CSA of copper and hence reduces the volt drop.

 

[SiliconS]

Thanks all.

Having read your replies I've cut the feed wire from the transformer (i.e. I've intercepted the feed from the transformer to the chime box within the hallway ceiling) to get back to clean metal straight from the source.
Unfortunately my multimeter is still not detecting an AC voltage in any combination of core pairs. Don't understand why! When I stripped some insulation from the four cores, two momentarily touched each other and there was a visible spark, so there was power although I can't now reproduce this.
I tried touching pairs of the cores in various combinations to a 12v (automotive) bulb but it didn't light up. (I assume that tungsten bulbs work with both DC and AC?)

The breaker in the consumer unit hasn't tripped. Perhaps there's a fuse in the transformer that blew when the wires touched. My next job will involve investigating the transformer, but that involves dismantling part of the kitchen so it's a bit of a hassle...

 

[oldbutnotdead]

Oops. You might be better off buying a new transformer and mounting it where you can get at it (depends how destructive your dismantling will be)- take it as a lesson about burying active devices

 

[SiliconS]

Yeah, in an ideal world the transformer would be more accessible but I wasn't going to compromise the design of the new kitchen for things that should (theoretically) never be disturbed. Just need to remove the top box over the fridge/freezer to be able to take the lid off the transformer and get my probes in there.

 

[SiliconS]

 

[Harry Bloomfield]

Some very early bell transformers did have fuses on the output, you might have blown one of those. Bell transformers are not able to supply much current, your 'automotive bulb', unless it was a very small one, might have tried to draw more than it was capable of supplying - hence no light.

You need to test with your meter first of all, at the transformer's output, with no wires connected at the output, to ensure that works. They often had three tappings, allowing for three voltage output options. Across what should be the 12v, likely there should be a little more than 12v.

 

[SiliconS]

Thank you Mr Bloomfield. Good plan.

 

[AndyPRK]

I would say your meter is faulty!!!

 

[SiliconS]

I would say your meter is faulty!!!

You could be right! I'll try it on the back of a 240v faceplate later on and see what it reads.

Having checked more closely I've discovered that the transformer does have a 1A fuse in it, according to its case, so potentially that blew when the wires brushed against each other.

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!

 

[333rocky333]

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!

You said earlier the bulb did not light, so unlikely that blew the fuse.

If you cut and shorted the cable between the TRANSFORMER and CHIME that would blow the fuse as there is a + and - in the cable.

Cutting and shorting the cable between CHIME and button should not blow the fuse as it is a + and a + switchline, shorting them would just ring the chime.

 

[Harry Bloomfield]

If you cut and shorted the cable between the TRANSFORMER and CHIME that would blow the fuse as there is a + and - in the cable.

It's ac, no +ve and -ve