Another doorbell question

[pete]

For reasons which won't become clear, I've been having a poke around inside my doorbell. It's a "ding-dong" chime type, with a single push-button near the front door. The unit itself is powered from the mains, presumably through a transformer.

The pokin' I've done so far shows that the voltage across the push-button is a slightly clipped sine-wave, around 40V peak-peak. I have to say, this surprised me as I'd assumed that the solenoid inside the dinger had to run off DC - oh well.

When I whipped the lid off the chime-unit (a normal looking commercial type) I found that it has comparments and contacts to take 4 * 1.5V batteries, which are empty. Power is supplied through a couple of wires coming from somewhere in the vicinity of the mains distribution panel - so I presume there's an old bell transformer behind a kitchen cupboard, somewhere.

Now, the guy I bought the house from was a complete bodge-it type. I've uncovered a mass of barely operational screw-ups, which I don't want to get into here. However, my question is: Given the door chime is designed to be battery operated, and seems to be powered off low-voltage AC, how the hell is it working now?

Pete

[Slider]

My door bell is mains powered via a transformer (16 Volt I think?) but the chime unit has compartments for the battery also. This was originally fitted by the house builders.

[pete]

Yes, that's interesting - thanks. I'm still wondering how the solenoid manages to operate off an AC supply. Do these things have an in-built diode to rectify the voltage?

[Adrian]

I don't think they need DC - I've used relays in the past with the coil fed directly by 240v AC.

I'm on shaky, half-remembered, A-level physics here - but if you imagine the coil as operating on DC, creating a magnetic field, and attracting the arm of the 'dinger', then if you put the batteries in the other way round you'd likely find that the 'dinger' still 'dinged'

When fed with AC you're just 'swapping the batteries around' - but fast

The 40v peak-to-peak does sound a bit high - ISTR that standard doorbell transformers are rated about (.....fetches chair, checks tx built into our CU) 8V AC. 40V does seem bit OTT - but if it's a 'ding-dong' chime then there's only a very brief time that the power's applied, so I guess it's survived the overload... so far....

Adrian

[TheOldFellow]

Most of these (usually Freidland) chimes were designed for either battery of mains operation. Oft times, people got fed up of changing the batteries and added the appropriate bell transformer. The transformer is usually either near the consumer unit (in which case look out for dodgy wiring in the back of the same, usually with bell flex), or buried in the ceiling near the chimes, and wired to a convenient back-o-socket, or even back-o-lighting-rose supply.

If they are fitted by a competent electrician during house build, then they might be OK.

The transformer drives the solenoid - an inductive load - which flings the chime striker one way to hit one bell and it flies back under a spring to hit the other.

R.

[pete]

Indeed. However, by driving the solenoid with AC, I'd expect the core to be moved in step with the input voltage/magnetic field, first in one direction then in the other at 50Hz. End results would be no net movement, with either not enough time for the "dinger" to get to the chime, before it's field was reversed - or for it to hit the dinger many times a second. Since neither of these happens, what's going on?

[Dave W]

The plunger is soft iron, not a magnet, so both polarities of field pull it in. Your 40V peak to peak (what are you doing with an oscilloscope?) is measured off load and is called 13V rms, probably about right for the tichy little transformer which may drop to 6V when loaded by the bell solenoid. Over voltage doesn't really matter for a

1 second push on the button.

Dave W

[Owain]

In very simple terms, there's a magnetic delay which prevents the solenoid 'chattering' on AC.

For more information google slugged AC relay

Owain

[pete]

Yes, I did make it just over 13V AC on my meter - it was also pulling just under 0.5A. for as long as the bell button was pressed (a little worrying as you say: it's prolly not a big transformer). The plan is to utilise the doorbell as a form of alarm[1] with a VFET attached to a PIC 12c509 to activate the bell. Strangely, an opto- isolated triac (plan B) doesn't work either: the triac never sees a zero voltage, and therefore never switches off - resulting in a single "ding" and a heavy current draw. Next step is to slip a diode into one of the AC power leads to the chime and see if I can get the original idea to run off an unsmoothed DC supply.

Pete

[1] letterbox alarm. We've had a few instances in the past of baddies trying to steal stuff through house's letterboxes by poking long sticks through and trying to hook wallets/car keys etc. The plan is that if the letterbox is wide open for too long - say 15sec, then the PIC rings the doorbell a few times. It all works, except for ringing the bl**dy doorbell, which I had initially assumed was running off a DC supply.

[Chris J Dixon]

Might give you time to catch and remonstrate with the free-paper deliverers, and the like, who seem unable to push their bumph right through.

Chris

[Dave Plowman (News)]

Solenoids - as relays - can be made to operate on AC. All older door bells can usually operate on either AC or DC. I've never seen one that needed a mains derived DC supply.

But there are also ones which are DC only.

[pete]

Free papers? haven't seen any of them for at least 10 years. I thought they all got tossed, en masse, over the fences of people (un)lucky enough to live near the distribution points. Anyway, back on topic. The actuating microswitch has it's lever adjusted (i.e. bent) such that the letterbox flap has to be fully open for the whole time, so the odd leaflet won't activate it. If something does wedge the letterbox open, and fire the alarm, it won't fire again until it's been reset by the flap closing - thus preserving the sanity of my neighbours if/when I'm out.

[Dave Plowman (News)]

You use a non permeable core - so it becomes a magnet of opposite polarity with each cycle of AC. The armature, of course, has no polarity.

I'm not sure of the actual type of steels - but ones designed for DC only must use a different core as they won't often work on AC but 'chatter'

[Appin]

The message from pete contains these words:

Standard bell voltage in the USA, but high for UK where most have traditioinally used 8v with the bigger chimes using 12v

[John]

If you use an illuminated button then you may blow the bulb if you set the transformer higher than the bulb voltage.

[Appin]

The message from "John" contains these words:

Quite. So if you're using 16v, then in practice you're going to need an American push. But illuminated pushes are a bit passé these days, what with most door chimes being electronic and rarely hard-wired.