doorbell wiki confusion / using switched mode PSU with door bell

[David Robinson]

I share this with the group in case the conclusion (wire the bells in series, not parallel) is useful to anyone, and to see if anyone has any better advice about all this! And whether to use a snubber or not...

I tried to follow this...

installing my new door bell.

One push, one transformer, two ding-dong bells in parallel. push:

the standard transformer from tlc... worked OK on the 12V winding, but with two issues:

1) while the door bell is pushed in (i.e. between the "ding" and the "dong") the AC made the bells buzz/hum 2) good old transformers use electricity even when the bell isn't being pushed

I though a switched mode DC PSU could solve both problems, so tried to figure out what to buy...

The bells were rated 8-16V. 9V batteries or 16V transformer were suggested on the packaging, 1A load. I was quite surprised they'd worked in parallel from the 12V 1A transformer, but they seemed fairly happy with it.

I referred to the FAQ, and saw that I needed plenty of voltage and current. Taking all things into consideration, I chose this...

15V.

Long story short, it does this:

1) one bell - works, but with such force that the initial "ding" is more like a "BANG" that's going to shoot the sounder through the wall 2) two bells in parallel - holding the door bell push in (i.e. you'd just expect a "Ding"), you get a ding ... dong-ding ... dong-ding ... etc. Seems the PSU has some sort of short-circuit or overload protection that kicks in. 3) two bells in series - fine.

So, that was the solution: put the bells in series. Works fine, but I'm wondering if a lower powered PSU with the bells in parallel might have been a better solution, or worse?

Also, for just one bell, when using a switched mode PSU, it seems you need far less voltage (or current) than the packaging or FAQ would suggest. Not that I've tried it - but with 15V 1.25A, I don't think the bell is going to last long, and while it lasts, it won't make a very nice "Ding"!

In all cases, the crackle in the door bell push itself is quite considerable. I could try a snubber, but I'm worried that's just going to draw a little current 24/7 - the whole point of the switched mode PSU is that it draws basically nothing when there's no load (i.e.

99.9999% of the time).

Skinflint that I am, I'm trying to do the mental calculation of the cost of replacing the door bell push when it's prematurely worn out due to the arcing, vs the cost of the current drawn by a snubber. But I can't begin to figure it out!

Cheers, David.

[brass monkey]

1.25A, 15V.

We could have a whip-round and get you a new bell-push ;)

[Tabby]

The main problem is your high voltage, 15v for doorbells is way OTT. Rewiring them in series gets you about 7.5v per chime, which is still high, but they should survive it. The thing is with 2 tone chimes is that they use a motion activated switch to chop the power they take, so when you wire 2 in series you get total chaos, as you've found.

By using a nonstandard approach, you've run into several problems at once. The voltage is excessive, affecting the chimes and switch, and the way chimes work means that seriesing them causes all sorts of wacky timing issues. You seem to have escaped the current issue, namely that smpsus will only put out their rated current, whereas tranformer supplies will put out well above that for brief periods, and some chimes demand that extra current.

The standard solution would be to put them in parallel and use something a standard 5v transformer to run them - or if your wiring's very long, go up to 8v. This will solve all the problems.

If instead you decide to keep it as is, definitely fit a snubber, or your switch wont survive, and there's a miniscule chance of someone getting a nasty shock off the system. Snubbers dont eat any current when the bell's not operating. I'd be tempted to put a snubber across each chime, to reduce the peak voltages the wiring an switch are seeing.

My gut feel is that including all this on the wiki page will only end up confusing people. Mayb a bit more could be made of putting mechanical sounders in series.

NT

[Tabby]

Once you've got it working, its not hard to make a good nice bellpush.

NT

[Man at B&Q]

It will, very fast acting in the order of milliseconds.

You need a higher current rated supply if you don't want it to cut out. The voltage from a transformer based supply (assuming no current limited regulation) will sag under excess load rather than cut out. You also need a lower voltage, which will reduce the current requirement.

I wouldn't assume that for cheap consumer stuff like you bought.

MBQ

[Bob Minchin]

1.25A, 15V.

Knockers are very effective and consume no power at all.

Bob

[David Robinson]

But it was in series that they worked!

And honestly, they really did say 8V-16V, with 9V suggested for batteries and 16V suggested for transformers.

I can quite believe that. But then either the packaging, or the wiki, is incorrect. However, both agree on the need for a higher voltage when using a transformer.

So I can only think that it's using a switched mode PSU which causes this issue. If I ever have a lower voltage PSU handy, I'll try it - though the current handling would have to be stupidly high to avoid cutting out, surely?

As well as the bell push?

Cheers, David.

[Dave Liquorice]

He he, likewise batteries. Can you stil get PP9's? or wander off the Maplin and get a 6 x AA cell holder and have 9v supply that way. When the push isn't pressed no current is drawn batteries will last for years.

[Tabby]

I guess youre understanding less than I thought. I'd wire them in parallel, use a sensible supply voltage with more current, and add snubbers

NT