Need to mod door bell

The plastic around the winding and plunger in my door bell melted when
the door bell button got stuck in while I was gone. The button is < 2
years old. I can get another, but it could easily happen again. It
seems like it could be a fire hazard too.

What kind of mod can I do to keep the bell from melting again if the
button sticks? Something I could add that would make the bell go
ahead and complete it's 'ding-dong sequence' when the bell is pressed,
instead of waiting until the button is released before it 'dongs'?

 

The bell is made for very brief use, followed by a long period of
cooling off time. If the bell got hot, the transformer probably also
got hot.

A slow blow fuse of an appropriate rating would offer protection but
still allow the brief overload current needed for normal operation.
If you don't mind replacing the fuse occasionally, this is probably
the cheapest and most reliable solution. The fuse can be put in the
low voltage button circuit, or in the primary side to also protect if
the transformer develops a mild short.

You could start with an obviously too small fuse and work your way up,
till the blow time gets long enough to prevent nuisance blowing, or
measure the current and select a fuse rated for about half that. This
will give you something like 10 seconds of operation before the fuse
blows.

 

How about a thermal trip bonded to the coil of the "dinger" ?? (naturally in
series with the supply to the bell unit )

 

---
What you need is a timer which you can trigger with the doorbell button
and which turns on for (say) one second when the button is pushed and
then turns off whether the button continues to be pushed or not. A
relay driven by the timer could then be used to activate the ding-donger
and give one complete ding-dong sequence every time the button was
pressed.

I don't know much about doorbells, but it sounds like what's in yours is
a solenoid which, when energized, pulls in a plunger which hits the
'ding' bell and, when it's released, hits the 'dong' bell and then
settles into a neutral "floating" position. If that's true, then
there's probably a 24V transformer somewhere with the secondary
supplying power to the solenoid when the switch is made which makes the
whole thing work.

If you can post how the thing is wired I'll post a schematic showing a
circuit to make it work, if you like.

 

Hi,

You need to post more information about the circuit
of the bell. For instance, is the voltage to the bell-push
AC or DC? If it is DC, I would be inclined to start by
placing a large capacitor in series with it. This would
ensure that there was only an impulsive current to the
dinger/donger.

I expect that the dong is caused by the plunger
going back to its resting position so that the interval
would depend on the actual time constant. If it is AC, you
could perhaps put a bridge rectifier across the bell-push
with a capacitor on the DC side.


Cheers - Joe

 

Good application for one of those auto resetting solid state circuit
(thermistor type) breakers. ding - variable to long pause - dong

or

have the button work a monostable timer that can't be retriggered
without pushing the button a second time. ding - timed interval -
dong.

 

You could replace the bell unit with a cheap electronic one. These
units dont draw high power or get stuck.

 

A 2200 or 4700uF capacitor does the job, in series with the bell. Add
a resistor across the C so it resets. Ensure ca is corrected right way
round regards + and -.


Regards, NT

 

Sorry it took so long. My camera is half broken.

transformer and wall box:
http://pics.apartment808.com/users/Inverted/db1.jpg
http://pics.apartment808.com/users/Inverted/db2.jpg
http://pics.apartment808.com/users/Inverted/db3.jpg

door bell unit:
http://pics.apartment808.com/users/Inverted/db4.jpg
http://pics.apartment808.com/users/Inverted/db5.jpg

button:
http://pics.apartment808.com/users/Inverted/db6.jpg

simple schematic I drew:
http://pics.apartment808.com/users/Inverted/db7.jpg
Sorry I couldnt get the text tool to work to label. Blue box is the
wall box (wires go to circuit breaker panel/door bell button), black
filled box is the transformer, black outlined box is the door bell
unit, gold wire is bare copper wire, light blue thing is where i
thought about adding a fuse.

Thanks!

 

The transformer says it's 10V and 10VA. So what size fuse would you
think I need?
Can I get slow blow fuses at a hardware store or auto parts store, or
would I have to go to rat shack?

 

What I often use is a Dc circuit with a big cap charged thru a high
resistance. Push the button to get the ding, hold the button and the
high resistance takes over and little happens. Release the button and
the cap recharges in 10-30 seconds.

 

a 10 VA rating implies that the transformer produces a maximum product
of volts and amperes of 10. Since it is also rated at 10 volts, it
supposedly can handle 1 ampere for quite a while (but perhaps not
continuously, since some ratings include an assumption about duty
cycle). But the other important fact to know is what current the door
bell coil draws from this transformer, since that is what you are
trying to protect. I think you will have more luck finding time delay
(slow blow) fuses at Radio Shack that at an automotive store, though
Walmart may have some choices.

If you don't have an AC ammeter that can measure the doorbell coil
current, you might use an ohm meter to measure its resistance and
assume that resistance is primarily what limits the current. Say the
coil resistance is 10 ohms. Then the 10 volt transformer will push 1
ampere through it when it is on (10 volts / 10 ohms = 1 ampere). In
that case, you would probably want to try a .5 ampere slow blow fuse,
since it will blow in about 10 seconds with twice its rated current
passing. But if it blows after a minute or more, you will have to go
down in rating, and if it blows in a couple seconds, you will have to
step up.

 

I'm afraid to check the new doorbell since I don't know how long I
have to check it with current before it melts like the old one.

Out ot curiosity, I manually stuck the button on and tested the old
doorbell that has one melted stuck plunger, and it first shows 3.2A,
but the longer you hold the DMM on it, the lower it goes (I never held
longer than ~10 seconds). If you wait several seconds and check it
again, it's usually around 2.9A.

 

Sounds do-able for a novice like me. Could you tell me more details
on parts and where to put them, or draw a basic schematic for this?

Thanks

 

I am worried that not only the coil form was damaged, but some of the
insulation between adjacent turns. The drop in current you are seeing
is what is expected as the coil warms up, because of the positive
temperature coefficient of copper (in both the coil and transformer).

That current sounds high to me, but like I said, many doorbells are
powered by intermittent rated components, so this may be what the new
coil draws, too. You should be able to power it just long enough to
take a current reading without damaging it.

 

Yes, and I got several different suggestions. I wasn't sure which one
to do. After thinking about it (and being busy), I decided the fuse
would probably be the easiest for me. If I had more details on the
other suggestions, I might could handle one of them, which would keep
me from trying to test several fuses to find the correct rating, then
have to replace fuses if it ever happens again. But if that's all I
can do, I'm satisfied with it.

 

Hooked up new doorbell (still using original transformer) and took
some readings.
It also first showed as 3.2A. Next time it started as 3.14A, and got
down to 2.99A in 10 seconds. If it's dropping this quick, I would
need a fuse lower than this wouldn't I?
The voltage is showing about 13.5V (the new bell calls for a 16V 10W
transformer, old bell was 16V also I think).

Should I fuse it, or use a cap & resistor as someone suggested?