64 December 2017
>>>YOUR ELECTRONICS QUESTIONS ANSWERED HERE BY N&V READERS
pulling current for more than (let’s say)
five seconds. I figure a “slow-blow”
fuse would be ideal, but not sure on
2) More complex protection — A
“time out” circuit that would cut
power to the chimes if the circuit was
live for more than five seconds, self-resetting either after the button was
released or after 60 seconds (chime
would activate again, indicating a
#1 The simplest (though
embarrassingly low-tech) method is
to put a power resistor (a few ohms,
five watts) in series with a normally-closed thermal switch (opening about
50°C) in one leg of the line from the
transformer. Epoxy the resistor to the
switch, wrap with a bit of fiberglass
pipe insulation, and tuck it into a
As current flows, the resistor
drops a few volts and gets warm,
opening the switch. The thermal
insulation keeps it from cooling off
too quickly, so the power doesn’t
cycle on as often. Each time the
switch resets, the chimes will remind
you to fix the button. Check the
current draw of the chimes, and select
a resistor that drops two or three volts
— enough power to get warm; not
enough to interfere with the chimes.
Use any “junkbox” parts on hand,
as values are not critical. BTW, keep
the switch assembly inside where it
won’t get too cold to shut off.
#2 The simple answer is a 1.25
amp fuse. I suspect, however, the
more complete answer is that the
inductive “kick” from the two chime
coils is welding the pushbutton
Weather usually makes switches
fail open, not shorted. To prevent the
welding, you need a surge suppressor
across the chime coils. A suitable part
is the Cornell-Dubilier Quencharc #
104M06QC- 22. Available at Allied,
Digi-Key, and a bunch of other places.
If money is critical, just one
should do. If you can afford it, put
one across each chime coil.
#3 It seems to me you don’t need
a circuit. If the switch contacts fuse
and short the circuit, your problem
is current flow. Too much and the
contacts arc, fusing the contacts. You
need to add some resistance to the
circuit with the contacts.
I would start with 50 ohms.
Just tie in series with one leg of the
switch. If the doorbell still rings, go
to 75 ohms; one watt should suffice.
You can go to 100 ohms if it still rings.
It really doesn’t take a lot of current
to ring that doorbell. What we want
to do is limit it, to protect the contacts
in the switch. If we put a timing circuit
in, it will only start ringing again after
the time-out. With 75 or 100 ohms,
you're going to cut down that current.
#4 The best approach: Scrap your
chimes and buttons and buy a set of
wireless chimes and weatherproof
buttons. No wires, no transformer, no
fuse, no time-out circuit. Amazon lists
several types for under $30.