IMT 140

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IMT 140

• Chapter 16
• Typical Bell Circuits

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Bell Circuits

• Nearly every electrical installation includes
  some type of signaling circuit.

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Bell Circuit Rules

• It is easy to wire simple low-voltage bell
  circuits if 3 rules are followed.
• 1. Connect a conductor from 1 side of the
  voltage source to the bell.
• 2. Connect a conductor from the other side of
  the voltage source to the control point (or
  pushbutton).
• 3. Connect a conductor from the pushbutton to
  the bell which is to be controlled.

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Bell Circuits

• Figure 16-1 shows a bell controlled from 1
  pushbutton using a transformer as a source of
  power.

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What type circuit is this?
This is a simple series circuit.
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What does the transformer do in this circuit?
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Bell Buzzer Circuits

• A bell may be controlled from more than 1
  location.
• The same 3 rules apply to these applications.
• These pushbuttons will be in parallel and the
  pushbutton combination will be in series with the
  bell.

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Bell Buzzer Circuits

• Several signaling devices, each controlled with a
  separate pushbutton, may be used on the same bell
  transformer.
• However, to distinguish between the different
  devices, it is common use different tones for
  each device.
• (Most homes use different tones for the front
  doorbell and the rear doorbell.).

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What type circuit is this?
This is a Series-Parallel Circuit.
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Bell Buzzer Circuits

• Combination units containing a bell and buzzer in
  a single housing are commonly used.
• This type of signaling device has 3 terminals.
  (See figure 16-4).
• The terminals are connected as follows
• Center terminal to the transformer.
• Right-hand terminal to the front door pushbutton.
• Left-hand terminal to the back door pushbutton.

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Pushbuttons

• Pushbuttons are used to open and close a bell
  circuit.
• When the button is depressed, the circuit is
  closed.
• This is basically a single pole, normally open
  switch.

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Doorbells

• Figure 16-5 is a typical doorbell.
• Two small coils of insulated wire mounted on iron
  cores form an electromagnet.
• Current passes from terminal A to E by the way of
  the electromagnets, contact points B C and the
  armature.

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Doorbells

• When the circuit is energized, the 2 coils become
  electromagnets and attract the armature toward
  the iron cores. This causes
• The hammer to strike the gong and at the same
  instant
• Causes contacts B C to separate by the action
  of the moving armature.

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Doorbells

• The circuit is now open and the coils no longer
  attract the armature.
• The cycle of operation is very rapid which causes
  the armature, contact spring and the hammer to
  vibrate rapidly.

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Adjustment screw
Coils and Iron Cores
Armature
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Buzzers may be used with bells to distinguish
between which pushbutton was activated.
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This coil is for the bell And this coil is
for the buzzer.
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Door Chimes

• Many homes use chimes rather than a door
  bell/buzzer.
• These are available in many tones and can be
  designed for multiple entrances.

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Bell Transformer

• Chime transformers are generally larger in
  capacity than bell or buzzer transformers.
• Voltages for chimes range from 10 to 24 volts
  with a rating of 5 to 20 volt-amperes.
• Bell transformers are generally rated at 6 to 10
  volts with a rating of 5 to 20 volt-amperes.

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Bell Wire

• Number 18 AWG is generally used for bell wire.
• Single or multiple conductor wire is available.
• Special staples are used with bell wire.

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National Electric Code

• In general, bell wires with low voltage
  insulation should not be installed in the same
  raceway or enclosure with lighting or power
  conductors.
• Check Article 725 of the NEC for further details.

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The End!!