Telephone Ring Detector circuit

Discussion in 'The Projects Forum' started by yong, Aug 3, 2008.

  1. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    Hi,

    Can anybody help to depict the circuit in below link. Refer to page 9.
    I would like to understand the purpose of each component, the technical aspect from the AC to DC signal and how the output relay is actually trigger, the purpose of R5 and C3 connected in parallel with Pin 5 and 4 of the phototransisitor of 4n27.Can someone please enlighten. Thanks in advance.

    http://www.vellemanusa.com/downloads/0/illustrated/illustrated_assembly_manual_k8086.pdf
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The ring-signal are pulses of 90-120 Volts.
    The signal is put back to proprotion for the 4n27.
    On pins 4-5 of the 4n27 the pulses will show up.
    This will let the relays attract and fall off again.
    R5 and C3 will hold the signal a little bit longer so the relays will not flap.

    Greetings,
    Bertus
     
  3. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    Thanks Bertus

    For R5 and C3, you mention that the presence of theses components is to allow the relay to stay in the continuous position after the first toggle. Are you able to explain in a technical way?Current will flow from base of PNP T1 right down and split between 2 paths using KCL concept.I believe current flowing thru R5 will charge up C3 but I am not sure how this will actually prevent the relay from flapping.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
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    Hello,

    This is what happens at a ringpulse :
    The ringpulse will let the LED of the 4n27 blink.
    The transistor if the 4n27 will conduct.
    When R5 and C3 are connected, C3 will be discharged over R3 and the transistor of the 4n27.
    When there is no ringpulse the C3 will be charged by R3,R4 and R5 (the timeconstant is much higher).
    At the next ringpulse C3 is discharged again and so on.
    When no ringpulses are coming anymore, C3 will be fully charged and the relays will fall off afther some time.

    Greetings,
    Bertus
     
  5. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    Any idea what is the 1 Meg ohm resistor connected in parallel with C1 for?

    I need help on how to derive the calculation for the values.....anybody can help?
     
  6. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    Can someone pls tell me what is the 1 meg resistor for?
     
  7. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Anti-static? Wave shaping?
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The 1M ohm resistor is to discharge the capacitor, so you will not get shocked when pulling the circuits from the line.

    Greetings,
    Bertus
     
  9. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    Any idea the reason for using such a large value resistor?
     
  10. bertus

    Administrator

    Apr 5, 2008
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    2,344
    Hello,

    The capacitor is a resistor for AC without the dissipation.
    The resistor will dissipate heat if the value is much lower.
    The resistor used across the capacitor is called "bleeder resistor" and is placed there for the safety.

    Here is the complete schematic :

    [​IMG]

    Greetings,
    Bertus
     
  11. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    How do we calculate the heat dissipation of the resistor. I would like to prove this theory using mathematical calculation.
     
  12. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    Can i also assume the cap will discharge slower if a large resistance is used here?Becos limited current is flowing thru resistor in this case.Therefore disspate less heat.
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    The capacitor will not disipate becouse the current and voltage are 90 ° out of phase.

    To calculate the power used by the resistor you can use the folloeing formulas:

    P = Vrms * Irms.
    P = Vrms * Vrms / R
    P = Irms * Irms * R.

    Here is the page for DC equations.
    http://www.allaboutcircuits.com/vol_5/chpt_1/1.html

    For AC you must use the effective values (rms) to calculate the power.

    If you want to know more theory take a look at the cource notes from this page:
    http://technology.niagarac.on.ca/courses/etec1120/notes.htm

    Greetings,
    Bertus
     
    Last edited: Feb 13, 2009
  14. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    any idea on the AC ringing current. what is the standard specification of AC ringing current?
     
  15. bertus

    Administrator

    Apr 5, 2008
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  16. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    I was jus wondering if since we are already using a 12VDC adapter, do we still need the peak rectifier circuit of D2 C2 and R4? I am not sure if that serves any purpose, correct me if I am wrong.
     
  17. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The diode D2 is placed there to protect the circuit against the wrong polarity of the power supply.

    Greetings,
    Bertus
     
  18. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    HOw about C2 and R4?
     
  19. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    C2 is a decoupling capacitor (normaly done afther a "polarity safety" diode).
    R4 pulls the base of the transistor T1 high when no signal is present.

    Greetings,
    Bertus
     
  20. yong

    Thread Starter Member

    Feb 23, 2008
    27
    0
    Just trying to understand, when there is no ring pulse and when c3 is fully charge i assume no current flows from the base of T1. i dun understand how did the relay fallout in this case. can explain the rationale behind?
     
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