summersnow Posted January 30, 2007 Report Share Posted January 30, 2007 Hi all..I have a problem here.i tried to build a ringing detection circuit.But when i connect the ring and tip of the telephone cable to this circuit,i found out that it automatically being off hook.As a result,it cant be used to detect the ringing signal and it get engaged when i call to the telephone.Is there anyone who can explain to me why it get off hook even there are no ringing signal?Thanks in advance Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 30, 2007 Report Share Posted January 30, 2007 The telephone ringing signal in North America is 90VAC. Your zener diodes are only 3.3V and the 330 ohm resistor creates a huge current with a peak of 370mA. I am amazed that your opto-isolator didn't burn out.Look at all the ringing detector circuits in Google that use 15V zener diodes and a 10k resistor like this one from a modem: Quote Link to comment Share on other sites More sharing options...
summersnow Posted January 30, 2007 Author Report Share Posted January 30, 2007 hi audioguru..thanks for reminding..actually,i write wrongly in the circuit..i'm using 16V zener diode..anyway,after i change the resistor to 10kohm,the ringing detector circuit works..Thanks for your help Quote Link to comment Share on other sites More sharing options...
walid Posted February 1, 2007 Report Share Posted February 1, 2007 Hi guruNow I'm have a question:in your circuit shown below:why using two back to back 15v zeners. What they exactly do?thank you Quote Link to comment Share on other sites More sharing options...
audioguru Posted February 1, 2007 Report Share Posted February 1, 2007 why using two back to back 15v zeners. What they exactly do?They block signals that have a peak-to-peak voltage of less than 15.6V such as tones and connection clicks. Then the ringer doesn't falsely ring.The ringing signal in Canada and in the US is 90V RMS which is 255V peak-to-peak. Quote Link to comment Share on other sites More sharing options...
walid Posted February 2, 2007 Report Share Posted February 2, 2007 Hi guruok thank you i understand it.another question: why D3? Quote Link to comment Share on other sites More sharing options...
summersnow Posted February 2, 2007 Author Report Share Posted February 2, 2007 hi..actually the D3 is to limits the voltage drop across the LED of the optoisolator to protect it from negative voltages,therefore avoid any possible damage.. Quote Link to comment Share on other sites More sharing options...
walid Posted February 2, 2007 Report Share Posted February 2, 2007 Hi summersnow thank u very much for your replycan u please explain it more with numerical example. Quote Link to comment Share on other sites More sharing options...
audioguru Posted February 2, 2007 Report Share Posted February 2, 2007 Nearly every LED has a max reverse voltage rating of only 5V. The ringing signal has a peak voltage of 127V so the diode conducts and protects the LED. Quote Link to comment Share on other sites More sharing options...
mvs sarma Posted February 2, 2007 Report Share Posted February 2, 2007 Hi guruNow I'm have a question:in your circuit shown below:why using two back to back 15v zeners. What they exactly do?thank you Hi we can also use a DIAC (conducts at 32 volts in each diection) and this will reduce comonent count. component is ST micro DB xx Quote Link to comment Share on other sites More sharing options...
walid Posted February 2, 2007 Report Share Posted February 2, 2007 Hi guruNearly every LED has a max reverse voltage rating of only 5V. It is a good point for me to put in my memory. But there remains a question: whether such a diode (4148) can bears this 127v ?Hi mvs sarma we can also use a DIAC (conducts at 32 volts in each diection) This point is excellent and wonderful, thank you Quote Link to comment Share on other sites More sharing options...
audioguru Posted February 2, 2007 Report Share Posted February 2, 2007 The protection diode conducts when its voltage becomes higher than only 0.6V. Then when the AC ringing voltage is reversed across the protection diode then the LED conducts and limits the voltage to only about 1.5V. So the 1N4148 has a max voltage during ringing of only 1.5V. Quote Link to comment Share on other sites More sharing options...
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