opto isolator Ring detector circuit transistors not fully activating


Posted on Feb 7, 2014

A ringer interface circuit, and I`m running into problems that are over my head. The purpose of the circuit is to buffer the output of a central telephone system that goes to several ringers spaced throughout the building. The reason for the circuit is that whenever the line rings, it overloads and needs to be reset, thusthis circuit will buffer the line and offer a lot more output capability.


opto isolator Ring detector circuit transistors not fully activating
Click here to download the full size of the above Circuit.

Now, my problem is this: when everything is on, and the line is ringing (4N27 triggered), I only get 12V out, instead of 24. I went through and checked voltages, and it seems like none of the transistors are turning on (or off as the case may be) completely. When I tie the base resistor of the first (closest to the 4N27) signal transistor to ground (turning it 100% off), I then get the correct 24V out. The text on your schematic is so small it`s hard to read, but it looks like the output of the opto is being pulled up to 12V. Why do you expect 24V there Actually it should be considerably less due to that node being loaded thru R3 and the B-E junction of left transistor (Q1 it`s hard to read). Olin Lathrop Jan 10 `13 at 19:01 I meant the output from the TIP32 power transistor at the end, sorry for the confusion. Also, I will try and upload a better picture. thanks! Scott Jan 10 `13 at 19:08 It would really make a whole lot more sense if you were to use a PNP at Q1 instead of an NPN. Take advantage of the gain of the transistor to reduce the current that the opto needs to sink. Dave Tweed™ Jan 10 `13 at 19:48 So, in that case, would that mean that since the power transistor is PNP, I could just hook that up instead of Q1, and do away with Q1 & Q2 thanks for the help! Scott Jan 10 `13 at 20:18




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