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AN-263 - Sine Wave Generator - Logarithmic Shaping - Error in App Note?

Ian Mac
Prodigy 10 points
Other Parts Discussed in Thread: LF356, LM311

I am trying to build and modify the Sine Wave Generator circuit in AN263, Figure 14, albeit with what I have on my bench (TL071s in place of LF356 and LF351, and discrete transistors in place of LM394 and 2N3810) but the circuit description is either unclear or I am doing something incorrect, or the circuit may have an error? 

The 1.2K resistor between the output of the LM311 and VCC (+15V) has 30V across it (Pin 1 of the LM311 is at -15V as per the * note, IN- is at GND) and as such it gets scorching hot. Is this the correct resistor value in this location?!?

Thanks in advance! LM311

  • +1
    TI__Guru**** 167546 points

    Hello Ian,

    LM311 being scorching hot is a sign that something is wrong. The LM311 may be broken at this point.

    It is fine for the collector output of LM311 (pin 7) to be -15V, that just means the IN+ input is negative. The output current is fine; 30V /1.2k = 25mA. The LM311 will not get hot, but the 1.2k ohm resistor should be quite warm to very hot. The resistor dissipates up to 750mW (use a 1W or greater for this resistor)

    Pull the LM311 out then measure all the resistances of all the pins (where LM311 used to be) to ground with power off. With power on, measure all the pin voltages where the LM311 used to be. 

    If you find an issue and fix it, use a new LM311.

  • +1 in reply to Ron Michallick
    TI__Mastermind 26460 points

    Ian

    Just chiming in to see if I can help but I assume the output should not be stuck in the low output state.  As Ron mentioned, much power is being sunk into COL OUT.  I am pretty sure it was not meant to be sinking this amount of current constantly and could damage the device, not to mention the resistor if not the proper wattage.  I am not familiar with the AN but it may be best to increase this value10x initially just to make sure your circuit is connected correctly.  this will at least limit the current to a more reasonable value as the circuit is debugged.

    Chuck

  • 0 in reply to Ron Michallick
    Prodigy 10 points

    Ron, Thanks for your reply! I'm sorry if my original question was unclear, it is the resistor that gets hot, not the LM311. For the time being I have replaced it with a higher value resistor and the circuit runs as it should. Cheers!

  • 0 in reply to Chuck Sins
    Prodigy 10 points

    Chuck, Yes, I had two problems, first, an error kept the circuit from running (VCC- on pin 4 was disconnected) and the resistor was getting too hot, replaced the resistor and all is well now. Cheers!

  • 0 in reply to Ian Mac
    TI__Mastermind 26460 points

    Excellent news. Thanks Ian for following up.  And thanks Ron for helping address.

    Chuck