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Keep Blowing lm13700's

Discussion in 'General Electronics Discussion' started by grahamdavidmoore, Jun 25, 2014.

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  1. grahamdavidmoore

    grahamdavidmoore

    9
    0
    Jun 23, 2014
    hi. I'm experimenting with using the lm13700 as a voltage controlled oscillator, using the data sheet i can get the vco circuit working ok but i'm trying to get it to work with an exponential generator circuit. I'm using bc337 transistors and tl072 op amps in the circuit for the expo generator but when I connect the osc output to pin 1 on the lm13700 I seem to blow it and it no longer oscillates, what am i doing wrong?
     

    Attached Files:

  2. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    Do you add a serie resistor on the OSC out?
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
    It won't work because the LM13700 requires a current source (that sources current into pin 1) and that circuit is a current sink (sinks current to 0V). The LM13700's control input on pin 1 goes straight to the base of a transistor (IIRC) whose emitter is connected to the negative supply rail. You can't feed unlimited current into that pin; if you do, you'll damage that transistor at least.

    With that circuit, the collector of the output transistor can't go more than about 0.7V below the 0V rail because of the collector-base diode. So when you connect the transistor's collector to pin 1, heavy current will flow from the 0V rail, into the base, through the collector-base diode, out the collector, into pin 1 of the LM13700, through the base-emitter junction of the input transistor, and out to the negative supply rail.

    Two diode junctions in series can't have more than about 1.4V across them without blowing something up. If the negative supply is -15V, that's what's going to happen!

    You need to change the exponential current generator so it sources current. Where did that circuit come from?
     
    OLIVE2222 likes this.
  4. grahamdavidmoore

    grahamdavidmoore

    9
    0
    Jun 23, 2014
    Circuit was from here, see below, thought I could substitute the oscillator with a LM13700 vco, is it possible alter the expo generator to work as a current source instead of a sink? thanks for the replies.
     
  5. grahamdavidmoore

    grahamdavidmoore

    9
    0
    Jun 23, 2014
    original circuit
     

    Attached Files:

  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,267
    Nov 28, 2011
  7. grahamdavidmoore

    grahamdavidmoore

    9
    0
    Jun 23, 2014
    no, that circuit works very well, but is beyond my understanding of how he uses the lm13700 and 555 together so i wanted to experiment myself by just using the data sheet to start me off using a basic lm13700 vco and a simpler expo generator and make a simpler vco to learn a bit doing it myself rather than just copy a design, problem is i dont really know enough as you can tell lol, thanks for your help, its much appreciated.
     
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