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LM2917-N automotive tachometer, breaker point style

mike knisley
Prodigy 20 points

Hello,

I am trying to build an 8 cylinder, breaker point automotive tachometer based on info of document SNAS555C, page11. My goal was to have 2.5 VDC output at max 533Hz. I have wired it for the input at pin 1 and 11 as the document shows using the exact values of components shown. I used 0.02uF at pin 2.  31K ohm // to 1.0uF at pins 3 with 4.

It all seems to work as expected at frequencies below about 400Hz, but much above that,  the output voltage measured at pins3,4  decreases.

V should be 7.56 x.02E-6 x 31E3 x 533Hz = 2.498

I don't know what I'm missing and any help would be much appreciated!

Thanks

  • 0
    TI__Expert 3920 points

    Hi,

    Have you made sure you are within the max attainable frequency?

    Fmax = I2 / (C1 x VCC)

    This equation is found under the section "CHOOSING R1 AND C1"

    Tim

  • 0 in reply to Tim Reyes
    Prodigy 20 points
    Hi Tim,

    Thanks for your response.

    The circuit I have built is almost exactly as in Fig. 23 on page 11 of SNAS555C. The only differences
    are, I have fixed 31Kohm for R1 at pins 3,4 and my output resistance at pin 5 to ground from the floating
    transistor is 5Kohm.

    C1 is .02uF, R1 is 31Kohm, C2 is 1.0uF

    I would think I2,I3min at worst case -45C would be around 125uA with Vcc 7.56V from internal zener regulator,
    according to Fig.13.

    So

    Fmax = 125E-6/(0.02E-6 x 7.56) = 826.7Hz (All I want is Fmax 533 Hz)

    Also I have made sure that R1 >= V3max/I3min (2.5/125E-6 = 20Kohm)

    So I don't know what Is wrong

    Regards

    Mike