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LM2917 capacitor value for tachometer

Nicolas sakvay
Prodigy 30 points

hello, I'm trying to do the circuit on figure 14 of the datasheet in the next link,

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.ti.com/lit/an/snaa088/snaa088.pdf&ved=2ahUKEwjdlKfYmM7jAhVjTt8KHTaGAAgQFjAMegQIBxAN&usg=AOvVaw2hSrnEgNSdTr0TmFzMHGvy

I need to know what value of capacitor is needed at the connection with the negative side of the ignition coil.

  • 0
    TI__Expert 7540 points

    Hi Nicolas,

    The link you included doesn't appear to be valid on my end. Could you confirm the link address?

  • 0 in reply to Alex Davis
    Prodigy 30 points

    http://www.ti.com/lit/pdf/SNAA088?jktype=tech_docs

     

    AN-162 LM2907 Tachometer/Speed Switch Building Block Applications (0.34 MB)

    Application notes

  • 0 in reply to Nicolas sakvay
    Prodigy 30 points

    now the link works.

  • 0 in reply to Nicolas sakvay
    TI__Guru* 93105 points

    Hello Nicolas,

    Sorry this didn't get addressed sooner. I was on international travel when it came in and I didn't see it until it was directed to my attention.

    None of the LM2907/LM2917 literature provides any hint of what that capacitor value should be for the gasoline engine tachometer circuit; however, I suspect its value is not particularly critical. You will note in the basic tachometer circuits the magnetic pickup directly drives these F-to-V converters so in some cases a capacitor isn't even being used. So it may not be necessarily needed in all cases.

    Figure 14 shows the LM2907 in the gasoline engine tachometer application. The LM2907 requires that the input pulse stream coming from the pickup cross below 0 V so that the input comparator trips and changes state. Adding the capacitor between the pickup and LM2907 input removes any dc component from the input pulse stream and centers it about 0 V. Therefore, the input stream will move above and below 0 V and the comparator will change state in response to the input pulse stream. That may be why the capacitor is being added.

    Since the lowest idle speed coming from the distributor needs to be detected it appears that making the capacitor reasonably large in value would be necessary. The Figure 14 circuit has the 10 k/(20 k||20 nF) divider at the pin 1 input. Based on a low idle frequency of a few hundred Hertz, the capacitor that drives the divider should be something on the order of 47 nF, or 100 nF. That may be larger than necessary, but it would be a good starting place.

    If you use the LM2917 one of the input comparator's inputs can be biased above 0 V. That is usually accomplished by connecting a silicon diode to pin 11 and forwarding biasing it. That moves the compartor voltage from 0 V up to about +0.7 V. The capacitor could still be used in this case, but the input voltage would have to pass through the +0.7 V level to trip the comparator.

    Regards, Thomas

    Precision Amplifiers Applications Engineering