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LM2907-N: NPN flow sensor circuit for LM2907-N

Alex Necker
Prodigy 20 points
Part Number: LM2907-N

Hello,

I want to convert the frequency signal from this flow sensor:

to a voltage from 0-10V

The input voltage is 12V DC

The max frequency is 80Hz the normal operating frequenc is 48,85Hz

At the moment i put an Pull-Up Resistor parallel to the NPN to make a PNP. The Amplitude Voltage is max 9,2V

Can someone help me, i don`t know how the circuit design for the input at pin 1, must look like with an open collector (or the PNP)? And how i can calculate then R1, C1 and C2 or other needed resistors?

I calculatet for the regular minimum component tacometer:

R1 = 100K Ohm
C1 = 0,1µF
C2 = 10µF

i get an Vout of 9,6V at 80Hz.

Thanks to everyone!

  • 0
    TI__Guru* 93105 points

    Hello Alex,

    Pin 1 the TACH+ input of the LM2907-N is the input to an analog comparator. The other input to the comparator is the TACH- input. If the 8-pin package LM2907-N is used that input is connected to the GND potential, usually 0 V. Any voltage comparison made at the TACH+ input will be with respect to the GND potential on the TACH- input. If the 14-pin LM2907-N package is used, the voltage applied to the TACH+ input will be compared to whatever reference level is being applied to TACH- input, pin 11. 

    Therefore, if you are driving the TACH+ pin-1 input with the digital output from the flow sensor you must make sure that its output range swings across the reference level you have applied to the TACH- input. The 8-pin device would require the TACH+ input to swing to 0 V, or maybe even a little negative to overcome the ~30 mV comparator hysteresis. The 14-pin device TACH+ input would need to swing through the dc reference level set on TACH- input. Datasheet Figures 20 through 22 show the addition of a series resistor connected from Vcc (B+) to a silicon diode, which is connected from the TACH- pin to GND. That establishes a reference voltage of about +600 to +700 mV on the pin. Any voltage within the comparator's common-mode voltage range can be used for the TACH- reference level. 

    The LM2907-N datasheet and accompanying applications report (AN-162) provide much information about selecting the R1, C1 and C2 component values. In particular, see Page 4 of AN-162. If you are still having problems determining the component values, let us know.

    www.ti.com/.../snaa088.pdf

    Regards, Thomas

    Precision Amplifiers Applications Engineering 

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Sorry I forgott to note that I am using the 8 pin version.
    The signal of the flow sensor swings to 0V , I measured it with a oscilloscope.

    Okay, that means I must only connect pin-8 to ground and the Frequenz signal direct to pin-1.

    In this case I can use my calculations for R1, C1 and C2.

    Thank you Thomas Kuehl