Hi

I'm having a lot of problems understanding the calculations on the datasheet for the LM1819 Air-Core Meter Driver.

I have the circuit off a 8 cylinder tachometer that goes to 5000 RPM that works. I am trying to adapt it to fit a 6 cylinder that goes to 8000 RPM.

The equations on the data sheet for calculating R2 and C2 look like a foreign language to me.

I know that the meter will move though 250 degree, pointer deflection is 3 degree, frequency at 8000 RPM is 400Hz.

I've been scratching my head for 3 days trying to work it out and even excel has gotten confused.

Thank you

 

Do you already have a meter that you want to use? Do you already have that IC? The LM2917 and LM2907 are very good tachometer devices. And if the pointer on your meter can go 270 degrees then I doubt very much that it is an air core meter.
For some actual values get hold of the application notes for the IC.

 

Do you already have a meter that you want to use? Do you already have that IC? The LM2917 and LM2907 are very good tachometer devices. And if the pointer on your meter can go 270 degrees then I doubt very much that it is an air core meter.
For some actual values get hold of the application notes for the IC.

Hi

Yes I already have the motor and PCB with the LM1819 on it. I know that if I adjust the values of R2 and C2 that the circuit works better for what i need but can't quite get it right.

 

That foreign language is called algebra. There is no higher math needed.

For starters, there are equations for two modes of operation. Do you know which your meter is using?

Have you tried going through the calculation for the 6000 RPM engine to see if you are understanding them correctly? That is what I would do first.


Bob

 

Hi Bob

The circuit I have matches the circuit for the typical application of a Automotive Tachometer on the front page of the datasheet. I have tried using the datasheet values to figure out whats happening but keep getting different values.

 

Well, first you need to find out which of the two sets if equations to use. I can’t tell you that.

Bob

 

It is the constant duty cycle one

 

I have the circuit off a 8 cylinder tachometer that goes to 5000 RPM that works.

Does you circuit monitor crank-shaft revolutions/min, or sparks/min from one cylinder or sparks/min from all 8 cylinders?

 

Ok, you are looking for what changes if you change the number of cylinders (n) or the top RPM (ω).

These appear in the equation for Rz. There is no Rz in the circuit that I can find, so, I think this is a typo and it means R2.

Note that R2 is inverseproportional to both n and ω. So, you had 8 and 6000, and you want to change it to 6 and 8000. Which should work the way it is, since the two products are the same.

Edit: change directly to inversely proportional.
Bob

 

It measures sparks from all 8 cylinders.

 

So looking at that equation, pointer deflection at redline is 250 degree.

so get 250/3.54*6*8000*0.01 = R2 which =33898

Don't know if im doing the maths correct

 

It measures sparks from all 8 cylinders.

In that case the number of cylinders is relevant to the calculation.

 

I cannot determine what is in the numerator and the denominator. Each time I look at it I get a different idea. And I can’t get a reasonable answer either way.

Your calculation is what I get if I put everything but the 3.54 in the numerator and use the capacitor value in uF, which I can’t justify. And it is off by a factor of 10 from what the schematic shows. So, I feel your pain.

Bob

 

I've tried the calculation for C2 and got 1.5uF but the R2 calculation doesn't give a reasonable answer.

I've now tried R2 as 330k with C2 as 1.5uF and it's looking promising

 

Given that the circuit is getting either 3 pulses per revolution from a six, or four pulses per rev from an eight, the simple solution is to use an accurate oscillator/frequency counter and an adjustable resistor, after determining the frequency for half and full scale readings. Then set the oscillator and adjust the resistor to get the desired reading.