# little trouble calculating lm2917

#### FerryKoole

Joined Jul 19, 2014
4
Hi,

As I understand the basics of a LM2917/LM2907 I know that I can calculate Vo/Fin. There are some 8pin examples in de datasheet which I can reproduce.

The trouble I have is applying the same formula (Vo = Fin x Vcc x R1 x C1) to the 14pin examples in the datasheet.
As I calculate the data from attached picture I get 9,6V instead of the mentioned 6V. What am I missing here?

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#### FerryKoole

Joined Jul 19, 2014
4
Can it have anything to do with the build-in zener of 7,5v?

#### gagwd

Joined Aug 2, 2014
6
Do not know how you got 9.6?
I used 7.5V * 400Hz * 100K * .02uF = 6V (assuming a K=1.0)

Interesting that the calculation given by TI would assume the pot completely to one extreme. Typically one chooses the pot setting nominal at 50% but that yields an output of 3V.

Take a look at Figure 21. There is shows .02u, 100K with zener regulation and shows the output to be 66Hz/V. There with an output of 6V would then mean 396 Hz. Divide by 8 to get single cylinder firings per second of 49.5. In a 4 cycle engine that is 2 rotations per firing or 99 RPS. Then times 60 gives 5940 RPM.

Does that help?

#### FerryKoole

Joined Jul 19, 2014
4
Yes this helps. I calculated 9.6 with the voltage of the car battery. Later on, I figured the zener out

#### FerryKoole

Joined Jul 19, 2014
4
I now made this schematic in real life, calculated R1 and C1 (120k and 0,047u) and hooked it up to my motorbike's coil wire. It now only responds a little when revving the engine; just 6mV instead of the expected 350mV or so.

Did I make an error in calculating? I figured 7,5 x 60 x 120000 x 0,000000047 = 2,3V and thats what I need... Revving the engine would put out 0,5V or so.

Can anyone help me on this?

Edit: When I simulate the input with my hand connecting/disconnecting to the 12V lead then it works fine. I cannot see the difference between this and the input for the coil...

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