LM2917 frequency to voltage regulator

[labview1958]

On page 12 of the national semiconductor LM2907/2917 datasheet there is a typical application of converting frequency to voltage. Why the need for the external 0.01uF and 0.1uF capacitors?

[labview1958]

Here is the link.

http://cache.national.com/ds/LM/LM2907.pdf

I am having some problem with my acrobat reader.

[dyarker]

cuz they said so! It should be no surprise that a charge pump needs capacitors.

[Robert Reed]

Labview
If you decide to go with the LM2917-be avised that this chip loves square waves. It will only work properly with duty cycles of 25% to 75%. The data sheets don't tell you this, I had to find out the hard way.

[labview1958]

I have a rotating disc with 10% reflective surface and 90% absorbtiive surface and another disk with 90% reflective surface and 10% absorbtive surface. If I rotate both at 1000 rpm would the voltage output be the same?

[JPKNHTP]

-JPKNHTP

[labview1958]

What I meant was should the voltage vary with the surface area of the reflective portion of the disk or just depends on the frequency only?

[Robert Reed]

Voltage? what voltage? Are you taking average output from the photocell or what? The capacitors are part of the charge pump and determine ripple at the lowest frequency you expect to encounter. The only thing the 2917 cares about is the output FREQUENCY of the optics. Thats not your real problem here because as I mentioned before about proper duty cycle. Your duty cycle on either disc is way out of bounds for the 2917. You need to pay more attention to the answers you receive on this forum.

[labview1958]

Actually I have a few disk that are painted matt black. Each disk has a reflective aluminium tape attached to it. However the width of the tapes are not the same. Some are wide while others are narrow. I have already calibrated the rpm using a disk with a narrow tape. Can I keep the same calibration for a disk with a wider tape?

[dacflyer]

sounds interesting,
can this chip me made to make a electronic governer for a generator that has no governor?

like interface this with a servo ?
and the pickup sensor just count shaft rpms? or what would be best to do ?
i gave up on a project long ago because i never could find a way to make a servo be used to control the throttle of a small gas engine generator.. its still sitting in the shed collecting dust..

[JPKNHTP]

-JPKNHTP

[labview1958]

"Actually I have a few disk that are painted matt black. Each disk has a reflective aluminium tape attached to it. However the width of the tapes are not the same. Some are wide while others are narrow. I have already calibrated the rpm using a disk with a narrow tape. Can I keep the same calibration for a disk with a wider tape?"

Any coments on my post?

[JPKNHTP]

-JPKNHTP

[dyarker]

Add a flip-flop (divide by 2) between the Schmitt Trigger and the 2917. Then the duty cycle to the 2917 will be 50% at constant RPM, and 40% to 60% during moderate acceleration/deceleration.

If a disc currently has one reflective tape, add a second reflective tape 180° from the first. The 2917 will see the same frequency as before the insertion of the flip-flop.

[Robert Reed]

As Dyarker mentioned, a bi-stable (flip-flop) can be inserted between the optics and the 2917. You will want this device to toggle, thereby giving you a perfect square wave throughout the whole frequency range, This can be done with a "D" type of bistable. Just connect the ' NOT Q' output back to the data input. However there is a tradeoff here as you have now cut your input ferquency in half.This presents no problem for calibration but will require more filtering on the 2917 as now it has to handle a lower bottom frequency. A better solution is this. I am assuming you have made up the disc yoursef from the description. Why not retape this with equal dark and reflective portions. This would be acheived by the spacing and width of your tape. Now you will have a much more balanced square wave directly from the optics.