Re: V/F converter: Lm331


DaveM wrote:


I don't know which data*** you're looking at, but the one I downloaded from
national.com has some great examples of a voltage-frequency converter, and
includes the design equations. Specifically what are you having trouble with?
You might post a copy of your schematic here (ASCII art) or on
alt.binaries.schematics.electronic (PDF/GIF/JPG images). You'll have a much
better chance of getting good advice if we know what your circuit is.


Hi.
First of all, thank you for your interest, I'm in real trouble and
knowing that out there someone has some knowledge of what I'm dealing
with makes me feel really better :-)
I downloaded this data***: http://www.national.com/ds/LM/LM231.pdf
And this AN: www.itiomar.it/pubblica/Telecomunicaz/
applic_notes/an_LM331-6.pdf

I chose to try the schematic on page 2 of the AN ("Precision Relaxation
Oscillator") because it seemed quite clear, simple and made up of few
components.

Now, I wanted something which could go from about 0 to a thousand Hz,
so I chose

1V = 220Hz (good old A2 :-)
5V = 1100 Hz
Then, I could determine the scale factor K in F = KVi as 220.

The AN say fout = Vi/Vr * Rs/Rl * 1/1,1*Rt*Ct
So, fout = Vi * K, then K = 1/Vr*Rs/Rl*1/1,1*Rt*Ct
The AN advices to use 1,9V as Vref, so
K = 220 = 1/1,9*Rs/Rl*1/1,1*Rt*Ct

K = 220 = Rs/(2,09*Rl*Rt*Ct)

I chose 10*10^-9=10nF as Ct and 10*10^3=10K as Rl and Rt, thus having

K = 220 = Rs/(2,09*10*10^3*10*10^3*10*10^-9)

K = 220 = Rs/(2,09* 10^3 * 10^3 * 10^3 * 10^-9)

Simplified as

K = 220 = Rs/(2,09)

So, Rs = 220 * 2,09 = 459 ohm.

Now, I don't have a 459 ohm resistor, but I do have a shiny 470 one,
plus a cookie tin box full of trimmers which can be put in parallel
with the 470R :-)

The AN doesn't say anything about Cl, so I decided to use another 10n.

So:

K = 220
Rs = circa 459 = 470//Trimmer
Rl = 10K
Rt = 10K
Ct = 10n
Cl = 10n
Vref = 1,9

I grabbed the components and started to breadboard it.
I omitted the trimmer in parallel to the 470 ohms, because at this
stage accuracy is the last of my problems.
Picture of the circuit:
http://img250.imageshack.us/img250/5208/sany16738si.jpg

I plugged in my rusty Meterman pocket meter and started to measure
frequencies.

Here's what i got:

0,5V => 2,22 Khz
1V => 2,86 Khz
2V => 3,92 Khz

I double-checked everything, but It didn't help.

Then, *that* hateful little voice started to whisper *you're wrong!
you're wrong! you can't even get a cookbook-like application note to
work! hahaha! loser!*

So, I would be happy if someone helped me to make the voice shut up.
With violence, if necessary.

A giant thank you in advance, any advice useful to get out of this s**t
is welcome!


Some days you're the dog, some days the hydrant.

:-)

--

Me.

.