# Voltage to Frequency Converter LM331 Problem

Discussion in 'General Electronics Chat' started by syee10, Mar 22, 2011.

1. ### syee10 Thread Starter Member

Mar 26, 2010
59
0
Hi all,

I had construct a voltage to frequency converter using LM331. The circuit can be found in Figure 3 in the LM331 datasheet which you can found it at http://www.national.com/ds/LM/LM231.pdf

I use Proteus to simulate through the circuit and it works fine with a DC input signal (0-10V). But when i change my input signal to a pulse signal (square wave 0-10V), the output frequency i get is incorrect. Why is this happen? Is this means the VFC LM331 only accept DC input signal but not a square pulse signal? Or is because of my circuit connection got problem?

Also when i connect the circuit and test it in practical, i cant get output frequency from the oscilloscope. I had check through many times the circuit, and i am sure the connection is correct when i connect it in my breadboard. And yet i still dun understand why the VFC LM331 is not functioning in practical. Why is this happen? Please help me

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2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
The output frequency is incorrect? What output frequencies do you get?
If the input is a square wave with 0v as low and 10v as high, you should get about 10Hz out when it's low (0v), and around 10kHz when it's high (10v).

If you expected something else, why? It converts a DC level to a frequency. If you want to feed it a square wave, then you will need to add more filtering on the input so that you get a DC level from the square wave.

Do you have bypass capacitors across the supply pins?
Are you certain that the capacitors are not shorted and not open?

3. ### syee10 Thread Starter Member

Mar 26, 2010
59
0
SgtWookie,

thanks for your reply. I expect a 10Hz frequency when it's 0V and about 10kHz when its 10V, but the result i get is not this. It is only around 60 in my frequency counter. And it is weird that the output frequency that shows in my frequency counter decreases as my input voltage increases. It should b input voltage increase and my output frequency increases.
however, if i use a DC level input voltage, i get the answer and the input DC level increases, the output frequency increases too.
What you mean by 'Do you have bypass capacitors across the supply pins?'? Which capacitor do you mean? Which capacitor do you refer from the figure?

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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Did you read this paragraph on p.7?

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5. ### SgtWookie Expert

Jul 17, 2007
22,183
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If you are using a breadboard aka prototyping board and have long wires running everywhere, that could be a source of your problems.

re: bypass caps

I suggest that you use a 0.1uF and a 10uF or larger cap across pins 4 (gnd) and 8 (Vs).

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
What is the frequency of your input square wave?

7. ### syee10 Thread Starter Member

Mar 26, 2010
59
0
Thanks for your reply.. i need some time to digest first =)

8. ### syee10 Thread Starter Member

Mar 26, 2010
59
0
I think the LM331 wont work with a square wave input. I had tried it many times. And i notice that the VFC LM331 working fine with a DC signal input. So now my question turns to how to convert a square wave into a DC level signal? Say if i have a +1V high level and 0V low level square wave, how am i going to convert it to a DC level +1V signal? Any suggestion so that i can feed this input into my LM331?

9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Why do you ignore the questions of people who are trying to help you?

Did you read this paragraph on p.7? See the attachment.

What is the frequency of your input square wave?

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10. ### syee10 Thread Starter Member

Mar 26, 2010
59
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Im sorry Ron, the square wave frequency is 50Hz. And yes, i already replaced the 0.01uF capacitor to 1uF capacitor. And now the circuit works fine with a DC level signal. But it is not functioning well with a square wave input signal of 50Hz. Sorry that i miss out to reply you =)

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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Read my attachment again, paying special attention to the part I highlighted in yellow. If you follow this advice, your VFC should work with a square wave.

12. ### syee10 Thread Starter Member

Mar 26, 2010
59
0
I have a square waveform as in the 1st figure which is not a smooth waveform. It has a high peak that i dun want in my real circuit. I want it to be a smooth square waveform as in the 2nd figure. So how am i going to convert the square wave with the peak to a smooth square waveform? Please assist me..

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13. ### Ron H AAC Fanatic!

Apr 14, 2005
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Repost your drawing with time and voltage scales.
What causes the big spikes?

14. ### syee10 Thread Starter Member

Mar 26, 2010
59
0
Ron_H,

The time is x axis and voltage is y axis. The scale is not important is this case because i just want to get a smooth pattern of waveform and the exact value is not important? The spike is coming out from an analog multiplier.. which receives two input signal and multiplied together

15. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Are syee10 and hasanferoz the same person?

16. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Time and voltage scales are essential if you want to have someone solve the problem for you.
You also need to post a schematic of your multiplier. Why do we usually have to tell people this?

17. ### syee10 Thread Starter Member

Mar 26, 2010
59
0
Ron_H,

Of course we are different people. The time scales is in milliseconds (at 50Hz) and the voltage scales is at 1-10V. Is that answering your requirement?

18. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Did you post a schematic of your multiplier, as I requested?

EDIT: Multiplier? I thought you were working on a V-F converter.

Last edited: Mar 28, 2011