Help me fix some problems in a V-F converter circuit

[Switch_639]

V-F Converter cct

I didn´t know where else to post, I am new here... if its wrong forum tell me... I am a student and have some questions on this circuit...

here it is...
**broken link removed**

how do I adjust the gain with the 2K rheostat and know its correct? also the optional offset adjust of the OPAMP, when is this correct? at moment I have the cct built... I am getting output at pin3 of LM331... its a small signal around +/- 1V...

any help will be appreciated... I can not get a square wave pulse on oscilloscope either, but I might be trying to read it in incorrect manner...

 

[hanif]

Re: V-F Converter cct

what you are trying to do is using a precision voltage to frequency converter circuit 100khz full scale. first you should try Simple Stand-Alone Voltage-to-Frequency Converter Using LM231/LM331 (refer link below);

https://freecircuitdiagram.com/2008...tage-to-frequency-converter-using-lm231lm331/

it gives the circuit as well as complete explanation. the circuit you refer is from LM331 document (see below link you can download it)

https://ninova.itu.edu.tr/tr/dersler/fen-bilimleri-enstitusu/133/mkm-510e/ekkaynaklar?g24520

 

[Switch_639]

Re: V-F Converter cct

I have a Robot which outputs its speed as analogue voltage ranging 0-10V... I have effectively 200mm/s for 1V... on the other side there is a frame grabber card... it needs the Robots speed to compensate amount of pictures it needs to take... so depending on frequency/speed of Robot it takes pictures at a rate we determine... I need 100kHz to increase resolution... so I opted with the Precision circuit... I have that pdf already, it does not tell me much... I have also seen the standard circuit...

all I need to know is how to adjust the offset voltage properly of the op amp and also the gain of the LM331... at moment I don´t know if its correct or not... how does that optional offset adjust work? and the gain is suppose to increase output or not of the LM?

 

[sunyuan]

V-F Converter cct

 

[Switch_639]

Re: V-F Converter cct

I was told this for the offset...

Connect Vin to ground.
Then adjust the offset adjustment pot until the output of the opamp is as close to 0v as you can get it.
The LF411 already has a pretty low offset voltage; under 2mV for the standard LF411, and under 0.5mV for the LF411A.

another question... do I still supply pin4 with -VS and pin7 with +VS when doing so? pin6 is the output correct? cause my output does not change when I adjust the POT... only the voltage on Pin3 changes... I can never notice changes in output voltage...

 

[karesz]

V-F Converter cct

Hi,
of course with +/- Vs, you must have the same supply conditions as in operation!
I would say; short Uin, adjust offset to zero, than apply some Uin (1V, 10V) and set Fout for exampl to 100KHz with 10V by the 2Kohm potenciometer.
By the way; with zero Uin (offset adjusting) you must have some very low frequency output signal (theoretically DC)...
Take National Semi`s LM331 datasheet - is more detailed! www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS005680.PDF
K.

 

[Switch_639]

Re: V-F Converter cct

what is the offset pin? cause I adjust the pot as much as possible... have tried 20K pot, 100K pot and 500K pot... none give me zero offset voltage on pin3... output on pin6 never changes... always the same value except if I exclude the pot from cct its value jumps by 100mV... but its not import I guess... cause that offset is optional as it says on cct correct?

 

[karesz]

V-F Converter cct

What kind of pin3 do you means_from Op or LM331?
Do you have +/- Supply Please?
Minimum +/- 5 or 10 up to 15V...

I would propose for you; vorget to first the offset adjustment!

Apply for ex. -1V to the input and check your output signal_ it must be such (around 10)KHz...
Otherway : your circuit is (maybe)dead.
Offset poti can be "a" value between 20K-1MOhm (20/50/100K or eevn 1M), as you see on the datasheet, thes value is interesting only for your supply current.
Please check exact your boards building/wiring! Works it?
K.

 

[Switch_639]

Re: V-F Converter cct

I agree with you on the offset... it does not change a great deal no matter which POT I use in any case... this is about the frequency... I am getting an output voltage on the LM331 pin 3... is there another way to measure the frequency without oscilloscope? cause I am using an analogue one but am not getting it to read pulses...

 

[karesz]

V-F Converter cct

Hali,
Ofcourse;
you can measure frequency with an oscilloscope, then you have a time calibrated scale & F=1/Time...
You must only read how much divisions has 1 period of your signal, + you know what is your "time unit"_I mean:
your scope is in 500usec/div & 1 periode of your signal is in 2.8 divisons finished Tperiode=0.5msecx2.8,=1.4msec,than +/Tper.=1/1.4=0.7KHz...
You can simpler measure a signal frequency with a dedicated counter, called too as frequency meter.
Moderner DMMs have often capabilities to measure some (relative low, but in your case enough) signal frequencies -for examp. up to 100KHz, 1MHz...
What do you meand pls: "cause I am using an analogue one but am not getting it to read pulses..."
K.

Added after 8 minutes:

Its possible to indicate the output voltage with a DMM if you makes changing on Uinp at your U-F Converter, I mean you will measure "some" output voltages, but the value is (possibly) not true, then for a DMM is regularly some freq. over 100Hz or such KHz too high..
Apply some inputDC so from minus 100mV to 1,2, or 5, 10 V you must see some changing on your output voltages(DMM setted in AC).
I belief, that you must have an oscilloscope to set/control your circuit, and if it functions properly; you need (as the best) a counter + a DMM to make fullrange + offset settings...

 

[Switch_639]

Re: V-F Converter cct

exactly as said... I don´t get any frequency pulses... I must be trying to read improper way or something... is there some setting to change also to read square wave pulses?

 

[karesz]

V-F Converter cct

You can check your scope with the Cal-out pulses!
Try it pls. It has usualy an amplitude at 0.5Vpp & 1KHz, also set the scope to 200/500mV/div + 1msec/div you must see some deflected signal, than trigger it...
Do you have probes/sondas?
Is the scope input not switched to GND_also = shorted?

Added after 2 hours 17 minutes:

Hi Switch_639,
I think you should chek first an simpler version(Nationals Datasheet): Fig1,10Hz-10KHz, than fig.3 or thes one: fig.4...
I belief, that you hase some basic problem on your (bread)board.
K.

 

[Switch_639]

Re: V-F Converter cct

should the scope GND not be connected to GND of the circuit?

 

[karesz]

V-F Converter cct

Hi,
Of course: it must have GND-connection, then this is your reference potencial!
In practicum, if both eyuipments are on same phase of mains; not so important, but your signal quality tells for you: make it better pls!

Apropos;
Is your circuit functioning yet?
Othercase-my proposal is: build pls. to first only an simpler U/F conv. with only LM331_no transistor & OP...
K.

 

[Switch_639]

Re: V-F Converter cct

this is the thing... I get about 3.3V out on Pin3 of the LM331... but I see no square wave pulses on the oscilloscope... actually no pulses... I have no other way of measuring it yet... but if the circuit is functioning, how would I know that?

 

[Switch_639]

Re: V-F Converter cct

i did not know the output is controlled by me... via that pull up resistor... my output is now at 5V, but that doesn't mean anything... what is the most important thing for a LM331 to pulse?

 

[karesz]

Re: V-F Converter cct

i did not know the output is controlled by me... via that pull up resistor... my output is now at 5V, but that doesn't mean anything... what is the most important thing for a LM331 to pulse?

Hi,
I proposed for you on 30. Dec.:
Hi Switch_639,
I think you should chek first an simpler version(Nationals Datasheet): Fig1,10Hz-10KHz, than fig.3 or thes one: fig.4...
I belief, that you hase some basic problem on your (bread)board.
K.
Hier is the datasheet with some simpler/to begin solution - I referred as fig. 1 & 3/4...
Did you mean thes version of problem, or what is your momentan circuit pls?
K.

 

[karesz]

V-F Converter cct

Of course, if you has a datasheet with internal (simple circuit) you will seethat it has an open collector out, so you MUST have an external "workresitor"...

Please CHECK VERY EXACT your PCB to EQUIVALENT with YOUR APPS CIRCUIT!

I think you will (must) find some differenc(ies)_or is your LM331 dead:-
I assume you applyed the correct Input voltage too(GND referenced to this circuit).
K.

 

[Switch_639]

Re: V-F Converter cct

how would I know if the LM is dead? I have two... one on breadboard and one on my built cct to VERA board... I have rebuilt the circuit 10times... every time checking and rechecking... I need the 100kHz out... that's why I haven't tried the simple one... I have no clue what is wrong... will need to get someone in person to help, but no one around... so the LM is normally simple?... no probs to see pulses or frequency out? is there a way to test the LM to make sure its not dead?

 

[karesz]

Re: V-F Converter cct

... I have rebuilt the circuit 10times... every time checking and rechecking...

Pls belief me, exactly_or latest_ at the moment is a mus to go on some other way(...
If you can not have another person to control your veroboard, take a simpler way to see a basic function...
Check all pins for voltages, are both pins Nr4(OP & U-Fconv) on GND (can you measure it as zero Ohm)?
Can you measure your + supply voltages on Pins8 &(7 at OP);how much are their?
What is on the basis & emitter, better with oscilloscope too?
If you tell: I have 5V on LM331/3(out) = this is not automatically good, then means, (maybe)your circuit isnt in function...
Did you MINUS 1-2 volts applyed to the Uin connector?
K.