zero / span circuit

[AdamMayotte]

Hello,

I am trying to design a circuit which uses op-amps and resistors that would be able to make zero and span adjustments to an input signal.
Can anyone here help me with the circuit design or maybe direct me to a site that has that type of info.

Thanks,
Adam

 

[AdamMayotte]

Skogsgurra

How do I use the potentiometers in any other way?

 

[Skogsgurra]

Easy. You want a variable voltage.

Now, you are using a fixed resistor (R8, for instance) and a variable resistor (Course and Fine offset) to get a variable voltage. The voltage range is 0 - 4.03 V. The result is also to some extent dependent on contact resistance between wiper and conducting surface of potentiometer.

It is more practical, and physically sounder, to use the potentiometer as a voltage divider. Connect one side to +12 V and the other side to GND. The wiper voltage will now vary linearly with wiper position, you can have full range if you want to, and the wiper resistance will have very little effect on the output voltage (the current is quite small).

Google "potentiometer" for more on this. The name "potentiometer" comes from the use of a potentiometer to divide an unknown voltage and compare it to a standard cell. The setting of the wiper when wiper voltage equals std cell voltage was used to calculate unknown voltage (Ux = Ustd/wiper).

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jimkirk]

Above comments plus:

I'd also suggest different values for R1 and R_load. You're throwing away huge amounts of signal there, and then recovering it in your GAIN opamp (which looks to require a negative supply, is that available?). Connect it as Gunnar suggests, the output will be more linear (assuming you haev a linear pot).

You could use R1 to adjust your output range so that your input op amp with feedback controlled by a pot (output to wiper, one end to inverting input, other end to ground) adjusts your gain appropriately. So for instance, if you want 5 volts maximum, select R1 to give 4 volts maximum or so to the input op amp, then adjust the (greater than one) gain to give you 5 volts.

Another pot & non-inverting op amp to generate an offset voltage. Sum them together with resistor to another non-inverting op amp for your final output. There will be another gain loss doing this, but if you make the resistor applying the offset a fair amount larger than the one applying the signal, this will be minimized, and easily made up for by adding a little more gain. So for example, using 10 kOhms for the signal input and 100 kOhms for the offset input, 11 volts will generate 1 volt of offset in the final output, and the signal will only be attenuated by about 9%.

You could probably combine that last bit into a single op amp. All this might make your adjustments a bit more interactive, so you might need a couple of iterations of adjusting the gain and offset, but the benefit is you don't need a negative supply like your current circuit does, and you save one, maybe two op amps.

Is your 12 volt source regulated? That will have a direct effect on the output.

Oh, and R14 serves no useful purpose (except maybe some SPICE simulators need things like that for convergence).

 

[AdamMayotte]

Hello again,

I am still working on the circuit and it's going well, thanks to all your suggestions.
I was wondering too if you guys might know of a place that I could find a cheap digital panel meter that will display an output in percentage based on an input of 1-5 vdc?
I have found some but they are all around $200 US and that's a little steep for me. I was hoping to find something around 50.00.

Thanks again,
Adam

 

[itsmoked]

Absolutely!

Check these out. I've used a few hundred to good effect. About $30.

Trumeter Company, Inc
DPM952-T

Talk to George tell him I sent you which may help connect you to this product.
They're in Florida so keep the time in mind.

Their site is funky, but here's the page to Panel Meters:

http://www.trumeter.com/Home.php?cl...ntation&app=Digital+Panel+Meters&lang=English

This is the one I have experience with:

http://www.trumeter.com/Home.php?cl...Digital+Panel+Meters&item=DPM952&lang=English

Note yo might have t paste these URLs together in your browser.



Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[AdamMayotte]

Thanks, those look they'll work great.

I have been doing some research on string potentiometers and have found a few but the cheapest is around $250.00
Does anyone here know of something that could work the same but for cheaper?

Maybe someone even has a better idea.
I'm trying to convert a traveling cable (of about 87" of travel) into a readable signal. All I can think of using is a string pot or something like that.

Thanks,
Adam

 

[MikeHalloran]

If the cable wraps around a pulley, you could put an encoder on the pulley axis. Absolute encoders tend to be expensive, but incremental encoders aren't so bad. With the incremental encoder, you need to generate an index pulse somewhere in the cable's travel, and count up/down from there. Simple with a micro, especially if you already have one. Doing it from scratch as a complete standalone module might stretch your budget. If it's safety- critical, you need a backup power supply so glitches don't reset the micro.





Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Buy an axis readout for a machine tool. They turn up on ebay fairly frequently. Search for 'Anilam' or 'DRO' as a starting point.


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Sometimes I only open my mouth to swap feet...

 

[MikeHalloran]

Readouts, yes. Linear transducers with 87" of travel are not common.



Mike Halloran
Pembroke Pines, FL, USA

 

[ScottyUK]

Mike -

True, but when they do come up they're either dirt cheap because no one wants them that week, or hideously expensive because someone really needs one!