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AdamMayotte (Industrial) (OP)
25 Mar 07 18:59

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.

MikeHalloran (Mechanical)
25 Mar 07 19:48
I'm sure someone here can help ... provided that you help, too.

For instance, it would be helpful to know the range occupied by the incoming signal, where you think its zero is, and how 'stiff' it is.

And it would be nice to know what span range you want on the output signal, and what you're going to drive with it.

And, it would be helpful to know how fast the incoming signal changes.

Mike Halloran
Pembroke Pines, FL, USA

VE1BLL (Military)
25 Mar 07 19:55
'zero' is offset.
'span' is gain.

To get a sensible answer, you'll need to provide more details regarding your exact application.

Comcokid (Electrical)
25 Mar 07 22:13
And also, what you have available for powering the opamp.  Example - if the input signal goes below ground you will need a negative supply.
AdamMayotte (Industrial) (OP)
26 Mar 07 19:22
I want to know the position of moving arm.
I was thinking of providing feedback of the arms position with a pot attatched to the moving section of that arm.
As the pot value changed I wanted a output, proportional to the change of resistance, in terms of a current of 4-20mA.
a change in input resistance(doesn't matter how much) to provide a 4-20 mA output.
I am using a 12 VDC battery as my source.
Speed of incoming signal would be 1 degree of rotation per second (max).
I can provide negative source if needed.

Thanks a lot for your help,
VE1BLL (Military)
26 Mar 07 20:18
If there is a computer involved (at the other end of the 4-20mA loop ?), then do the calibration in software.

1 degree of rotation per second (max). People will get very bored watching the arm move that slowly. winky smile

Skogsgurra (Electrical)
27 Mar 07 4:00
What you need is a very basic opamp circuit with offset and gain adjustment.

The good old (around 30 years now) collection of opamp circuits at National Semiconductor is a good start.

Use the second circuit (Non-inverting amplifier) and make R2 adjustable. Use it to set the gain (1/span) you need. Then use a couple of resistor to add your input signal and an adjustable offset signal before inputting to pin 3 (Vin).

Since you seem to have a 4-20 mA signal, you will have to make a voltage out of it forst. Use a suitable resistor for that. 250 ohms will give you 1 - 5 V, which usually is a good range.

The resulting output is a voltage, so you may want to make a 4-20 mA signal out of it. Use the precision current source on page 9 for that.

You may also find several other approaches if you study all the circuits given in the reference. It is a very complete collection of analogue circuits created by people that lived opamps years after years.

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

AdamMayotte (Industrial) (OP)
27 Mar 07 10:45
Thanks so much for that page of circuits, its awesome.

I was wondering.
After I am able to set my offset to 1 volt, if I adjust R2 and change the gain won't that change my offset as well?
I want to have an output of 1-5 volts regardless of my input voltage.
From what I can tell if I have a gain of 1 and an input of 1 volt, I will have an output of 1 volt...which is good for my zero (offset).
Then if my max input is say 7.5 volts and I adjust my gain to .666 in order to provide an output of 5 volts, that will change my offset to .666 volts when I have a zero input of 1 volt, won't it?

Thanks again for the help,
AdamMayotte (Industrial) (OP)
27 Mar 07 10:48
woops, I didn't do my math right...I guess I could only have a min gain of 1.
LionelHutz (Electrical)
27 Mar 07 11:00
Is this a 1-off circuit. If so, Phoenix Contact has a module that takes a pot input and provides either a 0-10V or 4-20mA output. I'm not going to do the search for it but I know it's there.

ScottyUK (Electrical)
27 Mar 07 11:27
PR Electronics also has one, the PR5111A. It is dead easy to use and they will configure it for you at the factory if you tell them what you want. We've been using them for a while now and they have been so reliable that we have almost forgotten they are there. Helpful and knowledgeable staff too.

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

Skogsgurra (Electrical)
27 Mar 07 11:38
"I guess I could only have a min gain of 1"

Yes, for the non-inverting, but a simple voltage divider on the input helps. An inverting amplifier can also have gains < 1.

Agree that there are quite a few midules that does almost anything you need. BASI is another make. They can cive you a version that takes 100 data pairs to produce any characteristic you need.

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

Skogsgurra (Electrical)
27 Mar 07 14:57
...and, yes. Of course. You get interaction between offset and gain. If you can't live with that, you have to combine an amplifier to get G*Vin and an adder to get G*Vin + Voffset. A simple matter and there are dual as well as quad opamps at no cost at all.

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

AdamMayotte (Industrial) (OP)
27 Mar 07 15:35

Thanks a lot for everyones help.
I just really like playing with this stuff but unfortunately I'm not too good at it yet. It doesn't seem to want to come naturally.

Thanks all,
AdamMayotte (Industrial) (OP)
1 Apr 07 14:15
Hello it's me again.

I have sat down and designed what I want and from what I can tell it works well. I built the circuit in a simulator and it runs as I would like.
I am wondering though. How do you know what parts to buy to make it? There are 1000's of different types of op-amps and it seems a little hellish to try and determine which one is the best.
I would like to post the circuit but can't seem to figure that one out either. All I've been able to find is a way to refer people to a website photo...which I don't have.

Skogsgurra (Electrical)
1 Apr 07 14:31
Good to hear!

You can post pictures in several ways. Keith Kress has written an FAQ about that: FAQ238-1161

Which opamp to use? Go for internally compensated "ordinary types". No expensive ones, no fast ones, and - if you only have one single supply - ones that work with a single power supply. That leaves quite a lot to chose from, the LM324, the TI 081/082/084 bunch, LM358. If you want to go nostalgic, you can always use the '741 and '301, but they will need a dual supply.  

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

AdamMayotte (Industrial) (OP)
1 Apr 07 15:40
Thanks for parts suggestions.

Here is the picture:

AdamMayotte (Industrial) (OP)
1 Apr 07 16:09
I will give an explanation.

to provide a 1-5 vdc output indicating an arm position

As R_load changes in resistance (indicating a change in direction of an arm) there is an increase in voltage to INPUT.
INPUT provides a voltage to the GAIN circuit which multiplies the INPUT voltage to provide a scaled output. If the GAIN voltage needs adjusting a voltage can be added by the OFFSET circuit.
Both the GAIN and OFFSET voltage are added to provide a summed OUTPUT to the voltmeter.
Skogsgurra (Electrical)
1 Apr 07 16:27
Can't see what R6 and R7 are feeding. Guess the summation point of yet another inverting opamp. I guess that the use of course/fine rheostats (yes, they are not potentiometers) is dictated by the simulator. You should not do that IRL. I have a few things to say about the practical side of the schematic.

1. Use the potentiometers as potentiometers. That will give you better range and better control.

2. Do not use 3 Mohms if you are going to use standard opamps. They have bias currents close to microamps - and that means temperature dependent errors in the volts range. Use resistors in the 1 - 100 kohms range. Much less errors and less noise pick-up.

3. Do the simulation using a 741 and then with a TI084. Compare results, you will understand what I mean with errors due to bias and high resistance values.

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

itsmoked (Electrical)
1 Apr 07 16:50
Nice posting job Adam!

Now do as Gunnar suggests and pick a few common amps to stick into your simulator.  Your simulator should have them already included.  You can run a Montecarlo on them and see the wild "outback" ends of your choices.  The cheapest is probably an LM358.  I go thru about 1,000/year.  I'm not thrilled with their warm up drift which is substantial, but if your circuit's application allows for a brief warm up you'd probably be good to go.

Keith Cress
Flamin Systems, Inc.-

AdamMayotte (Industrial) (OP)
1 Apr 07 17:18

How do I use the potentiometers in any other way?

Skogsgurra (Electrical)
2 Apr 07 2:22
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
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

jimkirk (Electrical)
2 Apr 07 7:56
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 (Industrial) (OP)
9 Apr 07 15:12
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,
itsmoked (Electrical)
9 Apr 07 16:09

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

Trumeter Company, Inc

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:

This is the one I have experience with:;app=Digital+Panel+Meters&amp;item=DPM952&lang=English

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

Keith Cress
Flamin Systems, Inc.-

AdamMayotte (Industrial) (OP)
20 Apr 07 14:59
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.

MikeHalloran (Mechanical)
20 Apr 07 17:38
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 (Electrical)
21 Apr 07 1:40
Buy an axis readout for a machine tool. They turn up on ebay fairly frequently. Search for 'Anilam' or 'DRO' as a starting point.

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

MikeHalloran (Mechanical)
21 Apr 07 18:37
Readouts, yes.  Linear transducers with 87" of travel are not common.

Mike Halloran
Pembroke Pines, FL, USA

ScottyUK (Electrical)
22 Apr 07 2:41
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!

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