I have a pH controller with a 0-1,400 mv DC recorder output that I need to connect to a chart recorder that requires 0-5 v DC input. I bought an op amp and some pots but I am not sure how to design a simple circuit to do this. Any ideas or can someone point me to somewhere for help? Thanks.

 

What DC supply voltage to do have to run it on?
What OpAmp?
What kind of load can the PH controller drive?

 

Thanks Mike,
I was probably going to use a 5v dc power supply. The pH range I will be in (6 to 10) will put out 600 to 1000 mv which I need to boost to about 2 to 3.5 v.
I have a 741 op amp.
I don't know know about the load, the 1400 mv is the only spec in the manual.

 

The 741 will not hack it. It will not work on 5V. Any modern rail-to-rail CMOS OpAmp or even a lowly LM358 will work in this circuit.

 

I really appreciate your help. I'm glad I got on here for advice before trying something that isn't my area of expertise.

 

You will need a supply of more than 5V to get a 5V signal out. The LM358 will swing it's output NEAR to ground (not completely) but the up swing only goes to about VCC - 1.5V. So you would need about 7V rail to get a solid 5V out upper range.

 

You will need a supply of more than 5V to get a 5V signal out. The LM358 will swing it's output NEAR to ground (not completely) but the up swing only goes to about VCC - 1.5V. So you would need about 7V rail to get a solid 5V out upper range.

Bounty, look at the pull-up resistor I put on the circuit I posted...

 

The way MikeMl did this was to focus on the 1000 mv input to get 3.49V out (+/- 1%).
Two point zero volts out for an input of .600 volts in requires a gain of 3.333
A dead accurate gain of 3.333 would require a potentiometer. I'd say, 2.26K 1% as R1 and a 200 ohm pot arranged as a rheostat (one leg shorted to the middle terminal).

OR...you could make R1 = 2.32k 1% for a gain of 3.32 instead of a gain of 3.33 (+/- 1%)

If temperature drift matters, about 6.8K 10% to 7.15K 1% in series between the Ph meter output (signal voltage) and the positive input pin will cancel most of that.
Depends on how accurate you need it. At least now you know which resistors to adjust.

I used to do top quality Ph meters and it seemed like hypocrisy to calibrate to tighter than 1/2% in the electronics when the Ph cells are so horribly affected by temperature, pollutants in the water, and etc. Just know this: We can do the electronics. You have to deal with the Ph cells.

 

I do need to calibrate at pH 7.0 and 10.0 to within 0.1 pH units, so the fixed circuit would need to amplify dead on at both voltages. Otherwise without pots in the circuit I could use the calibration and slope pots on the controller, which might give me different values on the controller and the chart recorder, but the chart recorder is my main concern.

 

Thanks to all for your help!

 

Bounty, look at the pull-up resistor I put on the circuit I posted...

Not that I would be one to argue with a simulation..... but I don't see how the circuit you posted can work as shown using an LM358 with a single 5V rail. The 3.3k pull up resistor means that at 0V output, it forces the op-amp to sink about 1.5mA. The LM358 can not pull down to ground sinking that kind of current. That circuit would need a negative rail to work. IMHO, the sim model is wrong. I think the data curve would stop going down at about 700 mV.

See performance curve from LM358 data sheet:

 

Not that I would be one to argue with a simulation..... but I don't see how the circuit you posted can work as shown using an LM358 with a single 5V rail....

I wasn't using the LM358 model in the simulation; I used LTSpice's OpAMP2, which is a pretty good proxy for a CMOS rail-to-rail out opamp.

The OP stated that he is only interested in ph ranges of 600mV to 1000mV, which constrains the output swing to a range where the LM358 with the pull-up resistor can handle it.

 

I wasn't using the LM358 model in the simulation; I used LTSpice's OpAMP2, which is a pretty good proxy for a CMOS rail-to-rail out opamp.

The posting was confusing. I looked at the sim which said on it that you could use an LM358 for the op amp. If you do, it won't give the performance shown.

 

I added the reference to LM324 as an afterthought. My first statement to the OP was
Any modern rail-to-rail CMOS OpAmp

 

So the LM358 will work? Also I have all of the components ready to solder and need someone to confirm which of the LM358 pins 1-8 correspond with the IC connections in the schematic that MikeML posted earlier in the post.

Thanks!

 

Download the data sheet for the specific LM358 you have on hand (maker and package). The data sheet will tell you the pin numbers for that package.

The LM358 operated at a 5V supply will work with the gain shown as long as the input doesn't exceed 1.0V (output is 3.5V or less). If you want to go all the way to 5V at the output pin, then you will need a supply voltage for the LM358 somewhere between 7V and 30V.

The bypass cap shown on the schematic should be soldered directly across the LM358's supply pins. The two inputs on the unused half of the LM358 should be grounded.

 

Thanks again.
I built the circuit and the output voltage is boosted, but the pH controller makes a single click or popping sound and the display on the controller changes from reading 7.8 for example to 1. Could there be current flowing back into the controller?

 

Thanks again.
I built the circuit and the output voltage is boosted, but the pH controller makes a single click or popping sound and the display on the controller changes from reading 7.8 for example to 1. Could there be current flowing back into the controller?

Yes. but that current is tiny. It would be the input bias current of the opamp, which is only 20nA.

I don't understand what you are describing happens. Does it work for a while so you can measure the boosted voltage? And then the PH meter jumps to a different reading?

You have a common connection between the negative side of your 5V supply and the internal ground in the PH meter?

 

I am just using a 9 V battery for power and a vom on the output right now. When I connect the battery ground to the circuit ground and the meter neg output the meter snaps and reads 1, like and error, but the vom reads ~2.5 V, which is where it should be.

 

Where are you taking the output from the PHmeter? Is it an output jack? If so what does the manual for the instrument say can be connected there?