Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Amplifier for pH measuring

Status
Not open for further replies.

igorlab

Newbie
Joined
Dec 17, 2021
Messages
2
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
37
For one of my DIY projects, I need a circuit for pH reading. I have seen a lot of different schemas but some of them are unstable, another stable but oversensitive to electromagnetic interference. And when I've found, seemed, excellent schema and use it in few devices one problem has emerged. If disconnects pH electrode from PCB, without disconnecting electrical power supply - all amplifiers (LMC6042) and bipolar power source (A0505S-1W) overheats and make "pshh" with gray smoke in few seconds... maybe someone could give some advice - how this scheme could be modified to prevent component's damaging on disconnecting pH electrode?
p.s. The signal from a pH probe has a typical resistance between 10 MΩ and 1000 MΩ. Because of this high value, it is very important that the amplifier input currents be as small as possible. So pH-measuring supposes the use of a device with great input impedance, so adding of resistor between input cascade and ground is not variant, because it will damage the pH-probe.

I cant add a link to the project on easyeda, due to the forum restrictions, but add the img of the scheme.

268791942_4665575966866509_4871964337806442129_n.jpg

Thanks in advance!
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
23,016
Helped
4,715
Reputation
9,446
Reaction score
5,087
Trophy points
1,393
Activity points
152,497
Hi,

Some issues I see:
* no fast ceramics capacitors to stabilize power supplies.
* no ESD protection on the electrode connection
* no suitable low pass filtering in the input stage
* you talk about probe resistance, but I don´t see a resistance measurement at all. I just see a voltage masurement. (What exact pH probe are you using?)
* did you read the datasheet chapter "COMPENSATING FOR INPUT CAPACITANCE"? --> stability
* PCB layout

***
So I think it´s very likely that
* the amplifier gets killed by an ESD impulse when removing the probe,
* the OPAMPs start to oscillate.

Btw: measuring high impedance does not mean you have to avoid resistors.
Why are there different symbols for 10uF polarized capacitors?
Did you check supply voltages with DVM and scope?
Did you follow the signal path with a scope? (I mean under true application conditions)

Klaus
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
50,786
Helped
14,600
Reputation
29,475
Reaction score
13,680
Trophy points
1,393
Location
Bochum, Germany
Activity points
290,842
A0505S is unregulated DC/DC. Did you check that the +/-5V supply stays within safe limits for LMC6042 under all conditions? Zener voltage suppressors may be useful.

Open buffer input may cause self oscillation, I don't see how it should damage the OP.

OP latch-up due to input transient is the most likely way to kill the device (besides supply overvoltage). You don't want to load the pH sensor with an additional protection circuit, current limiting resistors for U5 supply (e.g. 500 ohm to 1k) may avoid component damage in case of latch-up.
 

sae74

Newbie
Joined
Dec 17, 2021
Messages
1
Helped
0
Reputation
0
Reaction score
1
Trophy points
3
Activity points
9
I am very interested in this scheme. I am new to electronics.
Who can help me in reworking this scheme?
What do I need to add?

My thanks will be great!)
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
23,016
Helped
4,715
Reputation
9,446
Reaction score
5,087
Trophy points
1,393
Activity points
152,497
Hi,

yes, the 100n ceramics capacitors are essential.

The PCB layout is single side. The GND trace goes all around the PCB.
The loop to stabilize the power supply is maybe 150mm in length, while it maybe should be 5..10mm.
ph_loop.png

.. this means a lot of stray inductance and chance of noise pickup.

A GND plane (no copper pour) will make it much more stable and low noise.

Klaus
 

mikrovolt

Newbie
Joined
Dec 17, 2010
Messages
3
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,311
There is one video showing basic ph probe op amp circuit. The power supply would need to be stable and low noise.
The circuit will be very sensitive. The ph buffering solutions allows the probe to be calibrated.
That could be a ph probe amplifier. Some of the projects take analog signal to digital like using
Arduino it can take ph samples, take temperature make adjustments find the average and output to a display.
The low cost digital probe circuits are small not too much on breakout boards or hacks where as
the analog amplified probe there is more history and documentation that might help get started developing uses.

 
Last edited:

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top