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Pressure sensor and opamp failures

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Quantised

Member
Hi Guys - new to this forum and hope you can help.

I have a piezo resistive pressure sensor RS286-664 connected to an LM358 used as a pair voltage follwers and supplying an LM392 configured as a differential amp (with gain of 150) to relate the output to ground.

The comparator section is part of an alarm system when the pressure rises/falls. Its a 12v single sided supply.

The problem is the LM358 keeps failing? And always in the middle of the night!

I cannot see why but wonder if the supply may be erratic as sometimes the alarm flutters. Would a varying Vs cause a problem and how much variation does it need to be or not to be?

And do you have a way to smooth the supply if that is of course the problem.

Thanks Quantised
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi Guys - new to this forum and hope you can help.

I have a piezo resistive pressure sensor RS286-664 connected to an LM358 used as a pair voltage follwers and supplying an LM392 configured as a differential amp (with gain of 150) to relate the output to ground.

The comparator section is part of an alarm system when the pressure rises/falls. Its a 12v single sided supply.

The problem is the LM358 keeps failing? And always in the middle of the night!

I cannot see why but wonder if the supply may be erratic as sometimes the alarm flutters. Would a varying Vs cause a problem and how much variation does it need to be or not to be?

And do you have a way to smooth the supply if that is of course the problem.

Thanks Quantised

hi,:)
Do you have a circuit you could post.?
 

mneary

New Member
The LM358 is a terrible voltage follower for this sensor. Your sensor output is 6.7mV/PSI and the LM358 offset is 2mV(max 7) per section.

When you say "fail" do you mean it requires replacement, or that it causes an error in the reading. What is the failure mode?
 

Quantised

Member
Thanks for quick reply. Eric - the circuit diagram may be tricky as I dont know how to do that yet but will see what I can do. Anyway apart from the parts already metioned it has 2 off 0.47Kohm resistors from the O/P of the 358 into the 392 amp and a 70K Rf and Rb.
Thats it - do you still need a diagram?

mneary - I noticed the cicuit had an offset when the pressure was zero but it seemed too small to worry about. The failure is the chip fries itself, sometimes just one amp and sometimes the whole chip - I replace them much too regularly to trust it as an alarm and its important as if I lose pressure I also fry other more expensive stuff.

Quantised
 

Quantised

Member
This is the best I can do at short notice.

picture.php


The comparator part seems to be fine so not included.
 

ericgibbs

Well-Known Member
Most Helpful Member
This is the best I can do at short notice.

hi,
I have looked at the sensor, its a bridge configuration, you say the LM358 is powered from a single +12Vdc supply.

How have you connected the LM358 inputs to the sensor bridge.?
 
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Quantised

Member
The circuit diagram shows the basic connections.
The bridge output is fed to the + of the follower and the lowest voltage side of the bridge is the fed to the -ve side of the differential amp.

Works ok for a while and blows up? Actually blows up a lot.

Really strange
 

mneary

New Member
If the offset is acceptable, then your circuit meets your needs.

How long is your wiring? What is the environment? Is there static electricity?
 

Quantised

Member
Would appreciate what opamp you recommend to reduce the offset, as it has a gain of 150 it would be better to use something with tighter specs.

The wiring to the circuit board is about 1/2 metre for the power supply and the board is contained in an IP67 box together with the transducer. There is no static electricity around and the environment is always between 30C and 45C

So what do I need to do to make it more reliable?
 

Leftyretro

New Member
Do you have access to a scope? If the device is literally blowing up then there has to be lots of current going somewhere. Your loads seem to all be fairly high impedance so that kind of leads me to think you may have some parasitic oscillations going on under certain conditions. I suspect your circuit is in need of some bypassing and layout improvements. Anyway a scope can show you things that might lead to a solution.

Lefty
 

Quantised

Member
I will get one and have a probe about and see what comes up. It wont be the first time this has happened and I never thought of it - nice one.

Maybe a few capacitors would help. Suggestions?
 

mneary

New Member
I looked around for better op amps and see that a fairly cheap replacement is the NE5532 which is only about us$0.70. (qty 10 on ebay) It has a typical offset of 0.5 mV and worst of 2.0 mV; about 1/4 as bad as LM358. If I understand your application, it should be a drop-in replacement. This will give you a more trustworthy pressure measurement without trimming every time you replace the chip.

If you haven't done so already, I would try about 100 microfarads across your 12V supply (capacitor rating 25V or more).
 

Quantised

Member
Lefty - I have found a scope - a bit tricky in this neck of the woods and have found a 2v ripple on the 12v supply which seems to come from an LED elsewhere in the 12v circuit. The LED flashes quickly but I cant find the cause. The basic 12 v circuit is enable with the start key of an engine and only runs the voltage gauge, the pressure gauge and the thermal cutout sensor so its difficult to see why it should have this ripple. It is at 60Hz the genset frequency.

mneary suggested a 100uF across the supply and it was not clear if it helps so I may try to add a diode in front of the cap on the Vs to stop it feeding back to the 12v circuit.

Having now run out of 358's (probably a good thing) I will try to fit a quad op amp in to see if that works?

I have to get this thing working as it protects the generator from overheating and seizing and thats my only power source.:eek:

Any other ideas on how to smooth the supply? Maybe a resistor between the bridge and the follower?

Quantised:confused:
 
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