An adequate current source can be created using a 78L05 regulator with a 5K resistor between the common and the output, as I recall. Then use a 24 volt supply and you get one millivolt per ohm, allowing a high input resistance digital voltmeter to display resistance directly. Feed the BCD digital output from that meter into the control logic.

HOW ACCURATE does the testing system need to be??

I appreciate the idea, but I'd prefer to make a microcontroller device for flexibility reasons. I could generate a test report, for example.

 

hi Daniel,
One option is:
8 off jFET 100uA or 200uA constant current sources,

Use 8 of the 16 ADC available pins on a Arduino Mega, a 10bit count is more than sufficient for a 0C to 70C temperature range

Write a simple program for the Arduino, and output the Thermistor temperature and resistance value for each ADC channel, via the Serial output.
Say channels A thru H.

E

 

hi Dan.
This is an example of an Arduino Sketch I use.
E

 

hi Daniel,
One option is:
8 off jFET 100uA or 200uA constant current sources,

Use 8 of the 16 ADC available pins on a Arduino Mega, a 10bit count is more than sufficient for a 0C to 70C temperature range

Write a simple program for the Arduino, and output the Thermistor temperature and resistance value for each ADC channel, via the Serial output.
Say channels A thru H.

E

Since the manual suggests a multimeter with 2000 counts, I would need an ADC with at least 11 effective bits to have the same accuracy.

 

hi D,
You have a span of 0C thru 71C,

Assume 0C thru 100C, that gives 10 counts per 1Cdeg == 0.1C /bit.

E

 

hi D,
You have a span of 0C thru 71C,

Assume 0C thru 100C, that gives 10 counts per 1Cdeg == 0.1C /bit.

E

Thermistors are non-linear. One must choose a resolution that provides at least the precision required at the temperature where the curve is steepest.

Normally, I'd choose at least 4x (2 additional bits) the resolution at that point, leaving me some headroom for digital post-processing.

 

hi,
It really depends upon the required accuracy of resistance measurements.

So far we have only read 'approx' values, and notably the statement

we need to ensure that the thermistor resistance must be between 1854Ω and 3116Ω, for an ambient temperature between 22.8 ±5°C (+73 ±9°F).

which suggests a low accuracy system.?

Perhaps Daniel could post a more precise specification???

E

 

I appreciate the idea, but I'd prefer to make a microcontroller device for flexibility reasons. I could generate a test report, for example.

First, a microcontroller would receive the voltage reading, OR the meter could be left out and just input the voltage directly . By using a single current it becomes possible to have a single, constant, scale factor of one millivolt per ohm. That will avoid possible software scaling errors., and it will simplify routine calibration checks.

 

hi J,
This simple sim shows at approx 71C, it has a low resolution of approx 2 counts/Cdeg.

Let's hear what the TS really wants and what his budget is?

E

 

This is what I can do with 12 bits and $8 of hardware.

 

Thermistors are non-linear. One must choose a resolution that provides at least the precision required at the temperature where the curve is steepest.

Normally, I'd choose at least 4x (2 additional bits) the resolution at that point, leaving me some headroom for digital post-processing.

Exactly.

 

Now for another question: What exactly is the purpose of this test?? How much value does it add??? What sort of failures is it supposed to detect?
Knowing that every test has a cost, how much value does this test add, relative to how much it costs??
It is not that hard to produce a large number of 1.0 mA current sources, and while the wiring can get a bit complex, if the micro has 8analog inputs, then the battery assemblies could be tested 8 at a time. Only one temperature monitor would be required.
WHAT ARE the accept/reject limits??? What is the most common failure?? I have designed industrial testing machines and always the limits must be known.
OR, are the resistances at temperatures used to provide individual calibrations?? Is this data being taken to provide an actual calibrated battery temperature calibration??

 

hi Dan,
Using one of my interactive calculators, I estimate.
At +25C, R0=2200 ohms, with a Beta value of 3900

If you are to design an accurate testing method, I would advise confirming these figures with the Thermistor manufacturer.

Is it required that you only test measure at the two quoted values of 0C= 7355 Ohms and at 71C= 382 Ohms.???

E

What is the name of that calculator?
Looks very interesting.

 

If you use a 1.00 milliamp regulated current source, and a 24 volt DC supply, you can read actual ohms accurately. But that will require five decade measuring system to read tens of volts with one millivolt resolution. Probably more accuracy and resolution than the job demands. No matter what, I suggest including a digital voltmeter in the system because it will allow verifying that it is working with only a glance. a 4-1/2 digit meter would be the very best, but a 3-1/2 digit meter can be almost as good.

 

What is the name of that calculator?

hi @schmitt trigger
I wrote a number of user online interactive tools for another technical website many years ago, for which I had direct assess to the website software.

So it cannot run on the AAC site.

E

If you want to try it:
Unzip this folder [ 3 files]
Run the *.htm in your PC Browser

 

Use analog switches or MOSFETs controlled via MCU GPIOs instead of mechanical switches (S1–S8), for automatic control.
Among Analog Switch ICs, you can choose CD4051 8:1 mux, 74HC4051 or ADG708. Or use N-MOSFETs/P-MOSFETs (with proper gate drivers). You have to use 3.3V as ADC reference to match the top of your divider. Add a small capacitor (e.g. 0.1 µF) between ADC input and GND to filter noise. Following are some designs that you may use for references.
https://www.hackster.io/Costalegre/auto-ranging-ohmmeter-6fa4af
https://www.pcbway.com/project/shar...e_Meter_without_microcontroller_a98f1825.html
https://simple-circuit.com/arduino-auto-ranging-ohmmeter-lcd/

 

You need to understand that THERE IS NO SUCH THING AS A PERFECT ANALOG SWITCH!! All of them have some amount of charge injection and internal resistance. Of course there are ways to work around those effects, but they do not happen by themselves.

I see no answer to the questions posed in my post #52. Especially important is the purpose of the test. Without knowing the purpose of the test it is not possible to create any reasonably cost effective testing scheme. All we have is suggestions of circuits that may, or not, be applicable.
Common purposes of production testing include:
1. Verify that a part is within some defined specification at a single point.
2. Verify that the part response is within some defined specification though out it's entire operating range.
3. Create a set of part responses to allow a system calibration for that specific part.
4. Measure the part variation within specifications for the purpose of part production process control.

The hardware and software to carryout the four different types of tests are quite different, as are the times for the tests and the cost of both the tester and the testing.
So until the thread starter provides an answer as to the purpose of the test there is no reason to continue.

 

You need to understand that THERE IS NO SUCH THING AS A PERFECT ANALOG SWITCH!! All of them have some amount of charge injection and internal resistance.

As well as cross-talk, switching speed, parasitic capacitance, etc., etc., none of which would have any impact whatsoever on this application.

 

As well as cross-talk, switching speed, parasitic capacitance, etc., etc., none of which would have any impact whatsoever on this application.

OK, and that is why I did not even mention those variables.
The single IMPORTANT question is what the actual purpose of the testing . That really does matter a great deal.

I have asked that question three times, #31, #52, and again in post #57. We do get a sort of description about the type of test but still not one word about the actual purpose.
Is that omission possibly a translation problem???

 

Now for another question: What exactly is the purpose of this test?? How much value does it add??? What sort of failures is it supposed to detect?
Knowing that every test has a cost, how much value does this test add, relative to how much it costs??
It is not that hard to produce a large number of 1.0 mA current sources, and while the wiring can get a bit complex, if the micro has 8analog inputs, then the battery assemblies could be tested 8 at a time. Only one temperature monitor would be required.
WHAT ARE the accept/reject limits??? What is the most common failure?? I have designed industrial testing machines and always the limits must be known.
OR, are the resistances at temperatures used to provide individual calibrations?? Is this data being taken to provide an actual calibrated battery temperature calibration??

I apologize for the delay in responding. My work schedule is a bit hectic, and I don't have time to access and respond while I'm working.

This thermistor reports, in real time, the internal battery temperature to the aircraft's electronic systems. As you may know, this information is very important for flight safety. For safety reasons, there's also a thermostat. The thermostat also needs to be tested, but since it only displays open or closed states, it's easier to test.
The purpose of this test is to ensure that the thermistor still functions according to the manufacturer's specifications. I understand that, to an electronics specialist, the test data reported in the manual may seem incomplete or inaccurate, but aircraft maintenance, overhaul, and repair manuals are written for aircraft technicians. For an aircraft maintenance technician, the temperature vs. resistance curve of a thermistor doesn't matter, for example. The only thing that matters to him is knowing that, within a certain temperature specified in the manual, the thermistor must exhibit a certain resistance, also specified in the manual.
The test will be considered successful if the resistance is within the specified range. Any value outside this range results in a failed test, in which case the thermistor is destroyed and a new one is installed. The tolerance is already within the range specified in the manual, so no values outside the range are tolerated.