Actron AM24-1 Controller Possible Thermistor Replacement.

[wotnot]

Hmmm....as you've noted a discrepancy between the temp the unit now displays (as opposed to previously), and what temp is measured by standalone thermometer, one has to make a choice on which data-points might be deemed as accurate.

If we consider the multimeter reading to be accurate with 8K resistance, we'll guess at one of the more common thermistor values (10K @ 25C);

(what the unit says)-> 8K @ 20C would infer a 10K NTC thermistor @25C with a delta of 3.9% per degree C

(what the thermometer says) -- 8K @ 23C ...nothing really 'fits' the standard ranges ..8K2 is the next resistance value down but none have a low enough thermal coefficient, ergo it's likely a 10K @ 25C NTC as well , with a higher delta value...highest I can find is 4.7% per degree C.

'Just because' the component itself is unmarked, and you've noticed a discrepancy between two different controller boards wrt measuring temperature (with the same unmarked thermistors) -and- your controller has already lunched on one, you're allowed to speculate these thermistors are chinese bespoke, or else factory rejects, or anything else like this, and thus if something like the actual component tolerance is out to +/-20% instead of less than half that, these things would easily account for such discrepancies (and the fact they die in the first place =)

Of the 2 choices, I'll presume the unit is working in some uncalibrated state due to the board swap, but the part dimensions appear correct wrt thermistor types... ergo, replace dead thermistor with any NTC 10K @ 25C thermistor, and observe results... ie; how closely displayed temp correlates with thermometer readings.

[RFI-EMI-GUY]

He is measuring the part in circuit I would guess. In which case the 8K is probably what in circuit will read on a 10K part. I would solder a 10K 5% or better resistor in the old controller. See what temperature it displays and then measure it in circuit. Probably 8K depending on meter polarity. Then I would buy a pack of 10K thermisters and put one in the old unit to use as a spare.

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[tready]

Thanks wotnot,

Agree 10K seems most logical when I was researching values. There is no way I have found to adjust the calibration on the controller which is annoying I rang Actron and asked the question and the newer controllers have the ability to plus or minus a 3 degree difference. I guess they got complaints about them being out of spec from junky components want to give changing the thermistor a go just for my own satisfaction and self confidence in soldering, just didn't want to do it without some sort of backup in case the whole thing went belly up. Will report back on the success or failure of my efforts.

[tready]

Well time for an update, After a struggle I replaced the thermistor and it worked (note to self get a better iron) Bad result is this one is 2.0/5C out on the reading which is liveable going to order a few online and see if I can get a better tolerance variation then the Jaycar one I used. At least I know its not dead completely though.

[wotnot]

Uncalculated guesstimation based upon what the jaycar part did -->

[tready]

Uncalculated guesstimation based upon what the jaycar part did -->

Thanks, Planned on getting a few different ones as a trial and error but at least its getting closer went from dead to nearly working.

[tready]

Uncalculated guesstimation based upon what the jaycar part did -->

Thanks for the suggestion I thought of getting a few different ones and trying it out with varying margins. At least were getting warmer went from 0% working to 98& functional and I cant thank you enough for all your info provided.

[tready]

Thanks for the suggestion, Was thinking of getting a few different ones with varying margins and doing trial and error. At least we have gone from 0% to 99% functioning and I didn't destroy it during the process will keep cracking at it and see where we end up. I mean I could live with it quite easily but its my OCD that likes numbers to be perfect. Thanks for all the info you have provided would of been lost otherwise.

[Reschs]

There must be away of calibrating the unit. Manufacturing tolerances would need to be taken in to account at testing stage.

[hinekadon]

dunno but spoke to a mate he told me stick two diodes in series (one fwd/one rev in parallel) then keep adding them to get it right . He works on industrial boilers ?????

[wotnot]

There must be away of calibrating the unit. Manufacturing tolerances would need to be taken in to account at testing stage.

You'd think so, but the Chinese really have re-invented the meaning of the term 'close enough is good enough', and the testing regime (if any) might not extend beyond an accuracy of +/- 2C error margin wrt displayed temperature =)

Thing is, there's a few ways of achieving such calibration ;

*change the thermal coefficient factor in the firmware, of the algorithm that's doing the ADC measuring/reporting of the thermistor circuit, at testing time (or based on the tested performance of the unmarked batch of thermistors)

*install a 'shim' resistor in series with the thermistor, to make the thermistor temp/resistance curve closely match the firmware operations

If you hold that last thought for a moment, R67 (2K7) in series with the thermistor, may actually be that resistor shim...(or a current limiter...would be worth checking it's actual value with thermistor removed)

dunno but spoke to a mate he told me stick two diodes in series (one fwd/one rev in parallel) then keep adding them to get it right . He works on industrial boilers ?????

This is a thing, but not applicable here really due to the low temperatures/temp range involved ...tends to be a good solution when you need monitor high temperatures tho' ( >100C), like in boilers =)

[tready]

You'd think so, but the Chinese really have re-invented the meaning of the term 'close enough is good enough', and the testing regime (if any) might not extend beyond an accuracy of +/- 2C error margin wrt displayed temperature =)

Thing is, there's a few ways of achieving such calibration ;

*change the thermal coefficient factor in the firmware, of the algorithm that's doing the ADC measuring/reporting of the thermistor circuit, at testing time (or based on the tested performance of the unmarked batch of thermistors)

*install a 'shim' resistor in series with the thermistor, to make the thermistor temp/resistance curve closely match the firmware operations

If you hold that last thought for a moment, R67 (2K7) in series with the thermistor, may actually be that resistor shim...(or a current limiter...would be worth checking it's actual value with thermistor removed)






This is a thing, but not applicable here really due to the low temperatures/temp range involved ...tends to be a good solution when you need monitor high temperatures tho' ( >100C), like in boilers =)

I measured it when I was first troubleshooting and I think it was about 273ohms. I thought the value of the resistor is 270 Ohm so assumed it was okay I based the info from this site But your saying its 2.7k? If I was to put a resistor in series what values do you think I should look at?

[tready]

Sorry about the earlier triple post I was replying but the post was disappearing when I looked so I kept posting till one stayed the rest obviously posted some time later

[wotnot]

I measured it when I was first troubleshooting and I think it was about 273ohms. I thought the value of the resistor is 270 Ohm so assumed it was okay I based the info from this site But your saying its 2.7k? If I was to put a resistor in series what values do you think I should look at?

My bad - I was reading it as a 3 digit code -- it is indeed a 270R resistor