??? 08/21/07 17:29 Modified: 08/21/07 17:47 Read: times |
#143459 - >Dissipation constant< gives the answer Responding to: ???'s previous message |
Kaushik said:
I have chosen the thermistor with
Dissipation constant: 1 mw/degree celsius Maximum Power rating : 30 mw at 25 degree celsius derated to 1 mw at 125 degree celsius. +/- 0.1 Degree Celsius The thermistor is in Contact of solid aluminum block. + 37 Degree celsius The "maximum power rating" is only relevant for you, if you decide to use the NTC for fluid detection or similar, means when you use a heated NTC. But in your application, precision measurement at +37°C, only a non heated NTC can be used. The "dissipation constant" of 1mW/°C tells you the maximum power that is allowed to be dissipated in your NTC to provide +-0.1°C tolerance, which is 100µW. At +37°C your 10kOhm-NTC shows a resistance of about 6kOhm (assuming a Beta = 3890K). So, no more than U = SQR (100µW x 6kOhm) = 0.77V must drop across it. Would you want to measure up to +50°C, then the maximum voltage drop should be less than U = SQR (100µW x 3.6kOhm) = 0.6V. Normally, you build a voltage divider consisting of NTC and precision resistor (as you already plan to do), which shows equal resistances at the center temperature of your measurement. So, you should choose a 6.2kOhm metal film resistor. This arrangement shows highest sensitivity, means yields highest voltage change per temperature change. Sometimes, the NTC resistor is paralleled by a precision resistor to somewhat linearize the rather unlinear R/T-relationship of NTC. But for your precision measurement the result of this paralleling is not good enough. So, I would use a look-up table to compensate for the unlinearity in combination with a ratiometric measurement to first order eliminate fluctuations of Vcc, means make Vcc be your reference voltage of ADC. Unfortunately, your 10kOhm-NTC is too low ohmic to work at Vcc=5V. I would choose a 100kOhm variant. So, a 100kOhm-NTC in combination with 62kOhm metal film resistor at Vcc=5V would be a suited combination. Kai |
Topic | Author | Date |
Thermistor Self-heating | 01/01/70 00:00 | |
Error budget | 01/01/70 00:00 | |
Use a bridge configuration | 01/01/70 00:00 | |
Error Budget | 01/01/70 00:00 | |
measurement point | 01/01/70 00:00 | |
measurement point | 01/01/70 00:00 | |
Math | 01/01/70 00:00 | |
>Dissipation constant< gives the answer | 01/01/70 00:00 | |
Amplify | 01/01/70 00:00 | |
The Center is not the only factor | 01/01/70 00:00 | |
Linearisation | 01/01/70 00:00 | |
>Dissipation constant< gives the answer | 01/01/70 00:00 | |
Getting closer | 01/01/70 00:00 | |
Correcting non Linearisation of thermister | 01/01/70 00:00 | |
1721 | 01/01/70 00:00 | |
Nice link | 01/01/70 00:00 | |
Measuring thermistors | 01/01/70 00:00 | |
Braggard? Show your work | 01/01/70 00:00 | |
nope | 01/01/70 00:00 | |
ADC selection. | 01/01/70 00:00 | |
Depends on your application | 01/01/70 00:00 | |
The Components | 01/01/70 00:00 | |
Accuracy... | 01/01/70 00:00 | |
What BETA? 3890K or 3575K? | 01/01/70 00:00 | |
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