yan_cart
Newbie level 2
Hi, wondered if anyone could help with this.
I need to sort out an NTC Thermistor conditioner circuit to work with a Bosch automotive NTC sensor with a temp range of -40 to +130degC.
In the sensor spec it mentions Rs = 1kohm, I assume this is recommending using a divider circuit with 1kohm resistor connected to a 5Vref and in series with the Rthermistor to GND. The mid point of the divider will go to an Rc filter and then to a unity gain non-inv op-amp and then to the 16bit ADC. The ADC i/p range is 0 to 5V.
The problem i see with this circuit is that at 130degC the Rthermistor = 90ohms and @ -40degC Rthermistor = 45kohms. The current through the thermistor should be < 1mA or self-heating will be a problem, but with such a massive swing and Rs = 1kohm you get several mA. As the seems not to be a std thermistor (due to the lower than expected resistance at high temps) would it be better to use a constant current source type cct instead of a the voltage divider cct? See thermistor Rt table below....
Can anyone recommened a better circuit or explain what I am missing?
Any help would be most appreciated...
Bosch NTC Vref = 5V
Rseries = 1kohm
T(degC) Rt(ohms) Vpt(volts)
-40 45313.00 4.892
-30 26114.00 4.816
-20 15462.00 4.696
-10 9397.00 4.519
0 5896.00 4.275
10 3792.00 3.957
20 2500.00 3.571
30 1707.00 3.153
40 1175.00 2.701
50 834.00 2.274
60 595.50 1.866
70 435.70 1.517
80 322.50 1.219
90 243.20 0.978
100 186.60 0.786
110 144.20 0.630
120 112.70 0.506
130 89.30 0.410
I need to sort out an NTC Thermistor conditioner circuit to work with a Bosch automotive NTC sensor with a temp range of -40 to +130degC.
In the sensor spec it mentions Rs = 1kohm, I assume this is recommending using a divider circuit with 1kohm resistor connected to a 5Vref and in series with the Rthermistor to GND. The mid point of the divider will go to an Rc filter and then to a unity gain non-inv op-amp and then to the 16bit ADC. The ADC i/p range is 0 to 5V.
The problem i see with this circuit is that at 130degC the Rthermistor = 90ohms and @ -40degC Rthermistor = 45kohms. The current through the thermistor should be < 1mA or self-heating will be a problem, but with such a massive swing and Rs = 1kohm you get several mA. As the seems not to be a std thermistor (due to the lower than expected resistance at high temps) would it be better to use a constant current source type cct instead of a the voltage divider cct? See thermistor Rt table below....
Can anyone recommened a better circuit or explain what I am missing?
Any help would be most appreciated...
Bosch NTC Vref = 5V
Rseries = 1kohm
T(degC) Rt(ohms) Vpt(volts)
-40 45313.00 4.892
-30 26114.00 4.816
-20 15462.00 4.696
-10 9397.00 4.519
0 5896.00 4.275
10 3792.00 3.957
20 2500.00 3.571
30 1707.00 3.153
40 1175.00 2.701
50 834.00 2.274
60 595.50 1.866
70 435.70 1.517
80 322.50 1.219
90 243.20 0.978
100 186.60 0.786
110 144.20 0.630
120 112.70 0.506
130 89.30 0.410
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