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WXP80 analog measurements
This page shows the analog measurements of the WXP80 heating element connected to an operation amplifier. From the results a formula is calculated to model this.
PTC resistance and voltages WXP80
The tip temperature is measured with a PTC. Using a voltage divider and operational amplifier (opamp), the temperature is converted to a voltage, suitable for AD conversion. The schematic for this circuit is:
With increasing the temperature in steps, the following measurements of the resistance (TH1) as well as the output of the opamp (Vtemp) have been made:
| Vheat-element (in V) | TH1 (in Ω) | Vtemp (in V) | Temp (in °C) | : | Vheat-element (in V) | TH1 (in Ω) | Vtemp (in V) | Temp (in °C) |
|---|---|---|---|---|---|---|---|---|
| 0.000 | 108.5 | 0.399 | 22.5 | : | 2.750 | 133.9 | 0.850 | 76.2 |
| 0.250 | 109.0 | 0.405 | 23.2 | : | 3.000 | 138.0 | 0.917 | 85.2 |
| 0.375 | 109.1 | 0.411 | 23.6 | : | 3.250 | 143.3 | 1.014 | 97.2 |
| 0.500 | 109.5 | 0.416 | 24.4 | : | 3.500 | 148.5 | 1.112 | 109.2 |
| 0.750 | 110.7 | 0.438 | 26.8 | : | 3.750 | 153.5 | 1.202 | 120.4 |
| 1.000 | 112.3 | 0.467 | 30.4 | : | 4.000 | 158.6 | 1.289 | 130.8 |
| 1.250 | 114.4 | 0.502 | 34.5 | : | 4.250 | 163.9 | 1.381 | 142.3 |
| 1.500 | 116.9 | 0.550 | 39.8 | : | 4.500 | 168.8 | 1.464 | 152.9 |
| 1.750 | 119.7 | 0.598 | 45.8 | : | 4.750 | 173.8 | 1.553 | 162.6 |
| 2.000 | 123.2 | 0.659 | 53.7 | : | 5.000 | 180.7 | 1.670 | 177.6 |
| 2.250 | 126.7 | 0.724 | 61.1 | : | 5.250 | 186.2 | 1.767 | 188.4 |
| 2.500 | 130.1 | 0.783 | 68.2 | : | 5.500 | 191.8 | 1.866 | 199.0 |
Entering this data in libre office calc, reveals the following graph:
With the 'show equation' option in Libre office a formula is given:
$Rv = 0.465T + 98.139 \tag{1}$
where T is the temperature in °C and Rv the resistance of TH1 in Ω.
The circuit around the opamp looks like this:
Todo
The opamp works both as amplifier and level converter. Based on the schematics the following characteristics have been determined:
- Gain: 31.3
- Vbias: 0.0504