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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 is heated up in small incremental steps using a DC power supply. After waiting for the temperature to stabilize (for each measurement about 7 minutes) the tip PTC temperature as well as resistance was measured. The small PCB in the WXP80 has a biased gain amplifier, With this the temperature is converted to a voltage, suitable for AD conversion. The WX2 supplies the opamp and the AD converter on the PCB with 4.66v. For these measurements the voltage was supplied externally and adjusted for small changes during warming up.
The schematic diagram for the biased gain amplifier is:
With increasing the temperature in steps, the resistance Rv of TH1 as well as the output of the opamp Vtemp have been measured:
| 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 opamp works both as amplifier and level converter. Based on the schematics the following characteristics have been determined:
- Gain: 31.3
- Vbias: 0.0504
Tools and measuring equipment
- Philips PE 1542 DC power supply
- Temperature: no-name kitchen-bread thermometer (max 199 °C)
- Fluke 87 multimeter