# Using 1N4148 as temperature sensor for absolute temperature

Please forget the "why" for now, but I am attempting to determine the temperature of a 1N4148WS-E3 part by doing two measurements at different current setpoints. I've done some tests and some calculations and I don't understand where the error is coming from.

The theory

Derived from the Shockley diode equation, the following formula should give me the absolute temperature of the diode:

$$T=\frac{\Delta V_D}{\ln\left (\frac{1}{N}\right )}\cdot\frac{q}{n\cdot k}$$

Source: This video by Arda Yilmaz

Where:

• ΔVD: the difference between two measured voltages over the diode
• N: the ratio between the two measured currents corresponding to those voltages
• q: Charge of an electron, 1.602176634 × 10-19 coulomb
• k: Boltzmann constant, 1.380649 × 10-23
• n: ideality factor of the diode, this is 2.60607 according to Vishay

The measurement

I've measured voltage and current over/through this diode using two setpoints. The measurement data I got is:

• U1: 0.5819 V, I1: 582.8 µA
• U2: 0.4886 V, I2: 78.3 µA

To get this data, I used two multimeters, a 1 kΩ resistor in series with the diode and a lab voltage supply to set the setpoint. The current measurement is in series with the resistor, while the voltage measurement is directly over the diode.

simulate this circuit – Schematic created using CircuitLab

Feeding this into the formula, ΔVD is 0.0933 V and N is (78.3/582.8 =) 0.134351

This results is T = 207 K which is obviously incorrect. I'm expecting something close to room temperature.

Does anybody have any idea where this error is coming from?

• Try taking a few more points and see if they track exponentially; and take a few at another temperature (as well calibrated as you can; can be tricky without a thermal chamber), see if you can solve for $n$. Depending on where you got the parameters from, the manufacturer may be prioritizing different parts of the curve, or have other parameters that modify the basic equation. Mar 23 at 14:25
• I think all you have done is prove that the ideality factor is under 2. See this web page for your information: 2n3904blog.com/1n4148-diode-forward-biased-i-v-curve See also this discussion of IF: electronics.stackexchange.com/questions/559667/… Mar 23 at 14:35
• I don't think you can use the same value of n for all 1N4148s ever produced. The fact that this value is quoted to 7 sig. figs. is a big red flag to me about that source, anyway. Even devices from the same batch will vary. A typical SPICE model uses 1.9, this answer calculates it to be 2.3 from Vishay's own data. Your best bet is to make some measurements and find n for your particular device. Mar 23 at 14:43
• Look at Linear Technology's application note 137 on how to do temperature measurement. A transistor is used for the sensor and $V_{be}$ is measured at two different currents. Also beware of self heating of the sensor.
– qrk
Mar 23 at 17:17
• Did you cover the diode to prevent light from making it conduct like a photo-diode? Mar 23 at 17:18