The LM335 Temperature Sensor

The LM335 temperature sensor is an easy to use, cost-effective sensor with decent accuracy (around +/- 3 degrees C calibrated). The sensor is essentially a zener diode whose reverse breakdown voltage is proportional to absolute temperature.

Since the sensor is a zener diode, a bias current must be established in order to use the device. The spec sheet states that the diode should be biased between 400 uA and 5 mA; we'll bias it at 2 mA. It is important to note that self-heating can be a significant factor, which is why I'm not choosing a higher bias current. The bias circuit is as follows:

The temperature sensor's voltage output is related to absolute temperature by the following equation: Vout = VoutT0 * T / T0, where T0 is the known reference temperature where VoutT0 was measured. The nominal VoutT0 is equal to T0 * 10 mV/K. So, at 25 C, VoutT0 is nominally 298 K * 10 mV/K = 2.98 V (to be really accurate, we'd need a reference temperature and a voltmeter, but nominal values are OK for our purposes). Thus, the voltage dropped between +5 and the diode is 5V - 2.98V = 2.02V. In order to get 2 mA bias current, we need a 1 K resistor for R1.

A pinout of the sensor is provided below:

Note that the adj pin is unconnected. The adj pin is used to trim the diode to be more accurate.

This file adapted from [http://web.mit.edu/rec/www/workshop/lm335.html]