Add a thermometer to your digital multimeter

Posted on Feb 5, 2014

A digitalmultimeter is a very useful instrument that combines several measurement functions in one unit. A typical multimeter includes features of a variable-range ohmmeter, voltmeter, and ammeter. Some of them also include capabilities of testing diodes and transistors. In this article, I am going to talk about a technique of adding thermometer f

Add a thermometer to your digital multimeter
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eature to a regular digital multimeter. The technique is very simple and uses one temperature sensor along with two resistors and a DPDT slide switch. In my previous article ( Testing analog temperature sensors with a multimeter ), I described a method of using a multimeter to test analog temperature sensors like LM34, LM35, MCP9701, TMP35, etc. These sensors provide an analog output voltage that is linearly proportional to the temperature, and therefore, by measuring the output voltage with a multimeter, we can verify if the sensor is working or not. Now we are going to embed one such sensor inside a digital multimeter and use the voltmeter feature of the meter to display temperature on the LCD. The sensor will acquire power supply from the multimeter circuit itself. Although this sounds pretty simple, there are few issues that must be taken care of to make it work. Let`s look at the setup shown in the picture below. Here, the LM35 sensor is powered from an external 9V battery source and its output is measured with a digital multimeter setup as a voltmeter. The LM35 output is linearly proportional to theCelsiustemperature with a scaling factor of +10mV/ °C, which means if the temperature is 24. 5 °C, the multimeter will measure the sensor output as 245 mV. Now the question is will the multimeter output be the same if the LM35 sensor is placed inside the multimeter and powered from the same battery (usually 9V or 12V) that...

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