AD595 based temperature meter

This electronic thermometer reads from 0 to several hundred °C and is small enough to fit in small spaces where a conventional thermometer cannot. I made it because I needed some way to compare the internal air temperature of a KEF PSW2010 subwoofer before and after making modifications.

thermocouple amp schematic

The project uses a thermocouple as a temperature sensor. A thermocouple is a pair of wires made of two different alloys; due to the Seebeck effect, when the wires are joined, they generate a small but predictable voltage which depends on the temperature at the junction. There are various types of thermocouple, the most common being type K, which is what we use here.

(More on thermocouples here and plenty of other places if you search.)

The AD595 is a 14 pin IC, designed specifically as a thermocouple amplifier. With no external components except for the thermocouple itself, it gives an output of 10mV/°C. Simply connect it up and monitor the output with a multimeter. (PDF data sheet is here.)

(I bought the AD595 (order code 409194) and a thermocouple (7076150) from Farnell for about 26€, including VAT.)

Construction

thermocouple amp close up

The circuit is easy to build on a small piece of veroboard. You could put it in a case if you want, I didn’t bother. Two diodes protect against accidentally reversing the supplies and killing the IC, and a 9V battery supplies power. The schematic, lifted straight from the data sheet, is above.

Testing

After a quick visual check of the circuit, simply plug in a battery. The data sheet is not clear as to whether pin 1 of the IC should be grounded, so I initially left it open. I got very erratic readings, or nothing at all. Once I tied pin 1 to 0V (as in myschematic) everything worked fine.

The data sheet has a table of voltage to temperature on page 3, but it is not really needed - you can pretty much read temperature straight from the output voltage.

thermocouple installed

To perform some “sanity checks”, I boiled a pan of water. With the water bubbling away I could measure 90-100°C, depending on where in the pan I placed the probe. (Even a bit more than 100 right in the middle of the bubbles.) Inside my fridge is 5°. (I could have used a jar of melting ice too.)

Given that all I need really was to make relative before and after measurements on the KEF subwoofer, I figured that this was plenty good enough.

In Use

I taped the thermocouple to the back of the power amp enclosure - see the photo. (The other two taped up wires are the connections to the loudspeaker, which I don’t use for this test, as I connect my dummy load in place of the speaker.)

Simply allow the device to warm up, reading the meter every now and then to see when the temperature has stabilised, and make a note of the final reading. In this case, that was about 35°C.

Conclusion

Thermocouple based thermometers have several advantages. The sensor is very small, and can be placed inside an equipment case, as here, or even bonded to the case of a power device with heatsink compound. They respond quickly to temperature change, and have a wide temperature range. (A type K should work from -200°C to 1350°C.)

thermocouple amp in use

They seem to be quite tough - I put this one in a gas flame, where it glowed red hot, reading close to 200°C, with no problem. When I thought that I had killed it, I simply stripped the ends of the wires and twisted them together - provided that they remain in contact, this works fine. And, they can be easily and quite inexpensively replaced.

If you intend to do any real work on power amps or other devices where heat is a concern, you need some way to measure, and this little project does the job very nicely and inexpensively. You could even integrate the circuit into control or measurrement systems quite easily - there are plenty of potential applications.