Build a Diode Temperature Probe

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I’ve made this circuit available for those of you out there (including me) who still have older socket 7 based systems without temperature monitoring facilities. This circuit is also great for monitoring the temperature of your overclocked graphics card.

If you own a digital multimeter (DMM), it can be used to give temperature reading for a small expenditure in parts and effort. When a small forward bias is applied to a conventional silicon diode, the voltage drop across the diode junction changes at a rate of about 1.25 mV / degree Farenheit (2.24 mV/ degree celcius). Thus, a low cost and readily available diode such as the 1N914 [available from Radio Shack] can be used as a temperature probe.

Circuit Diagram

The bridge cicuit above works in conjuntion with the sensor diode and a DMM on the 2V scale. The displayed digits are the temperature. Note that in the above circuit, two values are shown for R2-R4 and R6-R7. The values in parenthesis are for Celcius operation, while the others are for Farenheit. Capacitor C1 is used to bypass stray signals that may be picked up on the leads.

Construction

The circuit can be assembled on a small printed circuitt or perforated board. The small circles at C1 indicate the need for a pc pad or wire-wrap pin to make the connections to the remote diode. To make the temperature probe safe for liquid immersion, the arrangement shown in Fig. 2 can be used.

Probe

Preform a short length of vinyl tubing, fill it with epoxy and ‘thread’ it up the diode leads to make contact with the diode body. Allow the epoxy to thoroughly cure. If desired, a length of heat shrink tubing may be used.

In either case, leave a short length of the diode lead exposed for soldering to the flexible cable. Slide a short length of heat shrink tubing over the covered diode leads, solder each diode lead to the flexible cable, and then fit the tubing over the solder joint. Shrink the tubing to make a tight fit.

Calibration

The resistance values for R2-R4 and R6-R7 are not critical, but their ratios are. Perform the following calibration tests before changing any resistance value.

Potentiometer R3 balances the bridge to indicate 32 degrees Farenheit (0 degrees Celcius) at this temperature. Potentiometer R5 is used to reduce the 1.25 (2.24) m/V degree to exactly 1mV / degree and is also used to set the upper range point.

With R3 and R5 at their center of rotation, immerse the probe in a container of finely shaved or crushed ice. Adjust R3 to produce a DMM indication of 32 degrees Farenheit (0 degrees Celcius). Place the probe in a container of boiling water; adjust R5 for a DMM indication of 212 degrees Farenheit (100 degrees Celcius).

If you find that R3 is at the end of its rotation, add a parallel resistor (in the megohm range) across either R2 or R4, depending on the location of the wiper of R3. If R5 is at the end of its rotation, add a parallel resistor (also in the megohm range) across either R6 or R7.

Since the DMM will also indicate negative voltages, it will similarly indicate temperatures below those at which it is calibrated.

There are many different ways to build the probe. Below is a picture of the one I constructed and use to measure the temperature of my Voodoo2 graphics card.

Circuit Diagram

If you have any problems building this circuit, mail me at garthr1@hotmail.com and I’ll try to help you.

HAVE FUN!

Garth RAUTENBACH


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