CPU Temperature Measurement Gadget

Thermal Diode Measurement Circuit

I have built such a thermometer for the 300a myself. Here is my Do It Yourself Guide:

l. Download the specs (in PDF) for the ICL7107 from Harris at

2. Get a ICL7107 or even better a 200mV or 2V DVM panel with the ICL7107 from an electronics store for about $15 and use it as differential input.

3. Get a 78L05 volt regulator for exiting the diode and as a reference voltage.

4. Now the circuit: You can use the diode thermometer sample circuit from the PDF but with one important modification: The B14B/B15 have input protection clamping diodes to max. 1.5V which prohibit using COMMON (floating at 2.5V) of the 7107 as a Voltage Reference and using the sample circuit.

COMMON must not be connected to the cathode, but only to REF LO. Therefore we need an external reference. Burr’s REF2OO would be ideal but I didn’t have one handy so I used a 78L05. With a 22K resistor from the 78L05 output to the anode (B14) and the cathode (B15) connected ground we will have the input voltage of the anode which is connected to IN LO.

The Reference Voltage of 0.726V (Celeron in Ice Water) connected to IN HI I got from the 78L05 using a voltage divider (33K in series with a IOK poti). It is important that no current returning from the ICL7107 flows through tbe voltage divider to GND (even through a ground wire). Best is to have two separate ground wires if you connect B15 to GND on the Motherboard. Else the readings will jump +/- 5 degrees.

Setting a multiplier voltage of 0.18V between REF LO aad REF HI will get a reading of 100 in boiling water.

My reference voltages should be accurate enough for Celsius readings. But if you want to have really accurate readings or Fahrenheit units you have to calibrate it. You must temporarily solder or somehow attach (maybe conductive glue) the measurement leads to the edge connector pads B14/B15 of the CPU PCB and place it in ice water and then boiling water. You would also need a lead from the CPU ground (e.g. the SMD Tantal Condensor ) to the ground pin of the 78L05. This is because B14/B15 have input protection diodes and there could be a leakage current to ground. To account for the effect of altitude in boiling point temperature use this simple rule of thumb:

Fahrenheit: 212°F less 3°F for every 1000 feet above sea level.
Celsius: 100°C less 5°C every 900m above sea level.

You need +12V for the 78L05, +5V and -5V (for those panels that don’t generate their own -5V supply) for the 7107.

For permanent readings solder the measurement leads (shielded wire) to the B14/B15 pins of the Slot 1 on motherboard from the bottom. There should be NO traces going to these pins. It is NOT possible to use my circuit on the QDI Brilliant as they already use the diode for temperature monitoring using the Maxim MAX1617 Chip.

Now you have a very accurate thermometer which changes instantanously with CPU load. It has nearly zero inertia. The circuit diagram is below.

Finally if one wants to use industrial panel meters which are already enclosed and not based on the ICL7107 one must use an external op-amp to convert the readings. See e.g. http://www.burr-brown.com/Products/DataSheets/REF200.html for sample circuits.

If you are not comfortable in building this circuit, drop me an email ([email protected]) and I will build it for you for $40 including shipping.


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