In my application, I am using a MAX6699 for monitoring the temperature of the PCB. The chip supports four channels, whereas I am using only 2 for monitoring the PCB temperature (through P-N diode). Now I have an additional requirement of monitoring the temperature of the metal enclosure of my system also. Since I have 2 spare channels in the above device, my first choice would be to use that to do the job.

However, the P-N diode which I used CMPT3906 is SMD and cannot be used on the chassis for monitoring. And I couldn't find any other component which is both chassis-mountable and can interface to my sensor through a cable or some discrete wires. Can anybody suggest me a solution?

  • \$\begingroup\$ Link to the pn diode you are using? \$\endgroup\$
    – Andy aka
    Jan 28, 2014 at 12:03
  • \$\begingroup\$ CMPT3906 \$\endgroup\$
    – Avin
    Jan 28, 2014 at 12:05
  • \$\begingroup\$ You can't screw a TO-220 diode to the case? \$\endgroup\$ Jan 28, 2014 at 12:36
  • \$\begingroup\$ @Ignacio Will any TO-220 diode work for this application? \$\endgroup\$
    – Avin
    Jan 28, 2014 at 14:34
  • \$\begingroup\$ I don't actually know. \$\endgroup\$ Jan 28, 2014 at 14:43

1 Answer 1


Diodes are not as good as diode-connected transistors in this kind of multi-current \$\Delta V_{BE}\$ temperature measurement circuit. I note that the part number you specify is actually a BJT, not a diode.

Recall the ideality factor that is close to 1 for a diode-connected transistor and more like 2 for a diode.

enter image description here

The manufacturer also has detailed recommendations for the type of transistor to be used, and recommends that large power transistors not be used.

You could try a small power transistor such as a BD159STU (That one is in the convenient TO-126 through-hole package), or simply make a small PCB with the SMT part on one side and no tracks or vias on the other, and attach that to the panel (that's probably what I'd do- it could be connectorized, and made as a snap-off bit attached to some other board).

  • \$\begingroup\$ you are right. What I have been using is a diode-connected transistor and not a diode as such. However the enclosure which we have is small and cannot afford to have one more pcb. But I can use the TO-126 transistor which you suggested. Can you tell me how to select transistor for this application too? \$\endgroup\$
    – Avin
    Jan 28, 2014 at 16:42
  • \$\begingroup\$ You could consider reading the data sheet... and come back if you have any questions.. \$\endgroup\$ Jan 28, 2014 at 17:40
  • \$\begingroup\$ I read the datasheets and got some doubts. Maxim directs us that the forward voltage at the highest expected temperature must be greater than 0.25V at 10μA, and at the lowest expected temperature, the forward voltage must be less than 0.95V at 100μA. However, the datasheet of the transistor doesn't tell anything about the forward voltage (which I presume is Vbe, correct me if I am wrong). \$\endgroup\$
    – Avin
    Jan 29, 2014 at 13:01
  • \$\begingroup\$ You will not find a part that meets your mechanical requirements and has those things guaranteed. You could measure a sample and see if it's well within that range or not. The other part you're using does not guarantee the 0.25V requirement either. \$\endgroup\$ Jan 29, 2014 at 13:05
  • 1
    \$\begingroup\$ Something like 2N2222A? Yes, it will be closer to what is recommended. \$\endgroup\$ Jan 29, 2014 at 13:42

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