Use of a 0402 Size Thermistor

This post and project is keeping with my On-Going Goal of working on very small projects as long as my eyes and steady hands allow.

The Thermistor Fan Control Circuit is working very well for my PA47 RF Power Amplifier (see selected links to previous posts). The PCB was re-laid out to include the circuit. Initially for testing, I just used an available 10K NTC Thermistor super-glued to the heatsink on a 1 Watt PA47 configuration.

For this project, my plan is to attach a small 10K NTC 0402 size thermistor directly to the 2N7000 for best FAN Control and cooling of the FET.

Here are some Microscope photos of the attachment process.

Shown is the 0402 Thermistor super-glued to the back of the TO-92 2N7000, along with the tip of a standard X-Acto Blade (for additional size reference).

Attached 0402 Thermistor on a TO-92 2N7000

A Wire Wrap Wire (30 AWG) was then soldered on each end of the Thermistor.

Thermistor with Wires Attached

A small drop of Epoxy was added to cover the connections and support the attached wires. Epoxy does not stick to the Kynar (wire insulation) very well (if at all). The wire connections may need better/more support.

Waiting for the Epoxy to Cure

I used Fast Cure Hobby Epoxy, but it will take over night to harden completely, in circuit testing maybe started tomorrow.

I am not sure of the thermal conductivity of Hobby Epoxy, perhaps something else would be better? Maybe another harder Epoxy such as JB-Weld?

This is ALL probably not necessary, but I wanted to see if I could do it. For future projects and use of Thermistors will probably be a larger size, and maybe not attached directly to the FET.

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