More 0402 Thermistors

In preparation of loading and testing the next revision of my PA47 Amp, I have started the process of joining FET and Thermistors, by mixing the glue - JB-Weld epoxy (see: previous posts).

For this build, I am using only JB-Weld, for the initial tack and then for the final cover. The JB-Weld web site provided some interesting data regarding strength and temperature resistant abilities. It suggests that JB-Weld is the right "glue" for this Thermistor attachment process.

It appears that a little too much of JB-Weld was mixed, but is actually is just a small drop, most of it will not be used.

Microscope at 7X

Here are some FETs with Thermistors attached with small mounts of JB-Weld. Soldered wires will be added after the epoxy has set.

2N7000 with Thermistors Attached

While under the Microscope I noticed that JB-Weld is not what it appear to the un-aided eye, it is actually gray with black flecks.

This the same as the First Image, only the
Microscope Set at 15X

After curing, this glob of epoxy was subjected to a test with a hot soldering tip, confirmed that JB-Weld is much more heat tolerant than Hobby Fast Cure Epoxy.

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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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PA-47 with Fan Control

Today I breadboarded a FAN Control circuit for my PA-47 RF Power Amp (see previous post). I used a simple NTC Thermistor for the sensor, which will be attached to the FET on the Heatsink. The FAN Control circuit also supports a slight temperature bias when the TR Relay is activated.

The Thermistor can be seen attached to the FET

The breadboard FAN circuit has be running for several hour - working as expected, it cycles the FAN ON and OFF (with speed control) as the Beacon Exciter transmits on its six minute schedule. The RF output is connected to a dummy load, as I have not looked at the PA signal on a Spectrum Analyzer yet.

The Chaos of the Bench
Testing in Progress

My goal was to build the new PA with the additional FAN Control circuit small enough that it would fit the same 1 x 1.5 inch layout of the current PA-47. I had to rearrange a few parts to make it all fit.

While doing some experiments with a new layout with DipTrace, I tried laying it out as a four layer board. Four layers makes the circuit layout much easier. but it would be a lot more expensive ( about 3 to 4X ).

New two layer PA-47 boards with Fan Control are now on order.

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