[sdiy] Potted expo converters

[Ian Fritz]

Hi Peter and list --

First: best wishes for a happy and prosperous new year to all!

Comments below.


At 07:24 AM 12/30/2003, Peter Grenader wrote:

>So, here's another lightbulb that appeared over my head that I'd thought I'd
>bounce off you all  that I'm also willing to bet ain't original, either...
>
>I remember when I was an engineering tech two billion years ago, we used to
>glue thermocouples to the windings of 1/2 horsepower electric motors using
>thermally conductive epoxy for UL lock-rotor tests. the material seemed to
>be was astonishingly heat conductive. But...we were dealing with
>thermocouples which could detect a match being lit in the next room (not
>really of course, but you know what I mean).
>
>Along the same lines, has anyone tried potting the tempco/expo converter
>pair in conductive epoxy to A) improve heat transfer from one another and B)
>aid in isolation from ambient conditions?

This is an interesting question and one that becomes especially important 
if you are trying to get extremely low drift (< 100 ppm/K).

The key goal is to make the thermal coupling between the transistors and 
the tempco resistor tighter than the thermal coupling between the 
components and the ambient environment.  Otherwise, drafts and/or thermal 
gradients will cause the converter and the tempco to be at different 
temperatures.

If you are attaching the tempco to an IC transistor array, then the thermal 
coupling is going to be mainly determined by the plastic of the IC 
package.  So it doesn't matter much how you attach it.  Thermal grease or 
ordinary 5-minute exopy will work about the same.  There would be no major 
advantage to using conductive epoxy.

However, when you glue a tempco to an IC the tempco will generally respond 
more rapidly to changes in the ambient than will the transistors.  There 
are several approaches to solving this problem.  One would be to carefully 
machine away most of the ICs package to reduce the spacing between the the 
transistors and the tempco.  I don't know if anyone has ever tried this.

Another method is to attach the tempco and then add a potting layer to 
increase the isolation of the tempco from the ambient.  The drawback of 
this method is that self-heating of the tempco could become an issue.

A third method is to condition the ambient environment.  This involves 
putting either the transistors and tempco or the whole circuit board inside 
an enclosure that reduces drafts and temperature gradients.  While 
developing the dial-a-tempco converter, I found that simply mounting the 
whole converter circuit inside an aluminum enclosure seemed to work quite well.

A metal enclosure provides a relatively isothermal environment, while at 
the same time shielding the circuitry from external drafts.  As long as a 
minimal amount of ventilation is allowed, there will be no heat buildup 
within the enclosure.

A small on-board metal enclosure should also work well.

Hope this helps.

   Ian