[sdiy] Was PTC/NTC intelectuall level of SDIY!

Bruce Duncan modcan at sympatico.ca
Fri Mar 21 02:16:16 CET 2008

At 06:37 PM 3/20/2008, you wrote:
>Hi Bruce --
>Many thanks for the detailed reply.  The dual SMT transistors have 
>been mentioned here several times, but you may be the first person 
>using them seriously. I wonder if their series emitter resistance is 
>different from the usual thru-hole parts.  Those require a fair 
>amount of compensation.

On the data sheet the figures for hie look similar to a thru-hole 
2N3904 at 1-10K. I used to use CA3086 and have noticed that they 
require less comp. than the 3086 in the same application.
I went with them because the Vbe matching is stated to be typically 
better than 1mV.

>I have spent some time with the SMT tempco's.  I measured them in a 
>special bridge/heater setup I made a while back.  The temp coeff was 
>very close to optimal.  But I had trouble getting steady 
>readings.  I found the same thing recently using one in an 
>oscillating filter.  Whenever the CV is changed, the pitch takes 
>several seconds to stabilize.  I think I have been seeing a bit of 
>self heating.  Maybe your potting method and the good coupling 
>betwen the devices  alleviates this.

I haven't noticed this phenomena but I do use a fair size glob of 5 
min. epoxy which may be sinking any minor self heating.


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