The merlin project. Update and tip.

[Rene Schmitz]

Yell "Oh", Batz 'n' Folks :)

At 11:13 11.02.1998 +1030, you wrote:
>Y-ellow Juergen 'n' diodes.
>
>At 01:15 PM 2/10/98 +0100, Haible Juergen wrote:
>
>>Trimming (filing) in circuit would be the big deal. Imagine tuning
>>a filter bank. Measure capacitors, then do some calculations
>>and select resistors, or combinations of 2 resistors in series.
>>Lot of measurment and calculation involved. If you can trim
>>the resistors in the living circuit, just feed in a sine with the right
>>frequency and file until the amplitude at the output reaches the
>>peak. Same as with trimpots, only that you have better long term
>>stability
>>and tempco.
>
>Not to mention a hell of a lot cheaper. :) Especially when you need to trim
>a lot of them. And the idea of being able to trim them in circuit could be
>particularly useful. But I also recal that there are intergrated circuits
>that have their resistors trimmed up by laser. A-D's etc. Precision
>multipliers and the like. They trim the circuit up by shaving off the
>surface with a laser before packaging the dye. I guess it's much the same
>thing. Which begs the question. "What happens to all the microscopic
>material they trim off with the laser?" Perhaps it just burns away or
something.

Maybe one can trim them incircuit when you use one of those little vacuum
cleaners 
they make for computer keyboards.

>I guess there's only one way to find out if you can trim up a cap and that's
>by doing it. Unfortunately I don't posess a capacitance meter of any kind. I
>guess I could build up some kind of oscilator and use my counter but I'd
>still have to work out how to trim it up.
>
>If anyone's got a few spare caps and a cap meter perhaps they'd like to run
>the experiment and tell us. :)

I accidentially broke one of those ceramic disc-capacitors (if I remember
right it was a 10nF)
into two pieces. The top half was gone, and I could still use it in a little
555-based oscillator.
So it is actually possible to "trim" them, but the question that remains, is
whether one can actually file them and how precise. Juergen mentioned only
that they break little corners off, and I don't know how precise one can
break the material, of course it would be more precise, if one could grind it.
Hmm, I remember experiments in my not so long gone youth, where I used high
voltage pulses 
(from a reversed transformer) on a 50V polystyrene cap (are those self
healing!?), watching the sparks as they went thru the dielectric. Now every
spark oxidates a little of the metal foil, so the capacitance should go
lower when repeating this procedure. Fortunately the hole in the dielectric
is smaller than the hole in the foil, so the cap doesn't get shorted. 
This could also be used to trim caps. Not in circuit obviously...

Bye for now, 

Rene - who knows the secret of makeing a LED from a 1N4148 :)

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