piclist 1998\11\17\121553a >
Thread: Question
www.piclist.com/techref/index.htm?key=question
picon face BY : Peter L. Peres email (remove spam text)



On Mon, 16 Nov 1998, Scott Newell wrote:

> >For the processor however, I think that I've come up with a truly evil way
> >to read the die temperature without opening a chip. Here is the idea:
>
> I dreamed up a very similiar idea about a year ago.  Never did get around
> to trying it though...

What a man has once imagined...

{Quote hidden}

Good idea ! Now, how can one chop AND switch the current with RA4 + coil ?

> I never bothered to check, but it might be easier if you could turn on a
> diode with a negative supply.  At least then you'd have one end as a ground
> reference.

I was trying to avoid that because of the higher currents in the GND chip
bonding and grid. Anyway negative voltages are hard to make unless there
is one around or one uses the sense pin proper to drive a coil to GND then
turns it tristate at which moment the coil will kickback and generate your
reverse voltage on that output's lower protection network ;)

A series resistor is in order. I think that a 200 uH standard coil for low
current and R=470 will do fine.  The following amp must include negative
voltages in its CMMR and be fast enough to sample while the coil is still
alive.  

This looks truly evil. I think that the LM324 will accomodate small
negative voltages at the input in inverting amp configuration with the +
input GND-ed. I'll try this later. The 324 is not fast however. Hmm.
Starts to look like a PIC711 + one section of 324 + one 200 uH coil + 3
R's ;) The R in series with the coil may be replaced by a CR???
current generator. This one acts as a diode when reverse biased. We'll
see.

You can obtain a positive voltage (larger than Vdd) using the usual
infamous RA4 + coil solution. The scaling and division is required in this
case because the 2.2 mV/degree is easily getting lost between A/D
non-linearity and other temperature drifts (f.ex. the PSU regulator's and
the opamps offset drift). If the GND is clean enough, the -V method
described above may be better.

Peter

<Pine.LNX.3.95.981117180055.301B-100000@plp4.plp.home.org>

In reply to: <3.0.1.32.19981116195303.00a6c530@mail.cei.net>
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