"John Fields" schreef in bericht
...
On Mon, 5 Apr 2004 14:05:15 +0100, "soundman" wrote:


"John Fields" wrote in message
.. .
On Sun, 04 Apr 2004 23:09:12 GMT, "James" wrote:

I am trying to design a circuit that will vary the temperature of a
heater
dependant on the temprature of an external input.

The circuit would involve two thermisters (one on the temp input and
one
on
the output)

The logic needs to be something like this:

Outside 20deg - Inside heated to 20 deg then switch off.
Outside 10deg - Inside heated to 50deg then switch off.
Outside 0 deg - Inside heated to 80deg then switch off.

There also need to be an adjustable "shift" on the curve so that it
can
be
varied downwards to something like:

Outside 20deg - Inside heated to 20 deg then switch off.
Outside 10deg - Inside heated to 30deg then switch off.
Outside 0 deg - Inside heated to 40deg then switch off.

Hopefully the ascii chart below makes it clearer!

Would an adjustable resistor on the thermister be a way to acheive
this?

IIRC correctly for my design days a thermister produces a variable
resistance which does not change proptionally to temprature, but can
be
made
"more linear" with an appropriate resistor? Is the temp range required
(40deg - 20deg to -20 for outside, or 60deg - 20deg to 80deg) small
enough
that a linear equation will work (would guess I need accurancy to
about
5deg
or so)

How would people here go about acheiving this? Hopefully the solution
is
simpler than it sounds!

EXT
80 x x
70 x x
60 x x
50 x x
40 x x
30 x x
20 x
xx20.....15.....10.....05.....0.....-5.....-10.....-15......-20
Heat to:

I know there aren't supposed to be any stupid questions on
sci.electronics.basic, but since you decided to crosspost this to
sci.electronics.design you need to know that this is one of the most
poorly thought out pieces of **** ever posted here. You say one thing
and your graph reflects something entirely different, your description
of what you want is severely distorted by what you think you know, and
your "hints" about how we should go about achieving your nebulous goal
are, at best, insulting.

Why don't you try to figure out what you want and just ask for that
instead of pretending you know what you're talking about?

For example, in one case you state that if it's 10° outside you want
to heat something to 50° and then switch off. Forever? Or do you
want to keep the thing heated to 50°? Or what?

GRRRR....

--
John Fields

One of the things about people posting questions here is that they don't
have all the answers and they don't have a full understanding of the
problem
they are taking on. Sometimes, this is reflected in a less than perfect
question or a question that doesn't fully cover the problem. But,
remember,
if a poster had the knowledge and understanding to fully describe his
problem, to know exactly what questions to ask and has the experience to
know exactly how much information to provide - then he probably wouldn't
be
asking the question

---
Bull****. A query is properly phrased, "Here's what I want to have
happen", can you please help me?" , not "Here's what I want to have
happen, and here's how I want you to make it happen because here's
what I think I remember. And, by the way, here's some meaningless
garbage to make it even more difficult for you."
---

John Fields, however, sees the partial knowledge and ideas of the
original
poster as insulting.

---
I find his assumptions irritating.
---

As I see it, he had three options: One is to ignore
the question, two is to reply and ask for more information and to clarify
some of the design parameters and the third is to reply with abuse about
the
original poster clear lack of knowledge and berate him for offering his
ideas for the group to consider.

---
The abuse wasn't about his lack of knowledge, it was about his
abyssmal presentation of the problem and his assumption that his
opinions were important. Moreover, if you weren't so intent on
playing the incensed bystander you would have noticed that I did ask
for more information. Not by kissing his ass for it though, which is
what you seem to espouse.
---

Well, John, I'm sure the original poster feels really good about his post
now you've put him right. He won't make that mistake again will he?

---
Let's just say he'll be less likely to.
---

By the way, I took a look at John Fields company website,
austininstruments.com where it says of their service: "Offered primarily
to
individuals who believe they have a good idea but who haven't the
technical
expertise to translate their idea into reality". And he responded to
this
poster with; "you need to know that this is one of the most poorly
thought
out pieces of **** ever posted here" Good advert for the firm!

---
Good advert or not, it's true.

Also, you "forgot" to mention that our Invention Engineering (SM)
service is only one of the services we offer.
---

Now, to look at the problem:

We know from the question that the temperature has to be raised to a
specific temperature. It is also safe to assume that once the inside
temperature drops to a certain temperasture, the proces will start once
more

---
No, it's not safe to assume anything, and by stating that it is,
you've just labeled yourself as an ass.
---

- but James (the orignal poster) can clarify this.

---
Go back and _read_ my post and perhaps you'll come to the realization
that I already asked for that clarification.
---

I may just be a one shot, but the process is similar regardless.

---
You may very well be, but the process is quite different in that there
is no servo involved.
---

As another reply sets out, this is an ideal job for a microcontroller
such
as a PIC.

---
Oh gawd, another one...
---

Use one with two analogue inputs to read the temperature sensors,
whatever they might be. You will need to get to a voltage range of 0 -
5V
so there may be some signal conditioning to be done.

---
Nebulous, cute catch-all.
---

Because the PIC will
convert the voltages into numeric valaues, linear operation might not be
important. If you know that specific values represent certain
temperatures,
you don't need to know the shape of the slope if you are prepared to
accept
specific temperature ranges to trigger the heating process.

---
It's called a lookup table, genius...
---

Next, you need a single output from the PIC to turn on the heater using
relays or similar. Finally you will need an inut to vary the heating
process. A simple switched input might be sufficient for high and low
temperature settings, but another analogue input could be used if you
need a
sliding scale.

Using a PIC means that the hardware can be reduced to almost a single
chip.
If you find there are changes needed, they can be put in with a simple
software change and the process is repeatable as it doesn't rely on a lot
of
analogue components which can drift with time and temperature. The
effort
of researching the PIC methods would be repaid very quickly.

---
You're a ****ing idiot. Here's this poor boob who doesn't know his
ass from an opamp and you're sending him off to look for PICs.
---

The challenge for you is to find out how to write the software for the
PIC.
Alternativley, you can get this done by system designers or developers
who
have the expertise to write the code and test it quickly. There's a firm
in
Texas called Austin Instruments for example that might be able to help,
but
I suspect you won't be going to them.

---
The real challenge for him is to avoid taking advice from know-nothing
assholes like you who are willing to lead him down the primrose path
without the slightest intention of supplying him with a solid solution
for his immediate problem.

If he wanted to come to us we'd be glad to help him, and he'd leave
with a _real_ solution, not the kind of pie-in-the-sky-crap you
advocate.

--
John Fields

John,

You may be right or wrong, but *I* consider the abusive language you used
much more irritating then the posts you answered. Besides, you made your
opinion clear enough but what else - if anything - you want to achieve?

petrus


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