Re: Temp readings with thermistor question
From: John Popelish (jpopelish_at_rica.net)
Date: 10/17/04
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Date: Sun, 17 Oct 2004 12:36:13 -0400
rob wrote:
>
> John
>
> This projoct is a 120 sq ft (6X20') solar air heater. Prior to
> entering heater, air gets preheated by flowing between new black steel
> roof and old tar-gravel. After going through actual heater, heat is
> blown down 3 floors to basement to a room that is 6x7x8 and FILLED
> with 45 gal. drums and large plastic pop bottles full of water. This
> thermal mass of water allows the heat to be stored and released at
> night.
>
>
> > Are you talking about ambient temperature or some heated device? 125C
> > is well over the boiling point of water.
>
> Yea...we have burst steel drums, melted plastic, baked plywood to a
> golden brown color similar to Thankgiving Turkeys, and, broken LOTS of
> non-tempered plate glass...lol ....Our new mantra now is "high temp
> materials"
> My Thermistor is a radio shack unit....no idea what it is packed
> in...I can use different ones as I am still in design phase..lol..Here
> in Atlantic Canada night time winter lows can get to -25°. The heater
> was just built when we got the 110 but did not have fans hooked up
> soooo...now that I think about it a 10K should be fine....
It is probably dipped in epoxy. It will last a while at 100C. But for
the final design, glassed encased ones would probably last longer.
And the lead wires into the hot area should probably be teflon
insulated. You can change to cheaper wire at a terminal box that is
at more reasonable temperature outside the heater.
> > Inconsequential, unless there will be lots of movement of the wires.
> >
> > Phone wire or cat 5 cable (6 conductor network cable) would probably
> > work fine.
>
> Done
>
> > > can I power it with an old cell phone
> > > adapter??....5 VDC 260mA output or would a different one be
> > > better?...
> >
> > That is probably plenty of voltage, but it should be regulated or add
> > a regulator to it, so that changes in line voltage will not shift your
> > calibration.
>
> Ahhhh...startung to get a mind grip on this...
>
> Again...Thank you
> Rob
A hint about the selection of the resistors that set the range of the
reading. The network will have the most expanded scale (change of
current versus temperature) if the resistance of the network equals
the resistance of the thermistor at that temperature. So you can
expand the temperature range of interest and compress (but still get a
reading) for extremes far from that range.
Biasing the back side of the meter with a second divider allows the
meter to read zero (no current) at an arbitrarily low temperature,
even though the thermistor will still have some high resistance and be
passing some small current. This allows you to cut off the scale at a
low temperature you don't care about.
So the circuit might be something like this: (view with fixed width
font, like courier)
+5V ---+----------------------------+
| |
R1 Rth
| - + |
+-Meter--+-------------------+
| |
R2 R3
| |
0V-----+--------+
The divider R1 and R2 set the temperature where the meter reads zero
current. They can be a decade lower resistance than the thermistor,
to keep from reducing the meter sensitivity (adding series resistance
to the meter current path). If you want to reduce the sensitivity
(expand the upper end of the range) raise the values of these
resistors.
R3 (in parallel with the meter) sets the load resistance in series
with the thermistor, to determine what temperature is most expanded on
the meter scale. Rth is the remote thermistor.
To keep from pegging the meter negative each time you switch
thermistors, you could use a separate R3 for each thermistor and put
the selector switch between the meter + terminal and each R3. Break
before make contacts.
Once you have tentatively selected a meter thermistor temperature
range combination, you have a bit of math to work through to determine
what resistor values give you what you want, and even if all your
objectives are reachable. But it is just an application of ohm's law
and the table of resistance versus temperature for the thermistor.
-- John Popelish
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