Re: NTC
- From: "kell" <kellrobinson@xxxxxxxxxxxx>
- Date: 27 Oct 2006 17:26:08 -0700
John Popelish wrote:
kell wrote:
I got a heater with a ceramic heating element that has a negative
tempco. This kind of bugged me out, but I decided to make lemonade out
of the lemon and design a circuit to keep the temperature of the
heating element confined in a window.
It uses an op-amp (or a comparator, with the addition of a pull-up
resistor) and resistors in a bridge configuration is such a way that it
regulates current and keeps the resistance of the NTC conductor
constant, which will keep the temp constant also.
If the heating element ever gets up to heat (!), its resistance should
cycle in a narrow range about the value
Rs(R2/R1). The feedback resistor on the positive input of the op-amp
should have a much higher value than the resistors in the divider.
V in
|
|
,---------------+---------------------,
| |
| |
/ /
\ \NTC
/R2 /heater
\ \
/ /
| |
| |
| ,--/\/\/\/---, |
| | | |
| | |\ | |
+-----+--|+\ | 10 |--'
| | >------+--/\/\/\/--+--||
| ,---|-/ | |--,
| | |/ / |
| | \ |
/ | /15k |
\ | \ |
/R1 | | |
\ | | |
/ '----/\/\/\/--------------+------+
| |
| /
| \
| /Rs
| \
| /
| |
'--------------+----------------------'
|
ground
I haven't built this circuit yet. Shoot it down if it won't work.
Before I put too much energy into this circuit, I have a
question for you. How do you know the heating element has a
negative temperature coefficient? Have you checked this at
anything near the normal temperature the heater was intended
to operate at?
I ask this, because many heating elements that are intended
to be self regulating positive temperature coefficient
devices (and have a very high positive temperature
coefficient at some transition temperature), have a much
lower negative temperature coefficient at low temperatures.
I measured resistance at 32 F, room temp, and 212 F.
R at 32 F: about 2.5 ohm
room temp (74 F) about 1.85 ohm
212 .96 ohm
I using the Rankine scale I fitted the room temp and 212 deg data
points into the Arrhenius equation of the form
R = a exp (b/T) and got
R = .0762 exp (1700/T)
used this equation to predict R at freezing and got a result within a
tenth of an ohm of the measured 2.5 ohm. This equation is a good model
for the behavior of this heating element between freezing and boiling,
and probably quite a bit beyond, but the heating element is meant only
to heat vegetable oil to about 170 degrees F, very loosely. Ten or
twenty degrees higher than that wouldn't do any harm. Anyway, it
certainly has a negative tempco, hard as it may be to believe that they
actually manufacture and sell such a thing. I knew something was wrong
as soon as I took the thing out and put it on a car battery to measure
current draw. The current start out low and very gradually crept up.
So now I have a good mathematical model for the R-T characteristic of
the material.
I also pencil-traced this stuff on graph paper.
By the way, after I posted the circuit I saw a mistake. The positive
feedback makes the mosfet turns fully off, which makes a muck of
current sensing.
That circuit would work without the positive feedback, but
unfortunately I can't have this thing run linear, there's way too much
power going through it.
By the way, it operates on 12 - 14 volts (automotive application).
.
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