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resistor in series and parallel

D

daveatfernie

Jan 1, 1970
0
Hi everyone,
I've a NTC thermistor and I'd like to change its response curve.
Am I right in thinking the following:
I can linearise the thermistor using a series resistor.
A resistor in parallel will change the gain of the thermistor
A resistor in series with the thermistor will change the offset of
the thermistor.

Many thanks in advance

David
 
R

Rene Tschaggelar

Jan 1, 1970
0
daveatfernie said:
Hi everyone,
I've a NTC thermistor and I'd like to change its response curve.
Am I right in thinking the following:
I can linearise the thermistor using a series resistor.
A resistor in parallel will change the gain of the thermistor
A resistor in series with the thermistor will change the offset of
the thermistor.

The equations are known. Just pull
up the math package.

Rene
 
daveatfernie said:
Hi everyone,
I've a NTC thermistor and I'd like to change its response curve.
Am I right in thinking the following:
I can linearise the thermistor using a series resistor.
A resistor in parallel will change the gain of the thermistor
A resistor in series with the thermistor will change the offset of
the thermistor.

Many thanks in advance

David

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David,

Usually, I think of a *parallel* resistor as the thermistor's
linearizer. And a resistor in *series* with the thermistor (i.e. with
the linearizing resistor in parallel with the whole series combination)
will change the factor by which the temperature changes the overall
resistance. (Think of the series resistor as making the thermistor, and
its resistance change vs temp, a smaller percentage of their series
combination). Another resistor can also be added in series with the
entire parallel network, to obtain a desired total resistance.

A pretty good way to do what you want, that I've used recently, is to
use LTSpice to get the thermistor parameters and to verify your
linearization (or response-shaping) results. (Note that for an
existing (on-hand) thermistor for which you don't have an LTSpice
model, you could use actual resistances, measured at three
temperatures, with the equation below, to linearize it.)

Do the following:

Download LTSpice (aka SwitcherCADIII) from http://www.linear.com.

Go to http://www.yahoogroups.com and join the LTSpice group.

Go to the files section and get the ntc.lib file, which has lots of
Spice models for Epcos thermistors (which are actually available to
purchase, through Digikey.com), which also include self-heating
effects. There are also some very good thermistor examples/tutorials,
available there. (The ntc.lib file is, I assume, also available from
Epcos' website.)

Make a thermistor test fixture, in LTSPice, with a voltage source (v1)
driving your grounded thermistor (and, later, your thermistor/resistor
network). You can use a 1-volt sine at 60 Hz for the voltage source.

Label the ungrounded (i.e. driven) node as "OUT". Plot V(out)/-I(V1).
(i.e. Run it and then right-click on the axis label at the top and
change it to V(out)/-I(V1). Then save your plot settings.)

Use a TRAN statement like: .tran 0 .0002 .0001 .00000001

Step the temperature through three equally spaced temperatures, over
the range you want to linearize for, with a statement like: .STEP TEMP
0 50 25 (i.e. 0C to 50C in 25C steps).

Put the desired thermistor (or thermistor with series resistor) in the
circuit (i.e. from the voltage source to ground).

Run the simulation.
From the plot, measure the resistances at the three temperatures. Zoom
in if you need to. Place the mouse cursor where you want to measure
and then read the resistance at the lower left.

Use the following equation to calculate the value of the parallel
resistance that will linearize the thermistor's temperature response
(for resistances measured at 0, 25, and 50 degC, as stated; but change
the temps if needed):

Rp = (R0*R25 + R25*R50 - 2*R0*R50) / (R0 + R50 - 2*R25)

Put the calculated parallel resistance, Rp, in the circuit and re-run
the simulation.

Verify that the response is what you need at each temperature.

If needed, add a resistor in series with the whole thing, to get a
desired total resistance.

If you're trying to get a specific curve shape, or specific resistances
at specific temps, you may have to iteratively tweak the resistor
values and re-run the simulation.

If anyone would like a copy of my linearize_thermistor.asc LTSpice file
(described above), email me (tomg at fullnet.com).

Regards,

Tom Gootee

http://www.fullnet.com/u/tomg

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