Control of heater circuit part2

[james]

Thanks for all the help provided and pointing me in the right direction.

After some research I have come up with a schematic (posted in
alt.binaries.schematics.electronics) which I think sort of works! Is there
possible to improve the circuit? Can anyone suggest improvements.

The object of the circuit is to control a heating circuit to heat to a
temperature depending on the external temprature.

Above 20deg C the circuit is OFF, below 20deg the circuit starts to produce
an output to trigger the heating to a setpoint value, when this is acheived
the control output goes to low untill the internal temp falls below the
setpoint value calculated from the external value.

There is a gain control to adjust the ramp how quickly the internal setpoint
reaches max (i.e least sensative switch on below 20deg, setpoint reaches
80deg with ext equals minus 20 or below, most sensative switch on below
20deg, setpoint reaches 80deg when external equals 10deg or below)

The thermistor used has a known resistance calculated using the steinhard
and hart equation, from three know temp/resistances.

Temp Resistance
-20.000 45070.354
0.000 15280.000
20.000 5868.665
40.000 2505.081
60.000 1170.041
80.000 590.362


The range of external temps is -20 to + 20. The range of internal temp is
+20deg to +85 deg.

The basic principle is to reduce setpoint temp as external temp increases
rather than produce exact internal temps for external temps.

The gain circuit appears to do the job (adjustable from 3.3K to 20K to vary
the gain over the range), however the side effect is the it appears to
amplify the non linerity of the thermistor at the same time. The voltage
change/temp when the variable resistor is at 3.3k is fairly linear - but
less so at 20k

I have used resistors in the thermistor potential divider calculated to be
(resistance at higest temp*resistance at lowest temp)sqrt which places the
thermistor in the middle of its operating range which should help things.

The circuit works by taking the difference in the voltage from the
thermistor potential divider and and the voltage if the thermistor would be
at 20deg using an opamp. this output is then amplied by the adjustable gain
in the second op amp, finally the output is compared to the output from the
internal thermistor potential divider using a third op amp and and LED lit
if heat is required.

Are there any simple ways to improve this circuit? Any I on the right lines!

Any suggestions welcomed.

Cheers
James

 

[MV]

Hi James

you sound like you are on a reasonable track if you really want
internal temperatures to get higher when the external temp gets lower
but why not work to a set point ? Is it not the case that you want to
just work the heating element harder as it gets colder ? If so a pulse
width modulation scheme might make more sense. There are chips that
produce a triangular waveform that are DC shifted via an error signal.
When the tips of the triangular wave break through a certain trip
level you turn on the heating element - therefore as it gets colder
the error signal is greater turning on the heating element for longer
pulses. However as the desired temperature is reached the error signal
should drop and the pulse width will reduce - with feedback an
equilibrium is found. Have used this scheme well with peltier heat
pumps where a lower trip level is used to cool aswell.

If you don't mind telling what is the real world application for this
?

Sledgehammer approach for non-linearities etc - small micro-controller
i.e. PIC and a lookup table could make a one-off much more accurate.

MV

"james" <jamenospam@net.net> wrote in message news:<SDTdc.56806$Id.13909@news-binary.blueyonder.co.uk>...

Thanks for all the help provided and pointing me in the right direction.

After some research I have come up with a schematic (posted in
alt.binaries.schematics.electronics) which I think sort of works! Is there
possible to improve the circuit? Can anyone suggest improvements.

The object of the circuit is to control a heating circuit to heat to a
temperature depending on the external temprature.

Above 20deg C the circuit is OFF, below 20deg the circuit starts to produce
an output to trigger the heating to a setpoint value, when this is acheived
the control output goes to low untill the internal temp falls below the
setpoint value calculated from the external value.

There is a gain control to adjust the ramp how quickly the internal setpoint
reaches max (i.e least sensative switch on below 20deg, setpoint reaches
80deg with ext equals minus 20 or below, most sensative switch on below
20deg, setpoint reaches 80deg when external equals 10deg or below)

The thermistor used has a known resistance calculated using the steinhard
and hart equation, from three know temp/resistances.

Temp Resistance
-20.000 45070.354
0.000 15280.000
20.000 5868.665
40.000 2505.081
60.000 1170.041
80.000 590.362


The range of external temps is -20 to + 20. The range of internal temp is
+20deg to +85 deg.

The basic principle is to reduce setpoint temp as external temp increases
rather than produce exact internal temps for external temps.

The gain circuit appears to do the job (adjustable from 3.3K to 20K to vary
the gain over the range), however the side effect is the it appears to
amplify the non linerity of the thermistor at the same time. The voltage
change/temp when the variable resistor is at 3.3k is fairly linear - but
less so at 20k

I have used resistors in the thermistor potential divider calculated to be
(resistance at higest temp*resistance at lowest temp)sqrt which places the
thermistor in the middle of its operating range which should help things.

The circuit works by taking the difference in the voltage from the
thermistor potential divider and and the voltage if the thermistor would be
at 20deg using an opamp. this output is then amplied by the adjustable gain
in the second op amp, finally the output is compared to the output from the
internal thermistor potential divider using a third op amp and and LED lit
if heat is required.

Are there any simple ways to improve this circuit? Any I on the right lines!

Any suggestions welcomed.

Cheers
James