LM334 based tempreature gain circuit

[james]

Hi,

Thanks for the schematic for a circuit to do the job, certainly a lot
simplier than the multiple opamp and thermistor version.

I have decided to base the circuit on this design, but I have a couple of
queries.

The schematic specifies LM334 as the temp sensor, however upon looking at
the datasheet these sensors only have a operating range of 0deg to +70deg c.
The
required circuit operated beyond these ranges!

Does the LM334 not require an adjustment input, the sensor is a three wire
unit - or is two wire input ok?

Regards

That will work just fine, but when you consider the +/-20% typical
uncertainty in "thermistance"- think of all the calibration work you
will have to do- and given the $1 pricetag of the LM334- I don't see any
cost savings in using thermistors. In simplified form, the LM334
controller would look like this:
Please view in a fixed-width font such as Courier.

. V+
. |
. +---------------------+
. | |
. | /
. | Rb 20oC ADJ
. | / +--/\/\/\/\--+
. | \ | | |
. | | | | |
. | V(20oC)---> +-------------+----|<|-----+
. | | LMV431 |
. | | |\ com
. | +-------|+ \
. | | >-----+
. | +-LM334-+ +-------|- / |
. | | | | |/ _ |
. | |I(Text)| Vext | /| |
. | | |------+--o--o----/\/\---+---+
. +------| ----> | | cal / |
. | | | / Rg |
. | +-------+ Rc |
. | sensor E / |
. | \ |
. | | | |\
. | com Vsetpoint-> +---|+ \
. | | >-> RLY DRV
. | +---|- /
. | +-LM334-+ | |/ ON
. | | | | +---
. | |I(Tint)| Vint | OFF |
. | | |------+---------------------+ --+
. +------| ----> | |
. | | /_
. +-------+ \/| Rc,adj
. sensor I /
. /\
. |
. com
.
.
.
Calibration consists of submerging both sensors in ice bath, opening the
"cal" jumper, and adjusting Rc,adj for Vint=Vext. Call this voltage
Vint,cal. Then since the LM334's have no offset error ( by very clever
design making their bias current a fixed fraction of the temperature
dependent current), you will have Vint(T)=Vint(0oC)*(1+T/273.15) where
T=any temperature oC. From this you know that V(20oC) must be adjusted
to 1.073 x Vint,cal at ice bath 0oC. Replace the cal jumper and you're
in business. The circuit pivot gain will be G= Rg/Rc and Vsetpoint= G*(
V(20oC)- Vext) + V(20oC), and since the sensors have been adjusted to
identical voltage/temperature gains, this is equivalent to Tint= G* (
20oC-Text)+ 20oC which was the original requirement.

 

[Fred Bloggs]

How many times are you going to post this? I answered under your
"Control of heater circuit part2" thread.

james wrote:

Hi,
....

[...snip...]