LM317 Current Loop (LM34DZ)

[Bill Stock]

I built a temperature sensor based on the LM317 current loop in the LM34
docs. I went this route, as the sensor is about 100 feet from the input
device. But the temp is jumping up and down by a couple of degrees all the
time (noise?). I went with the current loop due to the length of the wire to
the sensor/317. The one note I did read in the LM317 docs suggested tieing
VOut to the case. I haven't tried this yet.

Has anyone else used this circuit? any suggestions? I'd also appreciate any
help with the math (resistance values) if possible.

 

[Fred Bloggs]

Bill Stock wrote:

I built a temperature sensor based on the LM317 current loop in the LM34
docs. I went this route, as the sensor is about 100 feet from the input
device. But the temp is jumping up and down by a couple of degrees all the
time (noise?). I went with the current loop due to the length of the wire to
the sensor/317. The one note I did read in the LM317 docs suggested tieing
VOut to the case. I haven't tried this yet.

Has anyone else used this circuit? any suggestions? I'd also appreciate any
help with the math (resistance values) if possible.

The diagram does not show it, but you will have to bypass that 317
supply with something like 0.1uF minimum. That application circuit is
out of spec for the 317 anyway- not enough minimum current worst case
for regulation. There are other diagrams on the LM34 datasheet
-immediately after the specification pages, that show how to set it up
for remote sensing through twisted pair cable- these are also
temperature dependent current sources that overcome line length voltage
drops and they are simpler than the 4-20mA loop circuit.

 

[Brian]

"Bill Stock" <me7@privacy.net> wrote in message
news:qJidnTbDia_hkTncRVn-vw@rogers.com...

I built a temperature sensor based on the LM317 current loop in the LM34
docs. I went this route, as the sensor is about 100 feet from the input
device. But the temp is jumping up and down by a couple of degrees all
the
time (noise?). I went with the current loop due to the length of the
wire to
the sensor/317. The one note I did read in the LM317 docs suggested
tieing
VOut to the case. I haven't tried this yet.

Has anyone else used this circuit? any suggestions? I'd also appreciate
any
help with the math (resistance values) if possible.

In the data sheet suggestions, it has a de-coupling circuit. Have you
tried this? If you are not sure how to add this to your circuit, I made a
drawing on how to do it. You can see it at
http://www.fncwired.com/Temperature/
Hope this helps,
Brian

 

[Terry Given]

Bill Stock wrote:

I built a temperature sensor based on the LM317 current loop in the LM34
docs. I went this route, as the sensor is about 100 feet from the input
device. But the temp is jumping up and down by a couple of degrees all the
time (noise?). I went with the current loop due to the length of the wire to
the sensor/317. The one note I did read in the LM317 docs suggested tieing
VOut to the case. I haven't tried this yet.

Has anyone else used this circuit? any suggestions? I'd also appreciate any
help with the math (resistance values) if possible.

Hi Bill,

Have you used twisted pair for your wires? CAT5 cable is very handy
stuff, and cheap too.

Cheers
Terry

 

[Fred Bloggs]

Brian wrote:

"Bill Stock" <me7@privacy.net> wrote in message
news:qJidnTbDia_hkTncRVn-vw@rogers.com...
I built a temperature sensor based on the LM317 current loop in the LM34
docs. I went this route, as the sensor is about 100 feet from the input
device. But the temp is jumping up and down by a couple of degrees all
the
time (noise?). I went with the current loop due to the length of the
wire to
the sensor/317. The one note I did read in the LM317 docs suggested
tieing
VOut to the case. I haven't tried this yet.

Has anyone else used this circuit? any suggestions? I'd also appreciate
any
help with the math (resistance values) if possible.



In the data sheet suggestions, it has a de-coupling circuit. Have you
tried this? If you are not sure how to add this to your circuit, I made a
drawing on how to do it. You can see it at
http://www.fncwired.com/Temperature/
Hope this helps,
Brian

The circuit output is not the terminal labeled OUT on the datasheet
circuit, that OUT refers to the LM34 OUT terminal. The circuit output is
the total current drawn from the 5-30V power supply and not a voltage.