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LM317 Current Loop (LM34DZ)

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.

Reply to
Bill Stock
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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

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Hope this helps, Brian

Reply to
Brian

LM34

input

appreciate

Thanks Brian, talk about service. :)

My understanding of the (4-20) current loops was that they provided a two wire interface immune to noise. But if I still have to decouple the sensor, then I might as well run 3 wire over the 100 feet?

Reply to
Bill Stock

What has to be done, depends on how noisy the environmet is, it has to work in. Sometimes, this can only be found by trial and error.

Reply to
Brian

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.

Reply to
Fred Bloggs

the

wire to

tieing

any

I ran 4-wire for a serial device, but it got RMA'd. So I'm using that for my LM317/LM34, while the serial device get's fixed up. I realize that this is not the ideal wire, but I didn't plan on this problem. I guess I should ground the two unused wires in the mean time.

Reply to
Bill Stock

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.

Reply to
Fred Bloggs

Hi Bill,

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

Cheers Terry

Reply to
Terry Given

Since this is a relatively slow dc signal you are looking at, probably all you need is a rc decoupling network at the receiving end. Something like a 2K resistor and a 4.7 uf capacitor. Brian

Reply to
Brian

I have no idea what "4-wire" is. Use twisted pair, and yes do ground unused wires

Cheers Terry

Reply to
Terry Given

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