LM34 not showing any reading | LabJack
 

LM34 not showing any reading

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embayweather
embayweather's picture
LM34 not showing any reading

I am trying to connect an LM 34 to my U3 HV to measure temperature, but the output is overwhelmed by the 0.4 volts showing on the AINx. I suspect it is down to an earthing problem. I have connected the sensor diretly to the pins of the Labjack and also via a 2k resistor, and I am still getting the same problem, making the sensor useless. I have alos tried connecting another AIn to ground which again did not solve the porblem.

Is there any further advice you could give me please?

 

Mike

LabJack Support
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Make sure you are using a low

Make sure you are using a low voltage analog input (FIO/EIO).

We always recommend a load resistor ... a 10k from Vout to GND.

https://labjack.com/support/app-notes/temperature-sensors

 

embayweather
embayweather's picture
Thanl you for the thoughts. I

Thanl you for the thoughts. I have an inputof approximately 0.6v from the sensor. i will try a 10k reisstor again, but last time it was no different.

 

Mike

LabJack Support
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I'm talking about a load

I'm talking about a load resistor, not a series resistor.  Sometimes you need both, but the load resistor is usually the key one.

Your observation sounds exactly like what we see when we connect a raw LM34 or LM35 to a high voltage input on a U3-HV or T4, without a load resistor.

 

embayweather
embayweather's picture
That is in fact what I had

That is in fact what I had tried following advice I found on the website/forum. I will try again in case I had not done it correctly. Many thanks for the clarification.

 

Mike

 

embayweather
embayweather's picture
I have tried that again and

I have tried that again and it simply wipes out all functionalilty. The background voltage disappers as does any voltage from the sensor. I have tried various resistors of the same value and get the same result. Is there anything else you could suggest please?

 

Mike

LabJack Support
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So you have the sensor

So you have the sensor connected to VS, FIO4 and GND, and FIO4 is always measuring about 0.4 volts when configured as low voltage input?  And then if you add a 2k or 10k resistor from FIO4 to GND you still see 0.4 volts?

My recollection is that the sensor works fine by itself when shoved right into the screw terminals (no added wires) with a low voltage analog input (FIO/EIO).  On a high voltage input (AINx, worse resolution) I recall they have to have the load resistor to work properly.  We will do a fresh test with a U3-HV and T4 here.

 

LabJack Support
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Are you sure the sensor is

Are you sure the sensor is working correctly?  If you pull the signal pin out of FIO4 (leave VS/GND connected) and use a DMM to measure signal versus GND does the DMM read different?

For that matter, use the DMM to measure FIO4 versus GND while the sensor is connected and see if the DMM reports different than the T4.  If so, look at the signal with a scope if possible to see if it is oscillating.

Also, note whether the T4 readings change when you connect the DMM.

 

embayweather
embayweather's picture
I have tried different

I have tried different sensors to be sure. I will try as you suggest although I am unsure what the T4 readings mean.

Thank you for your help.

 

Mike

embayweather
embayweather's picture
I have retried your

I have retried your suggestion again,  and I am still getting the same results. Removing the data pin I find the voltage measure on the meter is exactly the same as that on Labjack. The voltage varies considerable between about 0.2 v and 2.0 volts. It seems I am getting nowhere fast. Any further ideas would be most welcome.

 

Mike

LabJack Support
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We did some fresh testing

We did some fresh testing with an LM34 connected directly to a U3 as well as a T4 and a T7.  All connected directly to a device without any additional resistors between VS and GND.  Temperatures were correctly measured down to 0 degrees F which is what our datasheet specs.

Can you connect a DAC line to an input on your device and make sure the AIN values are properly reading the set "voltage".  On a T4 and U3-HV channels AIN0 -> AIN3 are high voltage analog inputs allowing them to easily read ±10V signals.  The other flexible I/O lines can be configured as low voltage analog inputs (0 to 2.5V).  If you are confused as to what an analog input is, our analog input FAQ page may help answer your question.

embayweather
embayweather's picture
I am beginning to think the

I am beginning to think the Labjack is faulty.  One of the VS terminals has no voltage at all, another is intermitent. Connect a new LM34DZ to the terminals directly I get a steady 1.4 v output measured using a Labjack Variable window. The DMM confirms this value almost exactly. The value does not change, even when warming the sensors with my fingers. Remove the sensor and I get 1.3 v volts which is the same on the other AIN*. I have tried the output of the sensor going into all the available ports with exactly the same result. I can tell you that the internal temperature channel works perfectly.

Is there any way to test this theory?

 

Mike

LabJack Support
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I have retried your

I have retried your suggestion again,  and I am still getting the same results. Removing the data pin I find the voltage measure on the meter is exactly the same as that on Labjack. The voltage varies considerable between about 0.2 v and 2.0 volts.

If you have 5V/0V on the correct pins of the LM34, but the signal pin is not giving you the expected ~0.7 volts at room temperature, and the DMM & U3 agree on this fact, it suggests to me your LM34 is not working correctly.

Make sure you have identified the correct pins for 5V/0V.  There is a picture on our LM34 product page that shows one connected right into a screw terminal block and you can see how the leads have to bend funny.

https://labjack.com/accessories/lm34caz-silicon-temperature-sensor

 

One of the VS terminals has no voltage at all, another is intermittent.

See the following:

https://labjack.com/support/app-notes/screw-terminals

 

I get a steady 1.4 v output measured using a LabJack Variable window. The DMM confirms this ...

1.4 volts is normal for a floating high voltage analog inputs:

https://labjack.com/support/app-notes/floatingunconnected-inputs

The signal line from a properly functioning LM34 has no problem overpowered that an driving the analog input to the proper voltage.

Also, note that a low voltage analog input on the U3 is better for the LM34.

 

LabJack Support
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"One of the VS terminals has

"One of the VS terminals has no voltage at all, another is intermitent"
I assume you are checking this with a DMM.  Please connect the leads of your DMM to the screw terminal input itself and make sure you aren't connecting it to the top.  Securely tighten the leads into the screw terminals when checking.

Regarding your comment about reading a steady 1.4V on the related AIN channel and then removing the sensor and getting 1.3V.  Are you using the AIN0-AIN3 channels?  Please try connecting it to only AIN4.  The LM34's output can easily go unstable when presented with a capacitive load or lower impedance AIN lines which is the case for the AIN0-3 lines as well as in your situation where you have connected the output of the LM34 to all of the AIN channels.  

embayweather
embayweather's picture
I apologise for the delay in

I apologise for the delay in replying but our internet went down in a storm.

I have tried all the things you have suggested and I am still getting not results. I connected to AIN 4 and got a voltage of around 0.3 v competely unaffecetd by connecting the LM 34 to it or not. The same applied to another LM34 DZ too. The power output from the Labjack seems to be there once I tried your suggestions, but I am still convinced this unit is faulty. I have set these up before on other Labjacks and never had this problem. I really am at a loss to now what to do save buy another on.

 

Thank yu for your contining help.

 

Mike

embayweather
embayweather's picture
Just tried another sensor so

Just tried another sensor so I now have LM34 sensors in three of the AIN ports, not respond to temperature changes.

 

Mike

LabJack Support
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I suggest you put in for an

I suggest you put in for an RMA and send your U3-HV to us so we can check it out.  Leave 1 or 2 LM34 connected so we can look at the same thing you are seeing:

https://labjack.com/about/returns