lm35 problems

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fisherman

New Member
I have a couple of lm35 temp sensors and they are inaccurate by 8 to 10 degrees. they are fed with 5v through a 4.7k series resistor. What am i missing here. I have not connected wires to the adj pin as I thought it would not be needed.
I have even tried a sensor without a cable, just the protoboard and still about 8 C off.

Fisherman
 
L

lanternfish

Senior Member
Okay. Couple of Q's.

1. What Picaxe are you using, and connections?

2. Can you post your program?

With a range to 150degC and 10mV/degC the maximum output voltage will be 1.5V (or thereabouts) . Assuming a 5V supply to LM35 and PICAXE, the adc will only be reading approx. 1/3 of its range, or 0 to 85. An opamp will help.

3. Have you read the datasheet for LM35?

cheers
 
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moxhamj

New Member
Are they definitely LM35s as the schematics I've seen for that part don't have a series resistor - they run directly off the 5V. Maybe that is the problem, and the series resistor means the IC isn't getting enough power?

http://www.national.com/mpf/LM/LM35.html says supply volts 4V to 30V
 
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fisherman

New Member
A couple of errors on my part Post should have read lm335( i assumed the two were the same but differnt range)
reading were in K and not an exactly a direct conversion to c
Have ordered the ds 18b20 direct readtemp sensors. It seems they will make life easier

tx
 
premelec

premelec

Senior Member
LM335 accuracy... with ADC

It is very important to note that the ADC reading on the PICAXE is dependent on the supply voltage - so if you want high accuracy you must have a stable known supply voltage...

Or at least stable since you can finagle your math in the PICAXE to deal with the actual stable PICAXE supply. Tghe LM335 should work fine within these parameters...

You are right that the DS18B20 will not have the supply voltage problem and will give a straight digital reading... however it is a bit slow on it's reading if that is of import to you.

Have fun!
 
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manie

Senior Member
LM35DZ wired with by-pass cap and R's as per the datasheet worked a treat for me in even in 'hostile' chook-house environment. Also for accuracy, you will require good thermal transfer to the sensor, possibly a heat soak fin attached for good transfer. I find they work well.
 
B

BeanieBots

Moderator
As pointed out by premelec,
How good is your 5v supply?
Is it from a regulator and do you have caps both sides of the regulator?
If your supply is low, the sensor will read higher than it should.

@manie, I wouldn't have thought a chook house was very hostile to sensor readings. Not compared to a nice clean office in the middle of town with all it's flourescent lights, mobile phones, internet routers, fax/copy machines, microwave ovens, nearby taxis and other local radio transmitters.
 
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manie

Senior Member
BB: Like you said "a nice CLEAN office...", don't do much farming do you ? :):rolleyes:And then with super-charged electric fences (just to keep "casual" entry at bay...:eek:) and the like, the word "hostile" does come to mind. I'm sure Stan can expand on this...;)
Edit:
super-charged - read +15kv.......
 
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