lm75a from ebay

About a half year ago I bought some lm75a temperature sensors from eBay.

I placed two in a power supply, to measure the internal temperature on two positions, to switch a fan when the temperature passes a certain threshold.

Now it appears that the measured temperature is completely off.

They drift off a lot, to high values, but the strange thing is that the both keep in pace. Also when the fan switches on, the measured temperature of both sensors drops down.

But the measured temperature is much higher than the real temperature.

Does anybody have similar experience?

Is it a penalty for buying components on ebay?

Any comments are welcome.

thanks,

Gerrit.

jw0752 over 10 years ago

Hi Gerrit, One possibility is that you are getting interferrence from your other circuitry. Make sure you have a 0.1 or similiar capacitor from Vdd to Vss as close to the chips as possible. The fact that they both act in a consistant manor leads me to believe that the problem is systemic in your unit or the way you are interpreting their output. Defective chips would not likely be consistant with each other. I have not used the LM75A myself and I see that they are rated + or - 2 degrees. If you haven't already start with the decoupling capacitors and see what happens.
John
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
vsluiter over 10 years ago

Hi Gerrit,

John's answer might be a good point to look at, but your own idea is worth investigating too. You could consider buying one or two sensors through Farnell (although for private use you'd have to go > EUR 50) to verify the sensors.
Otherwise, buying a logic analyser to see whether your I²C decoding works could be worthwile too. Depending on your budget you could buy either a bus pirate ($25 on ebay) or a *recommended by me* Saleae logic. From your name I guess you have Dutch connections, in The Netherlands you can buy the Saleae Logic from Antratek.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
gpolder over 10 years ago

jw0752 decoupling capacitors were there from the beginning, furthermore, I do expect erratic behaviour and not drift from interference.

vsluiter yes, I'm from the Netherlands, I'm thinking of switching to https://learn.adafruit.com/adafruit-mcp9808-precision-i2c-temperature-sensor-guide/overview, which is much more precise, and available here without the need to buy for EUR 50. Anyway its a good idea to have a look at the I²C decoding, a nice job for the BitScope Micro Oscilloscope & Analyzer which I expect soon in my mailbox to road test.

both thanks for the valuable comments.
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel
michaelwylie over 10 years ago

Gerrit;

This is just a thought, but is there a chance the IC has become thermally insulated after this period of time? Dust maybe?

Mike
Cancel
Vote Up 0 Vote Down

Sign in to reply

Cancel