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.

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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
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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
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