temp sensor using diode

[RenesasT]

Hello All,

I am new to this forum. Please consider my question even if it is a silly one.

In my project I want to detect temperature of particular vessel by using MMBT3904 transistor. I am using SMD transistor. I have connected base of transistor to the ADC pin of my micro controller and emitter of transistor to ground. Collector of transistor is open.

For getting temperature value at ADC pin, the emitter should require bias current. Once emitter will be biased, i will get ADC values of temperature.

Now thing is that, suppose I am biasing it with Digital multi meter then I am getting adc value as 145 at room i.e temperature of room (as my ADC is of 10 bits ) and the value in digital multi meter is 0.665 V. Generally digital multi meter has bias current as approx 100 micro Amp.

I don't want to connect digital multi meter. I have to connect constant current source of 500 micro ampere current with the same adc value i.e 145.

Please help me with this. If other information is required, please let me know.

Awaiting for your reply.

 

[schmitt trigger]

You could use a LM334 constant current source. Very simple to use........... Only a current-setting resistor is required.

But since the LM334 has a linear and accurate temperature dependency, perhaps it would be even better to remove the transistor completely and use the LM334 as the temperature sensor itself.
Your choice.

 

[KlausST]

Hi,

I recommend to additionally connect base and collector, then the voltage reading is more stable.

There are ready to buy circuits for this. Especially some processor supervisory circuits include them.

If you want to build it on your own, then I recommend a simple resistor to generate bias current.

Klaus

 

[c_mitra]

...perhaps it would be even better to remove the transistor completely and use the LM334 as the temperature sensor itself...

This is an ideal temp sensor in the sense that it gives directly the absolute temp without the need for any reference. In addition, because of the band gap nature of the device, the variation from device to device is rather minimal.

Accuracy may not be great but you will get excellent reproducibility...