[SOLVED] NTC based thermistor is not working with transformerless PS

[Mithun_K_Das]

I'm working with some control circuit which is driven directly from phase line but a transformer less power supply(DC 5V) is used. The full circuit contains live voltage. The micro-controller is working well. But when I wanted to interface NTC based thermistor. the thermistor is not working. its becoming colder(at the sensor head).

Can you tell me why? the sensor I'm using is:

 

[barry]

The sensor is becoming colder because it is snowing. This is obvious because the leads shown in your very nice, informative picture are blue.

Seriously, how are we supposed to know what's going on when you don't give us any useful information. "The full circuit contains live voltage"- what is that supposed to mean? The microcontroller is working well. Oh, you have a microcontroller? Does that have anything to do with your thermistor?

 

[Mithun_K_Das]

The circuit is like this. Hope you are familiar with this type of ckt.

 

[KerimF]

Sorry, but are you sure the supply is drawn properly?
I mean the loop:
Phase - R1 - C1 then back to Nutral!

 

[barry]

That sure doesn't look right.

 

[Mithun_K_Das]

That is not the main topics. A micro-controller based system is running well with that PS. The main point is the sensor.


***** Also you don't need to worry anymore. I've solved the problem.

 

[FvM]

That is not the main topics. A micro-controller based system is running well with that PS. The main point is the sensor.

So you mean, you neither can draw the power supply circuit correctly nor show us how the sensor is connected. But want to know why the circuit isn't working?

 

[schmitt trigger]

Mithun;

Both Kerim and Barry had a valid point. If your question is that a particular thermistor is not being read properly by the microcontroller, you should provide three things: a) the thermistor datasheet, b) the actual circuit connections to the microcontroller, and c) the code snippet where you read the ADC port.

Instead, you send an INCORRECTLY DRAWN power supply. And become upset when other posters point that.

 

[KerimF]

Please note that if making silly mistakes during a design is a sort of a crime, then you can be sure I am perhaps the greatest criminal on earth since my first design (35 years ago) till these days And I challenge you that you can beat me in this but, at the same time, being so did never stop me going from one design to another more complex while learning continuously from my new mistakes, silly or not.

 

[Mithun_K_Das]

The whole circuit is too much big to draw here. Also the main problem is with the sensor. What happened really is,

The sensor shown in the image is resistive type. The sensor is used as a voltage divider with a serised fixed resistor. So what should happen here? The voltage in the voltage divider should change with the change of the temperature. But the sensor was acting like an open circuit. That is the main problem. As the whole circuit having live voltage(215-220V) so I asked if there is any problem with the sensor working in this range although the circuit operating here is bused by 5VDC.

And I posted wrong circuit diagram, just my mistake. Sorry for that. Also I've solved the problem. Putting a zener diode across the thermistor. Its working well now.

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Also a R-C filter is added between the sensor and MCU. now the voltage is changing with temperature.

 

[barry]

Aside from everything else that's been said before, putting a zener across the thermistor sounds like the wrong thing to do if you are using it as a voltage divider. Basically, you'll be reading the zener voltage regardless of what the thermistor value is. But if it's working and you're happy...

 

[Mithun_K_Das]

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Hope all of your confusion has gone now.