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measure analog output (0- 135 ohm ) of a thermostat using mcu.

Can you be more specific about what you're trying to do? For example, it's hard to imagine a resistive sensor that goes to zero ohms for "normal" temperatures??? How accurately do you have to know this resistance? Is one end of the resistor grounded? You can put a resistor in series and stuff the middle into your mcu a/d. That will give you the ratio of the resistors. From there it's math. But, viability depends on accuracy required, how much current you can waste, does the sensor heat up when you put current into it?

IF this is the knob you turn to set the temperature, replace it with a pot. Sensing the center voltage is easy. mike

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Reply to
mike
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hi,

iwant to measure a resistance signal(o-135 ohm) from a thermostat using mcu.For that i have to convert this signal to 0-5v voltage signal.is there any way to do that? can i use current sourse or any resitance network? please help me..

thanks

Reply to
litty

Assuming you have a rheostat (not a pot- 2 connections rather than 3), the easy way is to use a 135 ohm resistor to Vdd. You then use Vss and Vdd as reference for your ADC.

This yields a somewhat nonlinear input with respect to shaft rotation which is really easily linearized algorithmically (with an very simple equation) in your firmware. You throw away a bit or two of resolution, but with the typical 10 bit ADC, you will still have plenty for this sort of application. It's also pretty idiot-proof wrt installation errors. You can detect a broken pot connection easily (eg. input > 2/3 of FS) and default to some reasonable setpoint.

That's what I've done in commercial products for similar HVAC applications.

Reply to
Spehro Pefhany

A current source. For the quoted resistance, nominally 37mA. However you should remember that the resistance will be nominal, and a small margin should be provided, so perhaps 35mA would be 'safer', to give 0-4.725v.

Best Wishes

Reply to
Roger Hamlett

Using 37ma may cause self heating that will change the resistance being measured

I would use 5ma followed by an amplifier to get to the voltage you need

Of course this only works if the resistor is not connected to any other circuitry

Dan

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Reply to
Dan Hollands

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