0-10V therometer

Hello. I bought a PLC with analog input(0..10V), and I need to measure temperature. Thermometers with this voltage output are expansive and i dont need the measurement to be much precise or linear. Does anyone have an idea how to solve it? Could I use perhaps a thermistor or something like that?

With this solution first check that the analogue input won't be damaged if it sees 24V, it never should see the full 24V but could if there is a fault with a resistor or the PT100 sensor. Now we come to the problem, for a PT100 sensor the maximum current to avoid self heating is 1mA, to limit the current to 1mA you need a loop resistance of R=V/I : 24/0.001 = 24000 ohms.

If you connect in series a 24000ohm resistor and a PT100 sensor you can measure the voltage across the sensor. The PT100 sensor has a resistance of 100 ohms at 0C and 138.5 ohms at 100C. Resistors in series share the voltage in proportion to their resistances. So with a 24V d.c. supply at 0C you have 100/24100 * 24V = 0.099V, at 100C you have 138.5/24385 * 24V = 0.137V. You get a 0.382V difference for a 100C change. It is unlikely that this will give you any kind of sensible result.

I would be looking at using a converter/transmitter like these:
http://cz.rsdelivers.com/product/lkmelectronic/lkm-214-3/transmitter-lkm-214-3/3707940.aspx

http://cz.rsdelivers.com/product/lkmelectronic/lkm-103-2/transmitter-lkm-103-2/3707748.aspx

Don't know what your budget is. you can look at these and maybe they will help you out.

http://www.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners/884114

and you will need this as well.

http://www.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners/884501

I have used these with good results.

Jordan

Thank you for the advices, but these solution are more expensive than a new thermometer with output 0-10V. So it seem that it would be a good idea just to buy it.

These are 4-20 mA loop powered, but quite reasonably priced:

http://www.electro-sensors.com/index.php/cPath/32

http://www.dillinstruments.com/dillfinalpic_007.htm

brian it is 0.0385 volts difference as low is 0.1 volt and 100 is 0.1385 volts.
if you have 10 volts max it means 0.4 percent of full scale
i use only a XTR105 costing about 10 euro/piece it has linearization and needs no external power, just 24 volts and you will need a resistor of 250 ohms.
it will be very precise
if needed just ask and i will send you more info on it.

That sound good. The output of this homemade thermometer should be voltage(around 0-10V) (the analog input on the PLC is 0-10V). Send me please some more info about that.

There are a number of inexpensive temperature sensing ICs on the market. Most have a limited operating range but they are very inexpensive.

Here is one that is less than US $2.00
http://www.smarthome.com/1523/Temperature-Sensing-IC-Chip/p.aspx

Try a Google search using temperature sensing IC.