# Electronic Circuits and Projects Forum

1. ## lm335az and resistor value

hi another question i ordered some lm335az temp sensors and after reading the datasheet i cant find out what the value of the resistor marked R is for the basic tempreture circuit shown on the datasheet does anyone know?? also i didnt realise it gave the tempreture as Kelvin so do i just convert it in software after i have done a ADC reading? thank you
logan
this is the link for datasheet
http://www.datasheetcatalog.org/data...onics/2158.pdf

the circuit drawing is on page 6 and is the top left hand drawing

2. hi Logan,
The LM335 works from -40C to +100C, output is 10mV/C rise for each 1Cdefg rise in temperature.

E.

3. Sorry my screen capture isnt working but if you look at the link in my first post and on page 6 under typical applications the drawing shows a resistor marked R1 but dosnt give a value, also it indicates that the output is in kelvin that is indicated on the same drawing
thank you mr gibbs

4. Originally Posted by Little Ghostman
Sorry my screen capture isnt working but if you look at the link in my first post and on page 6 under typical applications the drawing shows a resistor marked R1 but dosnt give a value, also it indicates that the output is in kelvin that is indicated on the same drawing
thank you mr gibbs
L,
You want to have ~1mA flowing thru the LM335
At room temperature [say 25C] the LM335 will have ~+2.98V drop across it.
So using a+5V supply, you need to drop from 5V to say 3V at 1mA, so that [5-3]/.001 =2K ohms =R1

E

EDIT:
NOTE: 298K - 25C = 273K, so from -273K at 10mV/K rise will give +2.73V at 0Cdeg and at +10C +2.83V, so it goes higher/lower by 10mV for every 1Cdeg high or lower,.

5. thank you mr gibbs that realy explains it well for me! so if i am using it at say a higher temp (i am not) then i look at the graph and get the voltage drop from that and redo the calculations thats very very helpful thank you
logan
p.s reinstalled snagit and now have screen capture again! no idea why it wasnt working

6. Originally Posted by Little Ghostman
thank you mr gibbs that realy explains it well for me! so if i am using it at say a higher temp (i am not) then i look at the graph and get the voltage drop from that and redo the calculations thats very very helpful thank you
logan
p.s reinstalled snagit and now have screen capture again! no idea why it wasnt working

7. so do you mean with the 2k resistor it will go up 10mv per degree C? so 25Cis 2.83v and then each C after that is another 10mv?

8. i sent a email to st electronics explaining about my robot project and they sent them and some other bits FREE!!!! but some look too small to use shame about the mics and the accelerometer they have also set me up a samples account realy nice of them

9. Originally Posted by Little Ghostman
so do you mean with the 2k resistor it will go up 10mv per degree C? so 25Cis 2.83v and then each C after that is another 10mv?
L,
The 10mV change per 1Cdeg is the typical value from the datasheet.

Yes, if you set it for 25C and 2.98V output, it will change by 10mV/C, BUT using only a 5V supply you will reach a point where the output voltage will limit.

Consider a temperature of +100C [its max value] thats 3.73Vout, say 4Vmax.

I would suggest you set it up and plot Vout versus temperature, over the temperature range you intend to use it.. lets know

10. it is just for making sure the transistors on the H bridge arnt getting too hot as i have to use a small heat sink for the robot so it should be no more than about 75C after that a very small fan will come on i will get this working then start on something else and hopefuly eventualy add them altogether
thank you logan

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