Voltage Divider

I have an automotive temperature sensor. Its datasheet states it can handle a maximum of 5mW. I will be running it using 5v (hopefully directly from the Arduino but you might suggest it is safer for me to use an external supply?).

The temperature sensor has a resistance of around 10k ohms at -5 degrees centigrade (that's 41 of your strange fahrenheit thingies) and 30 ohms at 160 centigrade (320F).

In order to measure the temperature using an Arduino I am going to use a good old voltage divider using an R2 of 5k ohms. This provides a voltage range of 1.68v to just under 5v and a max power of a hair under 5mW so all good there.

Question;

Using the planned voltage divider, when the sesnor is at its minimum resistance (highest temp) of 30 ohms, is the analog pin seeing high current (167mA)? If I understand it correctly, the Arduino pin has very high (>1M ohm) resistance meaning almost zero current flow. Am I right?

I have already killed one of these sensors (not my fault - poor info provided at sale) so am treading carefully and still have my L-plates on!

Cheers

The analogue input does not see current. It only sees voltage, which will be the voltage dropped across the resistor connected to ground. The Arduino will have no effect on this voltage.

-5 degrees centigrade (that's 41 of your strange fahrenheit thingies)

Have another try.

AWOL:
Have another try.

See I told you they were strange!!!!! Ok OK.....23

@Grumpy - I realise that the Arduino only sees the voltage but my concern is that current flowing into the pin would cause damage.

but my concern is that current flowing into the pin would cause damage.

With such a high input impedance current does not flow into the pin.

I think you need to brush up on ohms law.

Mk1_Oz:
I have an automotive temperature sensor. Its datasheet states it can handle a maximum of 5mW.

Are you sure that this is the correct figure? - It seems very low.
Can you point us to a datasheet for the device?

JohnLincoln:
Can you point us to a datasheet for the device?

Here 'tis - TAYAO Technology CO., LTD.

I hear ya Grumpy. I have read Ohms law and all the other laws so many times but it just doesn’t stick. I am trying hard though… lol :frowning:

If you struggling with ohms law... then you will have problems calculating temperature from that non linear device you have .
Why not look at DS1820 sensor s?

Linear , accurate, cheap...No ohms law needed for those .....

hammy, the sensor came with a detailed resistance to temp table but I can always slip the Steinhart formula into the Arduino if need be. This is an automotive sensor so will handle the extreme environment out of the box.