LM35 & Negative Voltages

[torblednamris]

Howdy,

I'm attempting to build a Temperature Sensor for a car radiator using the LM35 from National, to feed a linear temperature/voltage signal to a Digital Panel Meter. All works well as far as I can tell except once we start getting to below zero degrees Celsius.

The LM355 puts out 10mv per degree (I have it operating on +12VDC), and has an operating range of -20 to 150 degrees Celsius. The datasheets indicated that at -20 degrees, the output will be -0.2 volts. Sounds logical to me, but how to I go about supplying the LM35 with a negative voltage? Or does it achieve this somehow from +12V? I have no concept at all of negative voltages....can someone point me in the right direction?

Thanks for any help...

Rob

 

[BrianG]

Regarding your query about the concept of negative voltages, it's all a question of reference points. In automotive terms, when the car battery negative terminal is connected to the chassis (negative-earth wiring) the "hot" lead is then at +12V. If you connected the positive terminal to chassis instead (positive-earth), the "hot" lead would be -12V. (if you had two seperate batteries connected as one positve and one negative earth you would have +12V and -12V supplies with respect to the common point, i.e. the chassis. OK?)

I don't know if you have the full data sheet for the LM35 (you can download it from

http://www.national.com)but

it does not actually need a negative voltage supply rail to the chip to produce a negative signal for temperatures below 0 deg., but it does need a bias resistor on the output pin to a negative rail.

The recommended alternative method for single supply operation is to insert a couple of ordinary diodes in series with the ground supply pin to the chip so that the output signal is offset from ground (chassis) and so does not go negative.

On a practical point - surely you don't need to know if the car radiator temperature goes negative - unless of course you are going to run without anti-freeze!

 

[torblednamris]

Thanks for the reply,

I think my question was a little vague, but I think you put me on the right track...

As for the practical point...very little of what I do in my spare time is 'practical'...and I've found doing things that you have no idea about, is the best way to learn things you didn't even intend to!

Thanks for the help.

Rob