Using 5V and 3.3V pins in Series

Hi everyone I've got a quick question about the feasibility of my project using an Arduino Uno.

I am designing a circuit that can provide enough data to compute thermal conductivity of a material. To do this, I am using a Thermistor in a Wheatstone Bridge configuration. The idea behind this is to Pulse the thermistor with high voltage for ~3 seconds to utilize the runaway thermistor effect, then monitor the temperature decaying to equilibrium over a period of time.

Is it feasibile to power the Thermistor with the 3.3V pin and 5V pin, then use a timer switch to utilize just the 5V pin after a pulse? If not, would an external battery work to over volt the system for the duration of the pulse, without effecting the 5V logic of the arduino?

Here is an attachment of a rough circuit schematic.
Rp = Thermistor
Rb = Potentiometer
Rs = 10k ohm
Rh = 500ohm

Rough Circuit Schematic.png

Rough Circuit Schematic.png

I don't think you can connect the 3.3v pin and the 5v pin in series because they don't have independent GNDs. They are not like two separate batteries.

If you don't need much current why not use a separate PP3 style 9v battery with the battery GND connected to the Arduino GND.

...R

Robin2:
I don’t think you can connect the 3.3v pin and the 5v pin in series because they don’t have independent GNDs. They are not like two separate batteries.

If you don’t need much current why not use a separate PP3 style 9v battery with the battery GND connected to the Arduino GND.

…R

Thats what I was thinking. Would this mess with the programming logic of Arduino then? At least for the duration of the pulse?

Thanks for the reply.

What a fun and simple experiment!

@Robin is of course right, you do need a seperate isolated supply; however, if you change the configuration and ground the thermistor you can do it more simply.

Also you dont need one half of the wheatstone bridge - few arduino boards are set up to handle differential inputs

Forgive the simple diagram, I dont have a symbol for a thermistor.

I'm sure someone will explain how you can use a transistor or FET to switch +3.3 (or better +5) to heat up the thermistor.

Karma added for nice q.

thermistor.png

thermistor.png

So take the 5V supply.

Put a rsistor divider to get 3.3V, use a relay to shunt the divider for for 5V.

Why don't you just use PWM to control the effective voltage on the thermistor? You can e.g. shorten Rs to have the full bridge voltage on the thermistor.

I assume the thermistor is a 10k one, and operating around room temperature. At 5V the current is 0.5 mA, and the heat dissipation pretty much negligible. A 9v battery pushes through 0.9 mA, this is still negligible.

To get to say 1W dissipation, that sounds to me a reasonable ballpark number for such an experiment, you need to run the thermistor at 100V.

OP has to chip in with real numbers, but 5V or 9V is quite certainly not going to do the job.

Hi,
I like the experiment.
I service laboratory equipment and heating a sample and monitoring its cooling curve over time is used in quite a few places in food technology. For eg, how do you think your Mars Bar or Twix chocolate always tastes the same.

These units have a heating element around the thermistor.

In your diagram you would be better sampling the bridge output at the connections I have shown.

You could, to keep it simple have a DPDT relay to switch the thermistor from heating voltage to bridge circuit.
That way your two circuits are isolated from each other.

As for thermistors, there is a plethora of sizes out there to choose from.

Tom... :slight_smile:

In your diagram you would be better sampling the bridge output at the connections I have shown.

I cant see any, am I missing something?

Also what is the point of sampling a voltage that is fixed by two reisitors?

Consider to mount a heater element (resistor) near the thermistor and heat it that way.

Hi,
Sorry not sure why that image didn't take.

In your diagram you would be better sampling the bridge output at the connections I have shown.
Rough Circuit Schematicedit.jpg
Thats better

Tom... :o

Rough Circuit Schematicedit.jpg

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