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Topic: Trying to write a seemingly simple code based on a thermistor reading (Read 106 times) previous topic - next topic

redhatman

Hi all,

I am new to the Arduino world of programming/setting up boards, however I do have a pretty solid background with Matlab programming, as well as electric circuits. Where I'm lacking is the know-how of setting up the board, so any help is appreciated.

I am trying to setup an Arduino Uno to switch a 12V relay based on a thermistor reading. The relay is to be setup on the board such that it does some sort of interval checking of a thermistor reading (ideally a calibrated temperature, whether it be from a table of values or not doesn't make a big difference), and I'm totally okay with up to 3 degrees (C) variation if it makes it easier.

Once triggered, the relay will switch, and complete the circuit for the load (heater element). The heater element will be connected to its own power supply which is adequate (the heater draws ~26A at 12V). Essentially, this relay is being controlled to maintain a target temperature.

What I have for hardware (in addition to typical jumper wires and other misc. stuff):
-Arduino Uno
-Power supply for Arduino
-12v 30A relay
-100K Thermistor
-12v Power supply for heating element (40A)
-12V Power supply (1A) for relay switching side if needed
-An old CPU Power Supply for parts (transistors,capacitors,diodes,etc)
-Assortment of 1/4W resistors

I have found similar tutorials, however nothing I have seen is exactly what I'm trying to accomplish. Some also include transistors and diodes however not all of them do. I have posted a sample diagram as well.

Any and all input and help is greatly appreciated, thank you!
-Pat


Koepel

Keep the 40A seperated from the Arduino, that is good.
The Arduino can not drive such a relay. You need a mosfet, a resistor (or two) and a flyback diode.
Or use the 40A power supply also for the relay, but use an opto-coupler for the relay (the mosfet is still needed).

Do you know how to calculate the temperature ?
https://learn.adafruit.com/thermistor/


redhatman

Keep the 40A seperated from the Arduino, that is good.
The Arduino can not drive such a relay. You need a mosfet, a resistor (or two) and a flyback diode.
Or use the 40A power supply also for the relay, but use an opto-coupler for the relay (the mosfet is still needed).

Do you know how to calculate the temperature ?
https://learn.adafruit.com/thermistor/


Thank you for the prompt reply and information, that link is really good. I'm particularly interested in the "flyback diode" you meantioned. This whole desire of mine to build this circuit is because it would essentially isolate my current two systems (because this would replace one of them). What are the chances I could find a flyback diode in an old desktop PSU?

jremington

Diodes are everywhere. A flyback diode just needs to be able to handle the motor or relay coil voltage in reverse and (briefly) the motor or relay operating current in the forward direction.

Koepel

Perhaps you could buy a relay module with optocoupler.
Search for : relay module optocoupler

Or build this: http://howtomechatronics.com/tutorials/arduino/control-high-voltage-devices-arduino-relay-tutorial/

The diode DS1 is not needed.
Diode D1 is the flyback diode. It is not a special kind of diode (like a Schottky diode), its purpose is to eliminate the flyback current.
In that schematic, a transistor is used. A mosfet is more fun, but a transistor just as good.
The relais can then be powered with the same 40A 12V (the JDVcc in the schematic).

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