Hello, I'm working on a school project in which I need to heat a moderate amount of water to temperatures close to boiling, at most 90º or similar. I have searched a lot and have not found any models of variable heating resistors compatible with Arduino. I don't mind what type it is, I just want it to be submersible, with variable power, and not excessively expensive. Do you know of any model with the characteristics I am looking for? Thank you very much.
Welcome to the forum
Use any heating resistor that suits you powered by an external power supply and have the Arduino turn it on and off using a relay
An Arduino will not be able to supply the required power required voltage and current required by the resistor
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You need to be specific. How much water? how is it to be contained? How quickly must it be heated?
Eg 100mL of water in an insulated beaker to be heated from 15C to 90C in 6 minutes.
What is your power supply?
The power is V^2 /R so you choose a siuitable fixed resisitor and control the voltage.
Or
(this is clever)
you can use a pair of power transistors to make a voltage controlled active load.
What is your budget?
Here's a rough datapoint for you. A regular 1500W kitchen kettle will heat a liter of water (15C to 100C) to boiling in about 3 minutes. That's 13.5 A at 120VAC(I specify, because your domestic power conditions may be quite different). So, now we need to know your volume of water for comparison. If, for example, you want to heat 100 ml(10x smaller), and can do it in 9 minutes(3x longer), first approximation says you can apply 50W((1500/10)/3) for 9 minutes and boil your 100 ml. A bit less time will bring the water to 90C.
There are all kinds of approximations in that little example, but it's the sort of back-of-the-envelope calculation that gives you a good idea of what you can expect.
So,
- what's your volume
- what form of power can you use (it might not be appropriate to suggest you use 115VAC, you might be limited to 12VDC for example for safety reasons).
- what heating time is acceptable?
Over to you, @carlos_de_francisco.
A solid state relay and a small immersion water heater would work well for a fairly small amount of water, such as a coffee cup worth. There are multiple sources of the immersion water heater intended just for this purpose, and have been used in school projects going back decades.
If you are limited to 12V, there are portable versions of this type heater intended for use from a car battery.
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I planed to use a relay to control it. It is for a school project and the teacher doesn't really mind how long it takes or if it even works at all. It should heat up around 5 liters of water. I don't mind how long it takes. While the resistor is variable I don't mind. The budget is 50 max. It would really help me if you guys say me a model which could meet this characteristics. Thank you.
Please explain more about this variable resistance requirement.
Typically as mentioned in a previous post, the Arduino turns the heater relay on and off, but the resistance of the heater is constant.
It doesn't need to be variable. Virtually all heaters, including the heater/furnace in your house, or your oven, work by cycling on & off. Power is turned-on until you hit the target temperature, then it's turned off. And on again when the temperature drops. Refrigeration and air conditioning works the similarly.
Usually, there is some hysteresis, so for example, if your target is 200 degrees F it might turn-on at 199 degrees and stay on until 201 degrees. That prevents the relay from "chattering" or the thing turning on & off several times per second.
It's also possible to "dim" the power with PWM or a circuit like a light dimmer, which works similarly to PWM but it "chops" the AC waveform. But an on/off relay is easier to use and it actually works better for temperature control.
Of course, that means you have to monitor the temperature.
You just have to calculate-estimated the required minimum power (wattage), calculate the resistor value, and then select a power resistor rated for that wattage (or more). Some safety margin is recommended.
To keep it safe, you should use low voltage (less than 50V) and that will mean a "big" power supply. Even at low voltage, it should be insulated (with heat-shrink, etc.).
Or, you can buy a submersible water heater that's designed to be run from the power line (switched through a relay). Then of course, the power line is the power supply.
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For the heater you may want to not refer to it as a resistor but rather a Heating Element. A Google of "submersible heater element" which will bring up a few dozen examples in assorted sizes and voltages. Next to control the process you write some temperature control (process control) software for your Arduino. Your Arduino will not directly drive a relay coil so you want a transistor in the mix to drive the relay coil calling for heat. You also need a sensor to measure temperature, like just for example a Type K thermocouple and a MAX6675 breakout board to read the thermocouple and pass the temperature data along to your Arduino for your process control.
Anyway forget about using the term resistor as you will do much better using "heating element" and for your project "submersible heating element".
Here is an example of a 1,000 watt 120 VAC element. Depending on how much heat and how fast you could parallel a few. You could also add a mixer or bubbler to prevent stratification of temperature layers in your tank. The better you define exactly what you want/need the better the help you will get.
Ron
There are several ways to go about all of this.
Thank you for your advice about variability. I am trying to make something similar to a sous vide, were there is two containers, one where there is the heating element and another container were you put a bag with the food. Both containers are connected using tubes and at the beging of them there is a propulsion pump. I know it would be more eficient and better to only use one container but the teacher said we must use two.
Knowing this, could you tell me any heating element models please.
OK as we move along now we have two containers. I gave you a suggestion as to finding heating elements.
How you go about this is up to you. Nobody here will do an assignment for you. What is expected is you read the forum sections about how to get good help and you posting your code and proposed schematic or drawing of your project. A hand scribed drawing is fine.
Using an Arduino UNO for example you have DIO (Digital In / Out) pins which can easily drive a SSR (Solid State Relay). That powers your heating element(s).
There are several ways to get to where you want to be, your project and your choice.
I will add that you want to be concerned and control temperature from the container with the food in it so your temperature sensor goes there.
Ron
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