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Thank you for the answers.
I'd say I'm not a completely newbie, I know something about it.

@Techylah
I like your idea.
I will buy a LCD, so I can see the temperature. I pretend in the future make some modifications, but first I need to build a simple water heater to know more about it.

Do you think I can vary the tension applied to that heater with a potentiometer? To get a better tea, e.g. if I want some green tea, I'd would prefer to heat it slower.
Also, do you think I could touch that heater in a metal plate like a cooker, and use the plate to heat the kettle? I think with that arrangement I would make better teas since has some controversy about the ways electric heaters are made.

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do you think I could touch that heater in a metal plate like a cooker
No.  I think that is ill-advised.  It is made to be immersed, which provides a solid, low thermal resistance connection to the heat absorbing object, the water.  It would probably overheat and burn-up in flames if not immersed.

Instead, use any regular electric teapot.  The electric and mechanical connections to the heating element are done.

Your device can look like a stainless steel spoon that you leave in the pot.  Of course it has digits on top and a wire going to your Arduino to bring it to and maintain the temperature you want by cycling power to it via a relay.

I don't know much about tea, but if it is like coffee, heating time doesn't matter.
It is only the final temperature that will affect flavor.  I think the better flavors come out at lower temperatures; the bitter components take longer and require higher temperature.
If this is so with tea, making tea at lower temperature but longer steep time, would be better and worth the effort.

You'd still want to reach the lower temp as soon as possible, so the relay is best.
Besides dealing with SCRs and other high voltage, high current "dimming" circuits is tricky.

Your Arduino could also handle the steep time with finesse.
You put your "magic spoon" into any size electric pot, plug the pot into your switched power outlet, and press Start.
You get to watch the water get up to temp as soon as possible, (a settable 85 deg C), give you a "Bong" to add your tea,
and then another "BingBong" x minutes later when it's done!  Then it maintains that temperature.

With your pot, potentiometer, that is, you can set the default temperature and steep times.
« Last Edit: May 21, 2012, 06:17:31 pm by Techylah » Logged

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there was a good reason i advised the use of peltier junctions for this project as  the highest voltage  one may come in contact with  is dependent on the  junction used  but will  nearly always be below 60 volts dc
You know what happens when you drive an "AC" heating element with DC? smiley-wink
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You know what happens when you drive an "AC" heating element with DC? smiley-wink

It gets hot?
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controversy about the ways electric heaters are made

There is no controversy, just a warning that electricity is dangerous. Normally control circuits are separate and well insulated from power circuits, once you start introducing water there is a risk that people can come in contact with the effects of high voltages or currents.

You say you are not a complete newbie but it seems clear that english is our second language so just take care about safety.
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I'm planning to use a digital potentiometer with a relay to turn on my heater (YL205 -
http://www.amazon.com/Lewis-N-Clark-Immersion-Size/dp/B001U0PA7M/ref=sr_1_3?ie=UTF8&qid=1337571828&sr=8-3 ) and change the potency. I found some like AD5171 Digital Potentiometer. Do you think it is a good one or do you recommend me another one?

@TechylahYes, tea and coffee are pretty the same, but teas have more bitter components, so I think it is more difficult to prepare.

@radman Probably you don't know about teas, but tea specialists do not recommend to use any kettle with metal or plastic parts since the flavor gets worse.
I was talking about controversy among tea specialists, maybe I should explain better, but, even if English isn't my first language, it hasn't any relation with the fact that I have or have not any knoledge about electronics.
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I looked closer at the reviews of that YL205 water heater. 
US people beware - Its "dual voltage" means that it is meant for Europe's 220v and works at half heat in the US 110v!

I don't know what you are going to do with the digital pot chip.
What I think you want is to implement a thermostatic controller.

Side note:  I need to do a similar thing to control an otherwise extremely noisy water urn.  I don't want to use my Arduino on it.
I found and purchased this cool digital controller for under 10GBP, including thermistor.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=110880711979&ssPageName=ADME:L:OC:US:3160

Back to your project.  The immersion heaters look like they're all 300 watts, so you need a relay with at least 2A contacts.
You can tap into the (non-ground) thermistor wire of the Adafruit thermister/display.
Your Arduino can measure the voltage on that (analogIn) and you can scale the reading to a number that always matches the readout temp.
When that temp is below the threshold you want, you turn on the digital output, turning on the transistor, engaging the relay, and turning on the immersion heater.
Remember to use a back-biased diode across the relay coil.
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For me I will use the US 110v socket.

I'm trying to optimize my project, so instead of using the relay to turn on and turn off the heater I will change the potency. So to keep the water warm I just need to set up a potency that equals the heat produced with the heat dissipated. Or if I want to heat just a cup of water maybe it is better the heat it slower, so I have more control and, hence, more precision.
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Alright, then.   Go for it!
Despite what someone mentioned, I see nothing wrong with putting DC through an AC heater, if you're careful.
I would guess you are going to use PWM to a digital pin, with that digital pin controlling an NPN power Darlington, giving the heater you plug in zero to full duty cycle pulses of 120 volts DC.  The 120v DC would come from a full wave bridge rectifier on the AC line, with a large capacitor to smooth the ripple.
If so, I would only say make sure:
1 - Your power on/off switch is double pole to switch both 110v wires.  Off should not leave one side connected, since a reversed plug could leave things hot with respect to ground.
2 - You include a fuse or circuit breaker.  If you don't have one and there is a short circuit, much current will flow, producing one heck of a vaporizing flash.
3 - You include a bleeder resistor across the capacitor 120vdc to ground.  Otherwise, like in some very old tv sets, you unplug it to work on it and come to the shocking realization that the 120v remains for quite some time (minutes, even)!
4 - Use low enough gauge wire, not going through any breadboard pins.
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I see nothing wrong with putting DC through an AC heater, if you're careful.
it really depends on what is inside, if there is only a heating element I expect no problems if there is some electronics/ voltage convertor then ?????
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Z, It looks like you are going for a very unique tea maker.
For the taste issues mentioned, a metal or plastic tea kettle is out.

So we are dealing with a ceramic or china pot, which do indeed "feel" like they will make tastier tea.
You will immerse from the top a device with both a heating coil and temperature sensor on it.
You'd do well to put as much distance between the two as possible, so you get temperature readings of the overall water, rather than of the heating coil vicinity.

Without a temperature sensor, and just trying to adjust the amount of heat ("tension") may be problematic.
You will have to account for differences in the amount of water and the thermal capacity and heat loss characteristics of the container.
One setting of "tension" will not work well for both 1 cup and 1 liter, or for both a metal and a ceramic pot.

So therefore you should have a temperature sensor involved in the control of heating, not just for display of current temperature.

Now we are left just with the question of relay, as everybody including me prefers, versus variable heat, as you are enamored with.  :-)

Let me add this tidbit to the argument for full On/Off, i.e relay control, other than that it is easier and safer (which it is).
Did you ever do the experiment of heating a water balloon to boiling over a candle flame?
It works. All the time. The water averages the heat going in, never letting the balloon temperature get higher than the water temperature.
I think a similar thing will happen if you go through the trouble of putting pulses of heat or even variable voltage to the heater.
No one will know.  The water will only take more time to get to the desired temperature.
Additional problem if you are operating "open loop", i.e. without a temperature sensor as part of the control - once the water has slowly reached the temperature you want, there is nothing to stop it from increasing further since heat, even at a partial setting, is still still being applied. (Yes, there will be some difficult-to-determine equilibrium temperature it rests at)

If, after all this, you are still convinced you want adjustable heating, i.e. no relay, then make that visible to the user.
Give your user interface an additional parameter to "Temperature setting".
For example  "Maximum heating rate (1-10)",  or  "Heater power (1-10)",  or even "Slow cook enhancement(1-10)"

What are your thoughts now about this?   Relay or no relay?  Start with relay and add PWM later?
Additional aesthetic note: The PWM approach is elegantly quiet.  The relay will make repeated clacking as it maintains a temperature!
Best!
Techylah
 
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Alright, then.   Go for it!
Despite what someone mentioned, I see nothing wrong with putting DC through an AC heater, if you're careful.
I would guess you are going to use PWM to a digital pin, with that digital pin controlling an NPN power Darlington, giving the heater you plug in zero to full duty cycle pulses of 120 volts DC.  The 120v DC would come from a full wave bridge rectifier on the AC line, with a large capacitor to smooth the ripple.
If so, I would only say make sure:
1 - Your power on/off switch is double pole to switch both 110v wires.  Off should not leave one side connected, since a reversed plug could leave things hot with respect to ground.
2 - You include a fuse or circuit breaker.  If you don't have one and there is a short circuit, much current will flow, producing one heck of a vaporizing flash.
3 - You include a bleeder resistor across the capacitor 120vdc to ground.  Otherwise, like in some very old tv sets, you unplug it to work on it and come to the shocking realization that the 120v remains for quite some time (minutes, even)!
4 - Use low enough gauge wire, not going through any breadboard pins.

Could you recommend me any sites where I can read more about it? I can understand, for example, what is a wave bridge rectifier, but I think I need to study more to really know how to use it and to avoid any problems or lack or security in my project.
Could you explain me better about the PWM you recommend me to add later. Also, I know it is a Pulse Width Modulation, but I don't know yet how useful it is or all the possibilities I have with.

You convinced me about the use of a relay. Someting I still have doubt is why don't use both, relay and heat control together. For example, if I want to increase the water in just 1°C, I think it will be much more easy to do the fine tune with a heat controller.
I intend to control the machine "virtually", so I will need a relay.

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Ladyada has a good page on full wave rectifiers:
http://www.ladyada.net/learn/powersupply/transformeracdc.html

I just did away with the transformer, since you don't need to step down the 120v AC, just convert it to DC so it can be pulsed by a simple power Darlington on a digital pin.

Water heats slow enough by itself.  Bringing water to boiling temperature with even a large gas or electric heater takes over a minute.

But if that's not slow enough, and you want to lower the heat of an electric heater, there are two ways to do it.
One way is to lower the current through the heater by lowering the voltage applied to it.
The other way is to use the full voltage but "pulse" it.  For example leaving it on for one out of every five seconds only gives you 20% heating.

This second way is the pulse width modulation method. It is very precise and also built into the Arduinos.
The electronics needed are also simpler and cheaper since they are actinging as switches, all on or all off, dissipating little heat themselves.

Quote
if I want to increase the water in just 1°C, I think it will be much more easy to do the fine tune with a heat controller.
Perhaps if you didn't have any feedback.  You'd still need to take into account how much water there currently is.  Use half the water and whatever heat you are applying now warms the water twice as fast.  You then have to calculate when to shut off the heat, how fast the container cools off, and when to turn back on the heat.

In contrast, using full temperature feedback and full 0/100% heat only, is simple.
If your measured temperature is lower than what you want, you turn on the heat.
If it gets higher than what you want, you turn off the heat.
When more water is added or a window is opened putting a cool draft on the kettle, the heater just stays on longer.
If you pour out most of the water, it heats quickly and so the heat cycles to off more quickly.

There is no "both".  With a relay, though you can use DC, why bother? Put the relay contacts in series with the AC heater power and you are done.
You'd never pulse the heater through a relay - it's way too slow, way too noisy, and way destructive to it's contacts.

If you use DC and PWM, you don't need a relay.  0% pulse width is full off.
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Sorry, I noticed I was saying things you already explained me. I have not a lot of experience with electronics, but I'm studying to really understand all I need to do.

Basically, if I can work the heater with a DC current (if it's just a wire that produces heat by Ohm's Law probably it isn't a problem, I think) I can connect the heater to Arduino (where I can use the PWM) , and to a socket to supply the voltage and current needed, but to do that I need to use a full wave rectifier to transformer from 110V Ac to 110V DC. Isn't it?

1)So could you send me a link that explains me how to work with a power supply and arduino together? I have studied project of motor and light that work with PWM and power supply but it isn't completely clear about that connections that I have to do.

2)Do you have any recommendation about full wave rectifier or can I buy anyone that fits the requirements of my project?

3)If the heater can't work with a 110V DC what  have I to do? I thought change the current from arduino to AC, but that has, maybe, a lot of complication and risks.
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You are right about using DC and Ohms law.
A full wave rectifier is just 4 diodes connected so that either of the 2 AC input lines going high conducts current to the Plus of your DC output.
Similarly, 2 other diodes handle the negative path.   Scroll to the middle of the link I posted:
http://www.ladyada.net/learn/powersupply/transformeracdc.html
For some reason she omits the necessary smoothing capacitor; she shows it for the previous half wave rectifier (single diode) case.
This capacitor must be large and have high voltage rating.  Say 1000uf at 200v or more rating.  Not cheap. (Make sure you wire + to +).
 
Getting the 110v DC is problematic.
You need an isolation transformer.  This is a transformer that doesn't step up or down the voltage but keeps it the same.
The reason is safety. If you don't use one and someone plugs in the 2-prong plug backwards, instead of having a common ground (0 v) with your Arduino 5v power and a separate DC that is 110v higher than ground, you would wind up with a -110v DC level lower than your circuit ground. This would burn up all sorts of stuff, including the power transistor which would be wired for +110v, now very much wrong.
The transformer gives you an AC supply that "floats".  You can peg it or the DC derived from it at any voltage you want.

The "solenoid power" in this diagram would be your 110v DC and your heater would go where the solenoid goes.
You could then use PWM on the input.
http://arduino.cc/playground/uploads/Learning/solenoid_driver.pdf

Specifically you want a 0v common to the reference for your 5v, and 110v UP from there.
Isolation transformers are not cheap. This "inexpensive" one, for example, handles only 500 watts.
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_221331_-1

Yes, you can control AC with an Arduino, but it's not easy.  You have to use an SCR or a TRIAC.
http://www.kerrywong.com/2010/09/11/a-high-current-triac-controller-using-arduino/

Though you need to control only 3-5A AC, you still need to switch it at 0v crossings which is what the circuitry does.

Look at the costs for 110vDC PWM approach (isolation transformer, 200v capacitor, diodes, power Darlington).
Then look at the sophistication necessary for the 110v partial AC approach (triac, heat sink, circuitry) used for high current AC.
Then consider that basic control circuitry of either type will essentially bring the water up to speed quickly by using full 100% PWM or full AC cycle conduction,
and then shut it off (0%PWM or no AC cycle conduction).

This is the same as what a simple relay does, for much less cost and complexity and more safety.
http://www.arduino.cc/playground/uploads/Learning/relays.pdf

Since this post has a discouraging tone, it would be cool if other experienced circuit hackers would chime in with their take.
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