Questions about DC heating circuit

[SaraBMB]

Hello,
I am planning to make a heating circuit that works with a battery. I need it to generate a low heat; I want it to only heat my coffee when it gets cold not to boil it. So, what is a good simple circuit I can use?

My idea is to make the circuit heat up a small heating element that will be placed underneath the cup.
Any suggestions or ideas? I am a beginner so I don't know much about producing heat, I wish someone can help.

Thanks.

 

[MrAl]

Hello there Sara,

The simplest way is to use some resistors and a DC power source. Resistors generate heat as R*I^2 so that is the basic principle at work here. It also helps to distribute the heat over the surface a little.

So say you want 10 watts of heating. Just for a simple example, say you have a 10v source. A 10 ohm 20 watt resistor would draw 1 amp from that 10 volt source, so the heating in watts is:
P=10*1^2=10 watts.

Say you use a 5 ohm resistor, then since Ohms Rule gives us:
I=V/R=10/5=2 then the current is 2 amps now, so we would see a greater power:
P=R*I^2=5*2^2=20 watts.

so we'd have to use a bigger size resistor that can handle 40 watts to be safe. But it helps to distribute the heat too, so we would want to use more than one resistor and have them next to each other. So say we use four resistors in series, to get a 10 ohm resistor that means we need four 2.5 ohm resistors. If we connect in parallel, we would need four 40 ohm resistors. In both cases the resistors power rating has to be two times the working power divided by the number of resistors, so we have:
Pr=10/4*2=5 watts each resistor, using four resistors for 10 watts.

It would be best to mount them under a small metal plate, and have an insulating base and maybe sides too. You can use high temperature silicone or something else that can take the temperature to glue them to the bottom of the plate, or sandwich them between the bottom and top. You may have other ideas here too.

So that should get you started to think about what is involved. It's not hard to do at all, but it may take some experimentation to find out what level of heat (10 watts, 20 watts, etc.) to keep your cup warm depending on what kind of cup it is and what the ambient temperature is and how you construct it, and also if you want to make it adjustable to lower and raise the temperature as needed.

 

[SaraBMB]

Hello MrAl,
Thank you very very much! I appreciate it very much.
It is exactly what I need, can't thank you enough.

Only one more thing, do you by any chance have a sample diagram for such circuit?
It'll be great if you have one, but it's so okay if you don't; I think i can draw one myself from the information you provided me with.

Thanks again!

 

[ClydeCrashKop]

Automotive light bulbs are resistors, they get hot, are easy to find in various wattages and are inexpensive.

 

[alec_t]

Be aware thet you will need quite a powerful battery to run the heater. A little 9V one, for example, would be dead in no time

 

[MrAl]

Hello MrAl,
Thank you very very much! I appreciate it very much.
It is exactly what I need, can't thank you enough.

Only one more thing, do you by any chance have a sample diagram for such circuit?
It'll be great if you have one, but it's so okay if you don't; I think i can draw one myself from the information you provided me with.

Thanks again!

Hi again Sara,


Yes it is a very simple circuit either parallel or series, or a combination of those two.

Here is a quick diagram of series and parallel, as well as two different ways of doing the combination of both series and parallel:

E is the voltage source which can be a battery for example, and polarity does not matter with resistors only.


SERIES:

   +---R1---+---R2---+---R3---+
   |                          |
   E                          |
   |                          |
   +--------------------------+



PARALLEL:

   +------+---+---+
   |      |   |   |
   E      R1  R2  R3
   |      |   |   |
   +------+---+---+



SERIES AND PARALLEL 1:

   +------+---+---+
   |      |   |   |
   |      R1  R2  R3
   |      |   |   |
   |      +---+---+
   E          |
   |      +---+---+
   |      |   |   |
   |      R4  R5  R6
   |      |   |   |
   +------+---+---+



SERIES AND PARALLEL 2:

   +------+---+---+
   |      |   |   |
   |      R1  R2  R3
   |      |   |   |
   E      |   |   |
   |      |   |   |
   |      R4  R5  R6
   |      |   |   |
   +------+---+---+

 

[crutschow]

How long do you want it to heat and how big a battery do you have room for?

 

[Pommie]

Anyone think a peltier device on a big heatsink (source really) maybe more efficient?

Mike.

 

[Mr RB]

Yeah it would be.

But I think the main problem is the "heating" of coffee and the OP might have really underestimated the amount of energy needed to heat coffee from (say) room temp 20'C to "hot coffee" 70'C and the amount of power needed to do it quickly (say in 30 seconds)...

 

[Pommie]

Let's see, 250cc, 20C to 70C, 30 seconds = 250*(70-20)*4/30 = 1,666 joules per second or 1.6kW.

The 4 is the number of joules to heat 1cc by 1 degree.

I actually thought the OP wanted to keep the coffee warm which is more doable.

Mike.

 

[SaraBMB]

Hello MrAl,
Thank you for the diagrams. I know how to connect resetnces in general, but what I meant by my q what is the best way to connect them in order to reach my goal? To generate enough heat I mean.

 

[SaraBMB]

Hello crutschow,
I do not have a specific time but hopefully not more than 15 mins, the shorter the better. The main goal for me isn't the time, but the heat.
I do not have much space for the battery, but I can take two 9Volt batteries, the one battery size is H: 48.5 L: 26.5 W: 17.5 so i guess i can take double this size.

 

[SaraBMB]

Pommie and Mr RB, I didn't get your idea well. Can you explain it further?

 

[Mr RB]

We are saying that to heat your coffee up takes a LOT more energy than you might have considered.

Pommie's calc shows you need 1600 watts, for 30 seconds. Your coffee might be less than 250cc volume, and you might not need to heat right up to 70'C, but even in a BEST CASE situation you need many hundreds of watts to heat your coffee cup in 30 seconds.

As another proof I often use a 850 watt microwave oven to re-heat coffee, it takes maybe 20 seconds for a 3/4 cup to re-heat it. Again, that's hundreds of watts.

Your two tiny 9v batteries can supply a safe max of about 500mA total, or 9v * 0.500 = 4.5 watts! So they will barely put any heat into your coffee you would notice, AND would make your batteries dead after a few uses.

 

[SaraBMB]

Mr RB, yah I see your point clearly now. Thank for the clarification.
On your opinion, what is the minumum amount of voltage I must use to reach a reasonable heat amount that can heat my coffee?

 

[MrAl]

Hello MrAl,
Thank you for the diagrams. I know how to connect resetnces in general, but what I meant by my q what is the best way to connect them in order to reach my goal? To generate enough heat I mean.

Hello again,

First off, i dont recommend using a Peltier device as they bring in extra mechanical mounting and insulating problems that are just simply not needed here for just something that has to heat and not cool. It's only when you have to cool that you need a Peltier and then put up with the extra complexities.

It doesnt matter how you connect resistors as long as they are all working at say 1/2 their rated power or something like that. It's simplest when they are all the same value too.

So if you have several resistors in parallel you'll get the heat of one resistor times the number of resistors. To heat coffee you can probably get away with 10 watts but it depends on several factors which we dont want to have to bother to measure, so aim for maybe 20 watts and if you include temperature sensing you can cut back the power if needed. Feel free to experiment a little too...connect some resistors and see what happens when you try to use them to keep coffee hot by having them underneath. Maybe build a little test fixture.

 

[SaraBMB]

Hello again MrAl,
Thank you! You have been a great help.
You made it all simple and clear, I think I'll do exctly what you said.

One more question if you may, is there a specific kind of resistors that you prefer? Or it doesn't matter?

 

[Externet]

Hello Sara
-----> https://www.ebay.com/sch/sis.html?_kw=12V+Hot+Coffee+Tea+Soup+Drink+Pot+Heater+Car+Portable+Warmer

 

[SaraBMB]

Externet,
Yes the results of your serch are very similar to what i want to make. Thank you but my goal is to creat one not to buy it.

 

[MrAl]

Hello again MrAl,
Thank you! You have been a great help.
You made it all simple and clear, I think I'll do exctly what you said.

One more question if you may, is there a specific kind of resistors that you prefer? Or it doesn't matter?

Hello again Sara,


Wire wound resistors would probably be best for heating. Other types arent made to take the heat as well. But i think the resistors should be derated anyway maybe by 50 percent to make sure they last.

So you should shoot for at least 20 watts one way or the other. Maybe four resistors across the underside of a small aluminum plate, but that's up to you. Depending on how hot you want the coffee you may need as high as 30 watts so maybe go higher and make it adjustable some how with (maybe) an external resistor or just vary the voltage.

I'd very much like to hear how this turns out when you get it assembled and start to test it. I am thinking of building something similar for the purpose of defrosting a freezer.