Questions about DC heating circuit

[MrAl]

Hi,

Yes Li-ion cells are lighter but we were shooting for 30 watts which as im sure you know is 10 amps at 3 volts, 5 amps at 6 volts. With a 18650 cell setup, the run time would be less than 20 minutes for two cells in series. Maybe buy four cells and boost that up to 40 minutes, six cells maybe 60 minutes. Maybe someone has a sale.

Also, 5AHr should be the same as 4.5AH.

More ideas:
Insulating the cup would help reduce heat loss. Using a top cover would help too. Once all the heat loss is eliminated it takes only a few watts to keep it hot. That could make all the difference in a battery operated cup warmer (one that keeps the cup warm that is, not heats it up).
That also means it would be nice to have a way to adjust the power in the resistor, as with a pulse width modulator. That way if most of the heat loss is eliminated the power could be kept low and that means longer (much) battery run time and battery cycle life.

 

[SaraBMB]

Hello again Mr Al,
Thank you!!
I will do as u said, they are very useful ideas. I'll keep you updated with the project. Thanks again realy!

 

[MrAl]

Hi again Sara,

You're welcome.

Yes i'd like to hear how this goes. I recently did something similar with a small refrigerator freezer. I ended up using one power resistor, a 50 watt resistor that was 3 ohms. Run at 25 watts it still gets pretty hot, hot enough to defrost the small compartment.

 

[audioguru]

Lithium batteries are used in cell phones and in laptops.
They are also used in hybrid cars.
My RC model airplanes are powered by a modern Lithium battery.

Here is the spec sheet of a tiny Lithium battery and they have thousands of more powerful batteries:

 

[MrAl]

Hi again,


Wouldn't it be kind of expensive to buy that many Li-ion cells for her project though? Maybe you know where to get them cheaper.

But perhaps we can work on reducing the heat losses. That would mean less power required so smaller batteries would be OK i think.

 

[audioguru]

My radio controlled model airplanes use Li-Po batteries. I buy Name-brand American batteries that are fairly expensive but they are powerful and last a long time. Some of my friends buy very cheap batteries from China and they do not last.

 

[SaraBMB]

Hi again Sara,

You're welcome.

Yes i'd like to hear how this goes. I recently did something similar with a small refrigerator freezer. I ended up using one power resistor, a 50 watt resistor that was 3 ohms. Run at 25 watts it still gets pretty hot, hot enough to defrost the small compartment.

Hello again Mr Al,
I am thinking of adding something new, but I have no idea how it works or if it is a good idea.
I was thinking of adding a heat sensor or something similar to measure the coffee temperature and make the switch closes immediately when the coffee is warm enough. Is this possible?

 

[MrAl]

Hi,

Yes that is possible. Depending on how you want to do this, the circuit will be different. They actually make thermal switches that are complete without any additional parts. The switch 'opens' when the correct temperature is reached, and shuts off the heater.

We might have to look around for one that would work for you. You'd also have to figure out and decide WHERE you want to measure the temperature. Obviously you dont want to have to dip anything into the coffee itself.

Also to decide what temperature is the best temperature for your own personal taste. I could measure mine but you should measure yours.

 

[SaraBMB]

Hi,

Yes that is possible. Depending on how you want to do this, the circuit will be different. They actually make thermal switches that are complete without any additional parts. The switch 'opens' when the correct temperature is reached, and shuts off the heater.

We might have to look around for one that would work for you. You'd also have to figure out and decide WHERE you want to measure the temperature. Obviously you dont want to have to dip anything into the coffee itself.

Also to decide what temperature is the best temperature for your own personal taste. I could measure mine but you should measure yours.

That is really a great simple idea! I like the thermal switch idea very much.
I would measure my cup of coffee right now if I could but I don't have any thing to measure it
but anyways, if I found my preferable temperature, what should I do next? What are the changes I must do to my circuit?

 

[MrAl]

Hello again,


Well this would be much like a room thermostat, except you're would be measuring say the bottom plate and when it gets to a temperature that correlates to the temperature of the coffee then you break the connection to the heater element. This would take a small circuit that is set up to measure and turn off. Eventually if you wanted to use a thermal switch you could do that, but to determine what temperature you'd have to experiment a little.

I find my coffee to be around 125 degrees F when it is served, but the 'hot plate' under your coffee cup will have to be hotter than that.

The measurement circuit could be a thermistor connected to a comparator, and a couple other small parts. The drive switch could be a MOSFET.

 

[SaraBMB]

Hello again,


Well this would be much like a room thermostat, except you're would be measuring say the bottom plate and when it gets to a temperature that correlates to the temperature of the coffee then you break the connection to the heater element. This would take a small circuit that is set up to measure and turn off. Eventually if you wanted to use a thermal switch you could do that, but to determine what temperature you'd have to experiment a little.

I find my coffee to be around 125 degrees F when it is served, but the 'hot plate' under your coffee cup will have to be hotter than that.

The measurement circuit could be a thermistor connected to a comparator, and a couple other small parts. The drive switch could be a MOSFET.

Hello again Mr Al,
Thanks for all the info! but I am not sure if I am getting you well. I got kinda confused..
This seems hard to do. Is it better to stay with my current circuit instead?

I need to buy all the materials online tonight. so I must decide on what I need more before ordering. Does this new circuit needs lots of materials, if yes then what are they all?

 

[MrAl]

Hello again,


I am not sure if you want to have to deal with this kind of circuit or not, but it would require an IC like the LM339 comparator, a voltage reference IC chip, and a few resistors, maybe a cap or two also.

Another way to adjust at least for now is to just use a series resistor. If you can adjust the series resistor you can vary the temperature. It wont be as good as a thermostat but at least allow for some flexibility. There are variable power resistors you could look into with a slider on the resistor that you slide along the resistor body to adjust the resistance.

 

[SaraBMB]

Hello Mr Al and everyone else who helped to create my project,

I just wanted to tell you that I did the project successfully and that it worked perfectly! Thanks to you all that I was able to do it.

Mr Al, I really can't thank you enough. Without your tips and suggestions, I wouldn't have succeed. You helped me GREATLY and taught me everything I needed; I am really truly deeply grateful.

 

[Mr RB]

Photos?

 

[MrAl]

Hello Mr Al and everyone else who helped to create my project,

I just wanted to tell you that I did the project successfully and that it worked perfectly! Thanks to you all that I was able to do it.

Mr Al, I really can't thank you enough. Without your tips and suggestions, I wouldn't have succeed. You helped me GREATLY and taught me everything I needed; I am really truly deeply grateful.

Hi Sara,

Oh hey you're completely welcome, and i hope you have many a good cup of coffee because if your new device
I still drink coffee too so i know it is nice to have it around even when i go out sometimes.

It's interesting too that after we started talking about this i designed my refrigerator defroster which works on the same principle of resistance heating. It's now installed and working great. Mounting the resistors was a bit of a pain, but it's ok now.

As MrRB says, it would be nice to see some photos of your new device in action if you get a chance to take some pics. I have early pics of my fridge defroster in another thread but intend to update that soon with the more final pics. My app required two resistors one on the bottom and one on the side. Interestingly, the resistors dont get hot because they are being heat sinked so well with the freezer metal. But i'll say more about that in my own thread.

So some pics would be nice, if you get the chance of course.

 

[Bach On]

There you have it, folks - Another Satisfied Customer!!!

 

[MrAl]

Hi Bach,


<chuckle>

Simple resistance heating goes a long way
Funny, most of the time we try to avoid this.