Please help me design a Li-Po powered nichrome heating element (400°F)

[kought]

Hello everybody, hopefully I am posting this in the right spot.

What I am trying to do is design a heating element powered by a Li-Po battery (RC battery probably) to heat a heating element to a range of 360-420°F. I want the temp to be adjustable, probably on three settings, so one for 360, one for 390, and one for 420. Space is at a premium, so the smaller the better for the element and battery. I am completely clueless when it comes to electronics and resistance and all that good stuff so I am hoping you guys can help me. I realize I am being very vague here but I don't know what information is required to figure this out, so if you guys need to know some other info, just let me know. Thanks!

 

[K^2]

First of all, what is your experience working with electronics of any kind? This is not entirely trivial, and while it's not terribly dangerous, there is definitely room for injury and property damage.

Second, this is all going to depend on application. Temperature by itself doesn't tell the whole story. Heat flow is just as important. This can be a very simple build, one you might be able to do safely, but you need to describe application in as much detail as possible.

 

[kought]

I know how to solder and have made several kits using PCBs and stuff. I am confident I will be able to make this without personal or property damage.

And as for the application, I am trying to make a vaporizer. So the airflow will be the speed of a very slow human breath.

 

[K^2]

Ok. In that case, I don't think it's going to be power limited, but the required current flow is going to depend very much on conditions, so you'll need an actual temperature regulator.

The simplest and most reliable construction would include a relay, transistor, a thermistor, a potentiometer (for tunning), and maybe a couple more resistors. If you want multiple settings, you can run three different potentiometers through a three-way switch, letting you tune each one separately.

In simplest implementation, thermistor and potentiometer together bias a base of the transistor. Transistor current runs to control coil of the relay. You want to set it up so that when temperature is at desired level, the current at the coil is right at the switching threshold. Naturally, at higher temperature, you want that current to drop to open the relay.

Relay will simply open and close connection between coil and power source. You'll have to chose the length of the coil to be such as to not overheat too badly even if for some reason power doesn't cut out and to provide sufficient resistance to not overtax the battery.

Do you have a specific battery in mind?

 

[kought]

Whoa. That's a lot of terms that I don't understand. When I made some PCB kits before, I soldered each piece where it was supposed to go, but I have no idea what each one does. So I don't really understand what any of those things do. But as long as I know I need them, I probably don't need to know exactly what each one does.

And as for the battery, I did not have a specific one in mind. I have to decide whether I want the battery to be removable or not. If it is removable, I was thinking maybe a AA-shaped Li-Po like an 18650? Or if it's not removable then maybe something a little smaller and like this? http://www.hobbyking.com/hobbyking/store/__7649__ZIPPY_850mAh_20C_single_cell_.html

That's just based on the very limited research I've been able to do. If you have any suggestions I'm totally open to them.

 

[K^2]

(Before I forget, typical solder melts at under 400°F, so you can't use regular solder to attach your coil or anything else that will be at that temperature. You can get heat-resistant solder, but that will probably require a special soldering iron.)You effectively have two parallel designs. Heating element and regulator. Each one is relatively straight forward.

Lets start with heating element. This is effectively just a NiCr coil, (I agree, that's a good choice) and a battery. As an example, let's look at battery you linked. It has two vital parameters.

Capacity: 850mAh
Discharge rate: 20C

20C tells you that this battery can spit out its charge in 1/20th of an hour. The capacity of 850mAh tells you that for 1 hour it can hold current of 850mA. So the maximum current it can withstand is 20 * 850mA = 17A.

That's a lot. At 3.7V, which is the output of a single LiPo cell, that's 17A 8 3.7V = 62.9W of power. Think mid-range incandescent light bulb. We are definitely going to go down from here, but it's good to establish an upper limit.

Wikipedia article on NiCr has many useful tables. The current-temperature table is your first stop. You definitely want a higher maximum temperature, so that the heater can kick in fast enough, but you probably don't want to go past 600°F. Note that for none of the gauges that puts you over 17A, which is great. It means that you aren't going to overtax the battery.

I would also be a bit worried about overheating your wires. You want to keep the current through these relatively low. The 600°F at 22 gauge wire suggests 3.63A of current. That seems reasonable. It would require resistance of 3.7V / 3.63A which is just over 1 Ohm. So now you want to determine wire length that will give you 1 Ohm.

The resistivity is listed in properties in the first table on the page. It's given at room temperature, but the resistance only increases by about 5% in your range, depending on composition, so it's close enough. To figure out resistance per length, you need to know cross-section area of your wire. This table should help. 22 gauge is 0.33mm². And resistivity is given as 1 to 1.5x10^-6Ohm*m. That's quite a spread, so you'll need to look up what it is for specific wire you'll be using.

Lets go with 1x10^-6 for simplicity. Resistance per length is resistivity divided by area. Keeping in mind that 1m = 1,000mm, 1m² = 1,000,000mm². So 0.33mm² = 0.33x10^-6m². Dividing resistivity by area, you get 3Ohm / meter. So for 1 Ohm of resistance, you need 1/3m, which is just over a foot of wire.

These figures will change depending on specific wire you go with. So look up these numbers, follow these computations, and you should be able to derive the wire length for your particular project. Then coil it up, making sure it doesn't touch itself or any other metal anywhere along, and you should have your heating element.

Ended up with length that's too long? 1 foot is probably excessive for your application, resulting in rather large coil. What you need to do is go to thinner wire. Instead of 22 gauge, maybe try all these computations for 24 gauge. That will both reduce current needed to heat up the wire and increase the resistance, resulting in shorter wire needed and a smaller coil.

Ok, so that's pretty much what I can tell you about the heating element itself. Let me get some thoughts together and I'll write up something on thermal regulator.

 

[kought]

Wow. That is incredibly helpful, thanks so much!

I plan to use silver solder, and after doing some research it seems like I'll need to find some solder that's more than the standard amount of silver, maybe like 10% or so to get the melting point to where I want it do be.

When you say we are gong to go down from 62.9W, what do you mean? As in use a smaller battery?

I am a little confused when you say to look under the 600°F tables. I realize it will be very hard to calculate the temp because there will always be airflow going through it (and a very inconsistent and slow airflow at that), but I don't want it to ever be able to get above ~430°F without airflow.

And as for the Wiki tables, the highest required amperage is 15.91A for a 12 gauge NiCr wire. How come I don't want that? Does using a smaller wire mean that I will have longer lasting battery power?

What makes you say 22 gauge wire? Is that just a good standard starting place?

As of right now, I am thinking that I will wrap the wire around a glass tube, with probably about a .5" OD, and then the herb material will go inside the tube, so that it will be vaporized without coming in contact with anything but the glass. Do you think I might have any issues with the glass expanding/contracting with the heating coil wrapped directly around it?

Another thing I am concerned with is battery life. I imagine the battery will be engaged for however long it takes to heat up plus about 12-14 seconds, and then disengaged. So I want to find a battery that will last for as many of these as possible. As I understand it, the figure that has to do with battery life is mAh, and I want the highest value while still maintaining adequate space, right?

The last thing I have been thinking about is charging. I really want it to be able to have an AC adapter (so you can plug it into the wall and use it) and also be able to be used while being charged (sort of the same thing, either would work probably). I think this means that I want the battery to be close to 12v, right? So what about one of these batteries?

http://www.hobbyking.com/hobbyking/store/uh_power_search.asp?idCategory=86&strSearch=11.1v&location=

Thank you SO much for all your help so far! I have already learned so much. And of course all the questions I asked are not doubting you in the slightest, I am just trying to get a better idea of how it will all work.

 

[kought]

I found a Pyrex tube that has a 13mm OD and 1.2mm walls. I read that Pyrex has a low coefficient of thermal expansion, and especially when its only to ~400°F, I would think it wouldn't be an issue to wrap the NiCr wire around it.

And would it be alright to just wind the NiCr wire around the glass tube and hope that the wire would slide down and touch itself? What are the consequences of two loops touching each other?

 

[kought]

Here's a pretty crude drawing I made of the heating element. Starting with a 13mm OD tube, you wrap that with the NiCr wire coil. Then you put a silicone heat resistant O-ring around each end of it, and fit it inside of a 14mm ID tube so you have a nice seal and the wire won't be touching anything directly except for the silicone O-ring which should stand the heat.

Unless it would be an issue to have this air sealed in there... If this made a good seal and was heated to 400°F, would the air expand and make it explode? Just thinkin' out loud here.

:) Thanks