Heating element

[Rob_K]

Hi guys, getting a little stumped here. I need to learn how to make hot wire, but can find nothing on line as to how this is done. I want a bit of wire that will heat up safely, but I am assuming you need special wire or it will just break won't it?

I want to do some research on how to make a controllable heating element that is about 0.5m in length, and not more than 1mm thick.

Any ideas?

Kind regards

Rob

 

[duke37]

There was a thread here a month or two ago on making a hot wire to cut polystyrene.

The power supply can be AC or DC, it depends on what you have and the amount of control you need.

The wire should be resistant to high temperatures, constantan or kanthal come to mind. Also stainless fishing wire or perhaps stainless guitar wire or suture wire.

 

[Rob_K]

Ok, I am trying to make a heating element that will be eventually be embedded in rubber with the aim of heating it up to temperatures of about 20 degrees centigrade initially, though I want to be able to control the temperature, it is for an item of clothing for cold weather, so the temperature will need to be able to be safely controlled.

Initial research is just how it is heated. I will be adding a temperature sensor and most likely designing the system using an arduino, as arduinos are cheap, I have 3 of them and it is dead easy to hook up to Matlab to get some good initial data from.

I don't know the terms to use to get any search results. Could you post the link to the hot wire for cutting polystyrene that you mentioned, as that is a perfect place for me to start. I have searched for it, but it is not coming up for some reason.

Kind regards

Rob

 

[duke37]

I cannot find the thread either but it dealt with mains driven hot wires. This will not be suitable for you.

Electric blankets have heated elements and should be scrapped after a while for safety. These could be a source of insulated heating wire to be run on low voltage.

 

[Rob_K]

Ok, I have been able to do some research now, so I just want to get the finer points right so I don't start burning the house down. I understand what I am looking for is "nichrome resistance wire". Now, I understand that resistors control the current in a circuit, but you can't just hook up a resistor to a battery can you, it will just catch fire slower won't it? what do you put in series with this to be able to complete a circuit safely? Say I am running this system off a 9V DC supply, I am assuming I need something that will provide the necessary voltage drop to be able to ground this circuit safely?

Kind regards

Rob

 

[duke37]

You do not want any external resistance, this will only waste energy. The way to do it is to switch the heating element on and off, quicker than it can heat up, so the average power is reduced.

The battery will have to be substantial, a diddy PP3 wil not do much. The resistance will need to be matched to the battery voltage.

 

[Rob_K]

Ok, so would a Lithium ion battery work with this and can I just hook it up? will it not just short the battery out?

 

[(*steve*)]

The idea is that the resistance of the wire is sufficient not to short the battery out.

However, if you have a low resistance wire, one option is a switch-mode variable constant current source. This will allow you to regulate the current (and that is what causes heating via I^2R losses in the wire) without wasting huge amounts of heat in the regulator.

It's not a very pretty solution, mostly because the current source is going to be a rather expensive item.

Since the load is a constant resistance, an effective alternative is to power the element via PWM, but place a suitable inductor in series with the heating element and a diode to allow current to circulate in the loop. This is topologically identical to a buck SMPS regulator except that it has no voltage or current feedback

It would be better to have a high resistance element as it won't require high currents to achieve the same heating. As stated above. wire from an old electric blanket is great -- I had a heap I used for all sorts of stuff as a kid that I got from an old electric blanket that no longer worked. Once you have determined the resistance of a certain length of the wire, you can use ohms law to determine the length you require to dissipate the desired amount of heat from the voltage you have. If you do this right then you can do something not much more complex than just connecting it to a battery.

You want to investigate P = V^2/R

 

[Rob_K]

Right, thank you Steve, that is a big help.