I am using a nichrome wire, underneath a silicone skin to simulate the temperature of a human arm. I want to vary the temperature up and down by a few degrees Celcius. I initially wanted to power this all through battery however they were not able to supply enough power to get it to the desired temperature quick enough. The plan is to get the initial heat up from a wall plug and then sustain this heat using a 9V battery. My question is how can I do this is there any special hardware that I may need to do this?
Also in order to change the temperature of the device, how can I control the amount of power going into the wire, would I need to create some kind of negative feedback loop that can sense the temperature of the wire and turn off the power supply when it reaches that temperature. Would it be a constant process of turning on or off or is there a way of sustaining a desired temperature?
What is a desired temperature accuracy? What is your nichrome wire resistance? Are you sure your 9V battery will last long enough to fit your requirements?
There are 2 main methods to keep desired temperature Thermostat and PID. It's explained many times just google it.
Heating the wire is easy: let maximum current flow until the desired temperature is reached.
Cooling down might require a fan or other means.
Nichrome resistance is not very temperature dependent, you'll need a temperature sensor as well.
In a human arm the temperature is produced by buried muscles. Your model is similar because the temperature sensor is a bit apart from the wire and does not follow the wire temperature immediately.
A 9V block battery has no remarkable capacity. If you need a power supply for fast heating up you can use the same supply for maintenance of the temperature.
Let's discuss the real world of using nichrome wire for heat. The problem is how you go about connecting the copper wire feeding current to the nichrome wire. Chances are the resistance of the connecting mechanism will generate as much or more heat than the wire. Avoid the problem by silver brazing the copper wire to the nichrome wire. Do not use compression connections. If you don't care about the end being hotter, compression is ok.
alesam:
What is a desired temperature accuracy? What is your nichrome wire resistance? Are you sure your 9V battery will last long enough to fit your requirements?
There are 2 main methods to keep desired temperature Thermostat and PID. It's explained many times just google it.
The goal is to have three distinct temperatures, these ranging between 35-40 Celcius, These don't have to be exact values however, you could be able to touch it and know that is higher than normal body temp or lower.
The nichrome wire has a resistance of around 1.5ohms. The idea was that that the 9V battery would last long enough as it doesn't have to heat up all the way from room temp. The device would warm up with the wall in and then be able to hold onto most of this heat with insulation and only use the battery to top up.
DrDiettrich:
Heating the wire is easy: let maximum current flow until the desired temperature is reached.
Cooling down might require a fan or other means.
Nichrome resistance is not very temperature dependent, you'll need a temperature sensor as well.
In a human arm the temperature is produced by buried muscles. Your model is similar because the temperature sensor is a bit apart from the wire and does not follow the wire temperature immediately.
A 9V block battery has no remarkable capacity. If you need a power supply for fast heating up you can use the same supply for maintenance of the temperature.
How do you vary the maximum current, after it has been heated how would you prevent the device from continuing to get hotter and hotter?
The temperature sensor could be placed in different regions just as long as we know how it is proportionate to the temperature on the surface of the skin?
The initial design criteria was for the device to be wire-free when it was running, so we would get it up to temp and then have a battery power it. A 9V battery doesn't have to be used but do you not think that any kind of battery would be feasible?
Paul_KD7HB:
Let's discuss the real world of using nichrome wire for heat. The problem is how you go about connecting the copper wire feeding current to the nichrome wire. Chances are the resistance of the connecting mechanism will generate as much or more heat than the wire. Avoid the problem by silver brazing the copper wire to the nichrome wire. Do not use compression connections. If you don't care about the end being hotter, compression is ok.
Paul
That was something we encountered whilst we did some testing, sometimes the resistor we used would get hot. However, with connecting the nichrome wire, I don't think it is that significant if the ends are hotter as the device will have a larger surface area so most of the contact with the skin will not be done towards the ends. However, do you think that the resistance that builds up at the connection could have a greater effect on the whole circuit?
lenihan_the_man:
That was something we encountered whilst we did some testing, sometimes the resistor we used would get hot. However, with connecting the nichrome wire, I don't think it is that significant if the ends are hotter as the device will have a larger surface area so most of the contact with the skin will not be done towards the ends. However, do you think that the resistance that builds up at the connection could have a greater effect on the whole circuit?
Only your experimentation will determine that.
Many times a wire is hotter in the middle because the connecting screws act as heat sinks.
Paul
lenihan_the_man:
The nichrome wire has a resistance of around 1.5ohms.
At 9Volt it gives you around 55Watts which could be to much. I'd suggest to use 18650LiIon battery.
And 18B20 temperature sensor plus logic level MOSFET(s) to control a heater.
lenihan_the_man:
How do you vary the maximum current, after it has been heated how would you prevent the device from continuing to get hotter and hotter?
I turn off the current. Unless you have a very sophisticated heating model a two point on/off regulator is perfectly sufficient.
lenihan_the_man:
The temperature sensor could be placed in different regions just as long as we know how it is proportionate to the temperature on the surface of the skin?
The initial design criteria was for the device to be wire-free when it was running, so we would get it up to temp and then have a battery power it. A 9V battery doesn't have to be used but do you not think that any kind of battery would be feasible?
If your skin isolates heat well then it has environment temperature, with or without heating. If the skin shall have a higher temperature it must be permanently heated. The amount of heating energy depends on how much heat is dissipated over time. A 9V battery may be sufficient for 10 minutes...
I'd postpone the battery question until you can measure the required energy in real life. If you plan to add motors or other actors to your arm you may run into the opposite problem, when the actors consume much power and heat up the arm more than the heating wire.
Please reveal more about the purpose of your project. Why an arm instead of simply a heat sink?
DrDiettrich:
Please reveal more about the purpose of your project. Why an arm instead of simply a heat sink?
The project is to make a device that can be attached to a mannequin so that when a nurse touches it instead of feeling the cold plastic it is the actual feel and temperature of a human. The nurse normally touches on the forearm so my group is making a sleeve that can fit around the arm, the heating wire will be underneath an outer layer of silicone. I don't know if that answers your question about a heat sink, we are hoping that the silicone rubber will act as a heat sink and the heat will be dissipated through that so when a nurse or whoever touches it can feel the temperature.
You are approaching this whole project in a very childish way. Hoping for this and guessing at that. If you are hoping that your silicon will act as a heat sink, why don't you actually test it?
We are of course, all wondering what you had in mind for a "9 V battery". Clearly not one of these surely?
I do not think any of us would use a nichrome wire for this. If you want fairly uniform heat distribution, a matrix of small resistors would be more appropriate.
The other Paul is right.
I used to stuff four low value wire-wound resistors into circuit boards that provided heat for a green laser diode used in snow plow operation. These were 5 watt devices and ran at 12 volts. You can get physically smaller devices, but the idea is better than nichrome.
