Hi all,
I would like to play around with some heated elements for jackets / vests and I was wondering... how are these commonly controlled? Is it by constant current or constant voltage?
If using constant current, the resistance will increase when the element gets hotter and theoretically voltage will have to increase to compensate, correct?
If using constant voltage, resistance will also increase when the element heats up and then it will have to draw more current to maintain voltage?
So does it matter?
(Trying to do this without feedback from temp sensor - no closed loop).
Thanks!
Is it by constant current or constant voltage?
constant voltage.
If using constant voltage, resistance will also increase when the element heats up and then it will have to draw more current to maintain voltage?
No the effect is very small for the range of temperatures you will be using, it will make no odds.
If using constant voltage, resistance will also increase when the element heats up and then it will have to draw more current to maintain voltage?
Of course, you mean maintain current since we've stipulated constant voltage. 
As far as I know, that effect only "kicks in" when the heating (or lighting) element begins to glow. A jacket heating element isn't going to glow (hopefully).
I assume you're using PWM, so this would be constant-average voltage.
If you're adjusting the heat with a pot it won't matter because you'd have full-range and the "feel" is the feedback. If you have a high-low-off switch, you'll probably want to calibrate that by feel also.
One thing to remember is that heat is proportional to power and power is current x voltage, so power and heat) is proportional to the square of the voltage (1/2 the voltage is 1/4 of the power).
Thanks guys... I will give it a try with some adjustable voltage regulators and the feedback will be "feel" as DVDdoug suggested. Let's see what happens!
Using voltage regulators means the heat that is not generated in the jacket is produced in the regulator.
Since the first thoughts are to use nichrome wire as the heater, I would offer that you should spend some time reading the data sheets for that stuff.
I would also think that there are more than a few circuits for how to make heated socks and gloves, and car seats and mirrors, and steering wheels.......
Ah, I see. So rather than voltage regulator, a PWM that controls the amount of time on vs off would be better suited and not dissipate much heat.
Now, what if the heated pads are rated for 7.4V but I am using a 12V battery? Would this still not need some type of buck converter?
Thanks for all of the help so far!
Would this still not need some type of buck converter?
No.
This is a heater, in effect a piece of resistance wire. It has no inherent limit to the voltage you can apply to it.
what if the heated pads are rated for 7.4V
Is this real or did you make it up?
That would mean at 7.4V enough heat was produced to prevent it damaging itself, like melting through the insulation. So if you applied 12V that means it getting 1.6 times the voltage it need to operate within thermal limits. So basically don't go over a 50% duty cycle with the PWM.
Grumpy_Mike:
No.
This is a heater, in effect a piece of resistance wire. It has no inherent limit to the voltage you can apply to it.
Is this real or did you make it up?
That would mean at 7.4V enough heat was produced to prevent it damaging itself, like melting through the insulation. So if you applied 12V that means it getting 1.6 times the voltage it need to operate within thermal limits. So basically don't go over a 50% duty cycle with the PWM.
need to check the Data Sheet.
some LED's allow for 50mA pulse, but will burn out if more than 25mA continuous.
also, many FET's have really high values like 50A pulse, but 15A constant.
post a link to the data sheet if you have questions.
This is what I ordered... the listing says 7.4V ... and it seems that the optional battery is 7.4V nominal.
https://www.ebay.com/itm/7-4V-Heating-Pads-DIY-Thermal-Vest-Heated-Jacket-Outdoor-Mobile-Warming-Gear/322365880232?ssPageName=STRK%3AMEBIDX%3AIT&var=511293510701&_trksid=p2060353.m1438.l2649
I have some Milkwaukee 12V tool batteries that I would like to use to run the heaters instead of the optional battery in the ebay listing. It is a matter of convenience since I have a number of these batteries and chargers on the job site.
I ordered this PWM controller... I can 3D print a new housing for it to include the knob and button. Sounds like this should work for me then... I can just turn the duty cycle down because of my 12V power source.
https://www.ebay.com/itm/DC-6-30V-12V-24V-MAX-8A-MOTOR-PWM-SPEED-CONTROLLER-WITH-DIGITAL-DISPLAY-SWITCH/332081486336?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649
need to check the Data Sheet.
No you don't. This is not an active component like an LED, it is a heater, a piece of resistance wire.
Grumpy_Mike:
No you don't. This is not an active component like an LED, it is a heater, a piece of resistance wire.
Sorry, but I have to disagree. for residential wiring in the US, the NEC lays down the law based on the data sheets.
the thickness of the wire, the length, the voltage and the application are all in strict accordance. and a as you say, a piece of copper is just an element.
Nichrome can melt with a 12V power supply at 1 amp 100% duty cycle.
There is something to be learned about any thing from the data sheets.
and since it melts at 1,950 deg F, it would burn a human really painfully.
used in clothing, it would start most on fire much sooner than it would melt.
Yes but we are not talking about residential wireing are we? We are talking about a battery operated heater where you are not over driving it but giving the equivalent powe as it is rated at. The data sheet will say nothing about pulsing it with 12V instead of continuous voltage of 7.4V. In Europe this will come under the low voltage directive, it is nothing to do with domestic wiring.
To expect
- for this to be a problem
- for this to be mentioned in the data sheet
is the height of naivety and I am very supprised about this coming from you.
Grumpy_Mike:
Yes but we are not talking about residential wireing are we? We are talking about a battery operated heater where you are not over driving it but giving the equivalent powe as it is rated at. The data sheet will say nothing about pulsing it with 12V instead of continuous voltage of 7.4V. In Europe this will come under the low voltage directive, it is nothing to do with domestic wiring.
To expect
- for this to be a problem
- for this to be mentioned in the data sheet
is the height of naivety and I am very supprised about this coming from you.
Maybe I am looking at it in a different way, the OP is asking about voltage and why he should not use 7.4 and if he needs a booster to get to 12v.
in regards to the data sheet, with constant voltage, the wire temperature is dependent on two things found on the data sheet.
diameter and length.
from that, one calculates current.
one picks the heat desired.....
the voltage available
then selects the distance of the wire(s)
then the diameter.
then calculates current.
IMHO once the OP looks at the tables and starts the selection process, the questions of voltage will have a different perspective. And more questions.