I make my own heated clothing with this carbon fiber tape designed for clothing. It is basically a simple resistance heating strip.
https://www.carbonheater.us/

I used two strips and put them in series as seen in the picture and connected them to 12 volts.

The longer strip is 2.9 ohms, the shorter strip is 1.9 ohms.

For the life of me why is the longer strip hotter than the shorter strip when connected in series as shown in the picture? Shouldn't the strips both be the same temperature because they are in series, similar to if they were just one long strip? If I give power to just the short strip it gets way hotter by itself.

Basically the longer strip is much hotter to the touch then the shorter strip when in series. In parallel the short strip is way hotter.

Why I'm stumped is if the two strips were just one long strip and never cut, it would be the same heat through the whole strip.

Some specs. at 12 volts

Both heating strips in series draw 2.3 amps as shown in the picture.
The long heating strip draws 3 amps by itself. @ 2.9 ohms
The short heating strip draws 6 amps by itself. @ 1.9 ohms

Below are the two strips in series. I am missing the 12 volt battery in the picture. But you can see the circuit.

 

If you round the resistance values to 2 ohms and 3 ohms then then the longer strip will disipate 50% more power than the shorter strip. Looking at the picture its is not 50% longer than the shorter strip so the power per unit area will be greater so it will reach a higher temperature.

Les.

 

P = I^2 x R

With the same current through them, the higher resistance unit will produce more power, since it is higher resistance.

 

I make my own heated clothing with this carbon fiber tape designed for clothing. It is basically a simple resistance heating strip.
https://www.carbonheater.us/

I used two strips and put them in series as seen in the picture and connected them to 12 volts.

The longer strip is 2.9 ohms, the shorter strip is 1.9 ohms.

For the life of me why is the longer strip hotter than the shorter strip when connected in series as shown in the picture? Shouldn't the strips both be the same temperature because they are in series, similar to if they were just one long strip? If I give power to just the short strip it gets way hotter by itself.

Basically the longer strip is much hotter to the touch then the shorter strip when in series. In parallel the short strip is way hotter.

Why I'm stumped is if the two strips were just one long strip and never cut, it would be the same heat through the whole strip.

Some specs. at 12 volts

Both heating strips in series draw 2.3 amps as shown in the picture.
The long heating strip draws 3 amps by itself. @ 2.9 ohms
The short heating strip draws 6 amps by itself. @ 1.9 ohms

Below are the two strips in series. I am missing the 12 volt battery in the picture. But you can see the circuit.

They aren't the same size. In series, the longer one has to dissipate more current. In parallel, you have a venturi effect- same amount of current must flow through both strips, so shorter one will be hotter as it must dissipate more energy over a smaller surface area to be equivalent.

 

The long heating strip draws 3 amps by itself. @ 2.9 ohms

If you drive this with 12 volts then the resistance can't be 2.0 ohms--it has to be 12/3 = 4 ohms.
Or am I being careless again?

 

Sure .... you are correct both strips will be at the same temperature , you have made an error in measuring the temperature , or there is a high resistance point where you connect ...

Measuring temperature in the middle of the two strips it will be the same , if they are both laying flat on the carpet pictured .The way they lay will effect heat loss downwards ....

You will need a big battery to keep you noticeably warmer for any length of time ... probably only feasible for sitting in a vehicle when you can plug into the vehicles battery

 

Each strip will produce the exact same heat per inch.

The longer strip will produce more total heat.

 

Each strip will produce the exact same heat per inch.

The longer strip will produce more total heat.

It will ... but it will not get hotter since it has a greater surface from which the heat escapes

 

What if you rolled them up?

 

You will need a big battery to keep you noticeably warmer for any length of time ... probably only feasible for sitting in a vehicle when you can plug into the vehicles battery

Actually motorcycle attire is lined with heating strips as well as heated motorcycle seats. This includes jackets, pants and gloves which all interconnect and plug into the bikes accessory outlet or cigarette lighter outlet. The right gear will actually keep a person very warm in sub freezing temperatures. Pretty cool stuff or pretty hot stuff actually for the guys who like to ride in the winter in cold climates.

Ron

 

These heating strips are not quite an exacting science as to resistance. They come in assorted widths and thickness. Just for example the 22mm Carbon Fiber Tape has a resistance of about 12±1 Ohm/m ( 3.65±0.5 Ohm/ft ) the width is 22±2 mm (0.87±0.08 in). So there is a pretty wide tolerance. The connection methods are also important especially if you want the stuff to work well. You buy a long strip in the width of your choosing and just use a pair of scissors to cut to desired sectional length. Pretty slick if you want to make your own clothing.

Ron

 

Well P=I^2 * R and I is constant in a series circuit, so I^2 is a constant.

One strip is K* R1, the other K * R2

the bigger R, wins. Higher power.

 

I think Les nailed it in post #2. The original question assumes that both strips have identical ohms-per-inch, but the data and the photos contradict that.

ak

 

Well P=I^2 * R and I is constant in a series circuit, so I^2 is a constant.

One strip is K* R1, the other K * R2

the bigger R, wins. Higher power.

Yeah, that is true but and there has to be that but in the mix. Lets say I have some with a 35 mm width which is 6.0 +/- 1.0 Ohms/Meter (39.37 inches) so that gives us about 6.0 / 39.37 = 0.152 Ohm/Inch. So I cut a 6" and then a 12" section. The 12" section will have the higher wattage since P = I^2 * R so yeah, the bigger the R the higher the power for a fixed current. My 6.0" strip has an area of about 8.268 Square Inches and my 12" strip has an area of twice that at 16.536 Square Inches.

All in all I have 18" of material at about 2.736 Ohms my current will be about 4.385 Amps. So if we do the I^R routine we get about 52.6 Watts of which the 12" section will be 2/3 of total power or about 35 watts but that heat is spread out over twice the area of my 6.0" length. So which section is really hotter?

Ron

 

You probably want to calculate W/sqcm for different size material.

 

I make my own heated clothing with this carbon fiber tape designed for clothing. It is basically a simple resistance heating strip.
https://www.carbonheater.us/

I used two strips and put them in series as seen in the picture and connected them to 12 volts.

The longer strip is 2.9 ohms, the shorter strip is 1.9 ohms.

For the life of me why is the longer strip hotter than the shorter strip when connected in series as shown in the picture? Shouldn't the strips both be the same temperature because they are in series, similar to if they were just one long strip? If I give power to just the short strip it gets way hotter by itself.

Basically the longer strip is much hotter to the touch then the shorter strip when in series. In parallel the short strip is way hotter.

Why I'm stumped is if the two strips were just one long strip and never cut, it would be the same heat through the whole strip.

Some specs. at 12 volts

Both heating strips in series draw 2.3 amps as shown in the picture.
The long heating strip draws 3 amps by itself. @ 2.9 ohms
The short heating strip draws 6 amps by itself. @ 1.9 ohms

Below are the two strips in series. I am missing the 12 volt battery in the picture. But you can see the circuit.

You're not working it out correctly,, W= I x I x R

12V in series is 4.8 ohms = 2.5Amps.
2.9 ohms x2.5 x2.5 = 18.7W
1.9 ohms x2.5 x 2.5= 11.8W

 

Thermodynamics is not as simple as calculating wattage and dissipation area, there are other factors at work, such as mass and environment.

In the case of the strips my guess is the environment* is playing a major factor, by absorbing and giving back more heat than the smaller area of the shorter strip.

*The surface it's lying on.

EDIT:

Also the mass…every square of the strip in not only radiating outward but into other areas of the strip as well.

 

Hello,

As @ElectricSpidey said, you will measure different temperatures depending on the surface.
When it is laying on a brick floor, the temperature will likely be lower as when it is laying on a wool carpet.

Bertus

 

Given the apparent difference in lengths and the tolerances as suggested by Ron, there is clearly a connection issue and none of the reported values for current and resistance "add up." I expect resistance measurements in this range to be off substantially due to test lead issues and the like, however measured currents should be reasonably accurate and they suggest the actual resistance of the short piece is half that of the long piece. "Actual resistance" here includes everything in the path. The alligator clip leads pictured certainly don't look accurate for 6 A.

The currents can be measured at any point in the circuit that is convenient. With known current, the voltage across the strips, as measured between their connections on the strips themselves, must be determined. These two values should be meaningful.

I tried looking at the linked website. It is an abomination, so I gave up. The tape is described as carbon fibre and viscose (rayon, a regenerated cellulose fibre). I presume the warp (longitudinal fibres) are carbon and the weft (a.k.a. "fill" - the crosswise yarns) is viscose which is non-conductive. The weave design doesn't look conducive to easy connection to the carbon fibres, if it is make as I think it is.

 

Hello,

Did you see this table at the given website?



I have assumed you have the 15 mm version.
There are also tables for the 44 mm ribbon and cable.

Bertus