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# Nichrome Wire: I'm missing something

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### #1 StarBuddy

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Posted 14 October 2019 - 12:51 PM

In making dew heater strips for my 12" Dob I found that the strip for the primary doesn't generate enough heat to be effective. Originally using 28 gauge Nichrome 80 I decided to try making another (+/- 42") strip using smaller (40) gauge wire. When I compared the properties of the 28 against the 40 I found that in applying the same current to the same length of wire the 28 ran much hotter than the 40. This result was the opposite to what I expected. I'll readily admit my electrical ignorance, but I always thought that smaller gauge should run hotter than larger. Both are labeled Nichrome 80 so the only known variable is the gauge. What did I miss? Ohm ratings are included below:

### #2 JohnH

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Posted 14 October 2019 - 01:26 PM

One way you can do this is you connect up a bunch of resistors in series, and then what you do is you match the resistance and current to figure out your total watts of dissipated power and that gives you roughly how much heating that you generate

Edited by JohnH, 14 October 2019 - 03:51 PM.

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### #3 StarBurger

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Posted 14 October 2019 - 01:42 PM

Everything works according to Ohms law, the good old triangle:

V

I R

Reduce the resistance and for the same applied voltage the watts will increase:

Put 12 v across a 10 ohm resistor = 1.2 A     watts = V * I = 1.44 W

Put 12 V across a 100 ohm resistor = 0.12 A     watts = V*I = 0.144 W.

Probably the most useful formula in physics !

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### #4 StarBurger

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Posted 14 October 2019 - 02:13 PM

Also if you are new to multimeters be aware that the scale on the meter in photos should be read from right to left, i.e.  40 gage = 8 ohms  28 gage = 0.8 ohms.

I have to admit I was confused as i am used to meters that read from left to right.

Nothing's easy right?

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### #5 scottmm2012

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Posted 14 October 2019 - 02:28 PM

Its easier to wrap your mind around it if you use a water and hose analogy.   Think of the wire as two different diameters of hose.  Think of watts as a volume of water and amps as pressure.  The higher the pressure the more resistance, would you agree?

100 gallons of water through a 2" hose as opposed to 100 gallons of water through a 1/2" hose.  What do you think would provide the more resistance and therefore higher pressure?

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### #6 Exciton

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Posted 14 October 2019 - 02:41 PM

Starbuddy,

Your measurments are mostly in line with what you might expect.  28 gauge Nichrome 80 has a resistance of about 4 ohms/ft (you measured 8 ohms) and 40 gauge Nichrome 80 has a resistnace of about 68 ohms/ft (you measured 80 ohms).  For an inexpensive meter, this is ballpark close to the correct values.  It also assumes you properly zeroed out the meter with the "Adjust Ohms" dial on the right side of the meter.

Both StarBurger and kingjamez are exactly correct.  If you powered your wires with a constant voltage source (such as a 12v power supply), than the wire with less resistance will draw more current and get hotter (P=V^2/R).  If you drive the wires with a constant current supply (you would need a constant current regulator to do so), then the higher resistance wire will get hotter (P=I^2 * R).

A suggested, if you know how many watts of heat you want, you can calculate the desired current flow (for a constant voltage supply) and then cut the wire to the proper length to give you the desired resistance.

Happy heating,

Exciton

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### #7 kingjamez

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Posted 14 October 2019 - 02:52 PM

Lower resistance = higher amps. Your photos show that the 28 gauge has lower resistance as expected.

1 ohm at 12v is 12 amps.

Heat is the the result of the Wattage through the wire. Watts = Volts times Amps.

For the above 12 amps X 12 volts == 144 Watts.

If the resistance increases. Say 2 ohms. The amps decrease according to V=IR so 2 ohms,12volts yields 6 amps.

6 Amps X 12 Volts = 72 Watts. Half the heat.

I measure out my wire based on the Watts I think I need to do the job. I also use about 75% of the wire I need so it runs a bit hot. Better to have a little too much heat than too little.

A PWM 12V LED dimmer can be found cheaply on amazon that can then dial back the heat as required.

-Jim

Edited by kingjamez, 14 October 2019 - 03:01 PM.

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### #8 StarBuddy

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Posted 14 October 2019 - 02:53 PM

So far the much-appreciated replies agree with me that thinner wire should run hotter than thicker wire, all other variables being the same. All agree???

If so, does anyone have a clue as to why I can barely feel any warmth at all in the 40 ga Nichrome and at the same length of 28 ga I get my finger burned?

Again, thinking the wire compositions could possibility be different, they'e both labeled Nichrome 80.

Thanks again all!

### #9 Exciton

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Posted 14 October 2019 - 03:01 PM

Starbuddy,

I am afraid that Ohms law, and most of the responders to your post, agree that your fingers have it correct.  Thinner wire -> more resistance -> less current from a constant voltage source -> less power dissipated.  Go back through the equations provided and you will see that your fingers gauged it correctly (pardon the pun).

Exciton

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### #10 kingjamez

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Posted 14 October 2019 - 03:02 PM

So far the much-appreciated replies agree with me that thinner wire should run hotter than thicker wire, all other variables being the same. All agree???

Thanks again all!

No. Thinner wire = more resistance = less amperage = less Wattage = less heat.

Edit: Excition beat me by a minute!

Edited by kingjamez, 14 October 2019 - 03:03 PM.

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### #11 StarBuddy

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Posted 14 October 2019 - 03:57 PM

OK.. so... all things being equal thicker wire will generate more heat than thinner. That's what my fingers are correctly saying as I reach for the Neosporin and Bandaids..

My mistaken assumption goes to my limited knowledge of heavy loads needing heavier gauge wiring.. i.e. clothes dryers and ovens and such running 220 needing 10 or 8 gauge whereas trying to run those guys on 18 gauge speaker wire would cause a fire.. (thinner wire = heat in this case)

Therefore I need to get some heavier gauge Nichrome 80.. I built a 5-channel controler with those PWM 12V LED dimmers so I can control the temp to get just as much as I need..

Thanks to all for the education!!!

### #12 kingjamez

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Posted 14 October 2019 - 05:08 PM

My mistaken assumption goes to my limited knowledge of heavy loads needing heavier gauge wiring.. i.e. clothes dryers and ovens and such running 220 needing 10 or 8 gauge whereas trying to run those guys on 18 gauge speaker wire would cause a fire.. (thinner wire = heat in this case)

You got it. In the electrical wiring above the amp draw of the system is not limited by the short circuit resistance of the wires but the amp draw of the appliance / welder/ car charger/ etc. Thin wire would heat more given a ~fixed~ current draw because it has a higher resistance.

-Jim
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### #13 StarBuddy

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Posted 14 October 2019 - 05:36 PM

I need to make a strip around 42 inches long to get around the OTA at the primary mirror (12") location. A single run of 28 ga just barely got above room temp, and I really would like to double the run to warm a larger area of the OTA. Would 22 gauge be a significant improvement or should I go to 20, or even higher?

Again I'm using the dimmer controler so I do have control of how hot the strip gets.

THANKS SO MUCH!!!

### #14 kingjamez

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Posted 14 October 2019 - 05:53 PM

Parallel two strings ( or more!) of 28 gauge. Putting two pieces of wire in parallel will halve the resistance and thus double the heat output. Three pieces will triple it... etc.
-Jim
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### #15 KLWalsh

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Posted 14 October 2019 - 06:15 PM

It’s a common mistake to think more resistance = more power.
As many here have pointed out, for a fixed voltage, LOWER resistance will yield more heat when using ohmic materials.

P = V^2 / R. So, as R goes smaller, P gets higher.
And since the two wires have the same composition, the fatter wire will produce lower Ohms, R.
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### #16 mich_al

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Posted 14 October 2019 - 06:34 PM

Everything works according to Ohms law, the good old triangle:

V

I R

Reduce the resistance and for the same applied voltage the watts will increase:

Put 12 v across a 10 ohm resistor = 1.2 A     watts = V * I = 1.44 W

Put 12 V across a 100 ohm resistor = 0.12 A     watts = V*I = 0.144 W.

Probably the most useful formula in physics !

12 * 1.2 = 14.4

12 * .12 = 1.4

Post #1 OP said he applied same current to each wire..  Same current or same voltage?

Ohm's  I = V/R

Same Current

I^2 * R = W

@1A  1 * .8 = .8W   Fat wire

1 * 8 = 8W    Thin wire hotter

Same Voltage

V/R = I     I^2 R = W

@12V  12/.8 =15A  => 180W     Fat wire hotter

12/8 = 1.5A => 18W      Thin wire

Edited by mich_al, 14 October 2019 - 07:16 PM.

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### #17 Myk Rian

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Posted 14 October 2019 - 07:09 PM

Have you tried Vape wire?

https://www.ebay.com...e wire&_sacat=0

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### #18 kingjamez

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Posted 14 October 2019 - 07:16 PM

Have you tried Vape wire?

https://www.ebay.com...e wire&_sacat=0

Isn't that the same thing?

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### #19 Myk Rian

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Posted 14 October 2019 - 07:49 PM

I don't know. I have a roll of it. It's spiral wound instead of a single strand. Haven't tried it yet, but there was a thread earlier this year where the maker used a 3D printer to make a plastic strip to wind it on, A couple layers of a heat resistant tape, A base and cover layer, and it worked.

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### #20 StarBurger

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Posted 16 October 2019 - 07:22 AM

Thanks mich al for spotting my mathematical error!

Back to grade school for me!

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### #21 Jon Isaacs

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Posted 16 October 2019 - 07:59 AM

Thanks mich al for spotting my mathematical error!

Back to grade school for me!

Actually it was a conceptual error. As I understand it, you were thinking constant current (amps) but applying a constant voltage.

Jon

### #22 StarBuddy

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Posted 16 October 2019 - 11:29 AM

Thanks to all for the help! I learned something this week! At my age that's a cool thing.. I should have paid closer attention in my high school math classes to understand the formulas, but I'll study those until I do. In the  meantime if any of you ever need to know how to voice an F13 (+9+11) so it's sounds really cool in a large jazz ensemble let me know.. that I can help you with.

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### #23 Jim in PA

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Posted 19 October 2019 - 09:01 AM

Starbuddy,

Keep in mind, in the end, what you're trying to do is keep your optics a few degrees warmer than the ambient temp, right?  Not too hot, not too cold.  There's a way to calculate how hot the wire will get based on its mass, diameter, voltage, current and time...using integration... but unless you're into that sort of thing...I'll skip it

Simply calculating the power in watts isn't really going to tell you how hot the wire will get.  You're using your fingers at this point to figure out how hot the wire gets.  So Ohm's Law won't help you much.

What you really need to do is use something like an ambient temp sensor,  a thermocouple, and something like an Arduino.  Then all you have to do is tell the Arduino how hot you want the wire to be in relation to the ambient temp.  Doing this, you get to skip the integral calculus, and if you use something like a potentiometer (to adjust/control how much hotter you want the wire to get) and an LCD or series of red LED's(to display ambient temp and temp of the wire) the system would get fairly usable at that point.

This is a pretty simple task, but I'm new to the forums here and not sure how such projects are handled (is there a different thread for that, etc).  I'm an engineer and have all the parts needed...but I would imagine someone has already done this?   I have commercial heaters but I wouldn't mind building a few more just for fun, who knows they might turn out to be more efficient on battery power than the commercial ones.

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### #24 kingjamez

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Posted 19 October 2019 - 09:50 AM

It doesn’t need to be super complicated. We don’t need to be very precise either, as long as its keeping your system above ambient, it’s good... just don’t get too hot.

Rules of thumb work pretty well in this case. I like to shoot for 0.5watts per inch of strap. That gives me plenty of headroom to dial down with PWM but not so much power that if I leave it at full power things start to melt.

-Jim
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### #25 Jim in PA

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Posted 19 October 2019 - 11:04 AM

It doesn’t need to be super complicated. We don’t need to be very precise either, as long as its keeping your system above ambient, it’s good... just don’t get too hot.

Rules of thumb work pretty well in this case. I like to shoot for 0.5watts per inch of strap. That gives me plenty of headroom to dial down with PWM but not so much power that if I leave it at full power things start to melt.

-Jim

True.  The PWM dimmer is probably good enough to get the job done.

I use 26 gauge nichrome wire for a foamcutter I built...I run it between 600F-800F...drawing about 2A-2.5A.  Therein lies the primary concern for me personally, knowing I can generate enough heat to solder with, I personally wouldn't run a nichrome wire optics heater with a PWM dimmer unless I was absolutely sure there was no way for the system to overheat and damage the equipment, either by unforeseen failure/circumstance or operator error.

The secondary concern for me is battery efficiency.  If the heater is getting hotter than it needs to be based on conditions, then it's wasting battery power.  Something I've wondered about with my commercial heaters but never really checked into.

To me, using a thermocouple or two, temp sensor, display, and writing a little code is not a big deal or complicated...that being said, for most it probably seems a lot easier to use the commercial PWM dimmer and be done with it...sounds like you have enough experience with them to make it all happen without much fuss...

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