# How to heat a wire.

Discussion in 'General Electronics Discussion' started by poorfornow, Dec 19, 2012.

1. ### poorfornow

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Dec 13, 2012
Hi all,

I hope you guys have heard of Shape Memory Alloys. I wanted to find out whether it is possible to heat a short piece (around 10-20 mm in length) of around 0.5 mm NiTi shape memory wire to around 40 degrees centigrade, using a single button cell or such? Is there a circuit that can do that? The battery need not last more than one heating.

Regards to all who consider the question.

2. ### Raven Luni

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Oct 15, 2011
I looked into SMA a while ago. As far as I'm aware, you need to heat it to at least 70°C and the amount of shrinkage is very small.

3. ### poorfornow

35
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Dec 13, 2012
Hi Mr Luni,

Shape memory alloy can deform or perform at temperatures set at manufacture. The shape memory effect occurs dependent on the ratio of alloys in the metal. Thus you can get shape memory alloys that perform at lower temperatures than 40 degrees centigrade, or higher of course.

Regards

4. ### poorfornow

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Dec 13, 2012
Can a button cell, perhaps with some kind of circuit attached; produce 40 degrees centigrade in such a wire, even if all of it's charge were useable at once?

Regards all.

723
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Jan 21, 2009
6. ### poorfornow

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Dec 13, 2012
Hi Ken,

So there is no way to extract all of the power from the cell say into some kind of capacitor then fire it across the wire?

Regards.

723
75
Jan 21, 2009
No.

Ken

35
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Dec 13, 2012
Thanks Ken

9. ### BobK

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Jan 5, 2010
I have to disagree. An LR44 button cell has a capacity of 150maH. Use that to charge a capcitor at say 10ma for 1 hour and you could then produce 10 * 3600ma = 36 A for 1 second. And you could do this 15 times.

Bob

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
2,852
Jan 21, 2010
How large is that capacitor Bob?

I was thinking exactly the same thing a while ago. I thought the extra size and complexity (not to mention time) might be an issue.

If you charged 600 uF to 300V you'd get that amount of energy.

Firstly you need to efficiently charge it, then you need to efficiently deliver the power to an almost short circuit load over an extended period.

Almost a trivial problem from an AA cell (the charging at least) not so sure about an LR44.

The discharging would probably require a really clever buck power supply with synchronous rectification, but even so, I can't imagine you could do it without losses.

Perhaps applying AC to the load would be better, and use a transformer essentially to match the load. However the secondary needs to have perhaps an order of magnitude lower resistance than the wire.

11. ### BobK

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Jan 5, 2010
Oops, make that supercapacitor!

The 36A seconds is of course way overkill, I just threw out some numbers. The 0.02 in wire in the link above required 4A to heat. And it would certainly heat in less than a second.

So lets do some numbers, say we went with charging just to 1.5V and we wanted to 4 amps for 100msec. 1F would give you 1 Amp for 1 second with 1V of discharge. So 1F * 4 / 10 or 0.4F would be needed. Not entirely unreasonable.

Bob

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The only issue would be the internal resistance of the supercap. You need to ensure that most of the heat is dissipated in the load.

13. ### poorfornow

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Dec 13, 2012
If one preloaded/precharged a supercap, could it hold that power for use say 1 month after being charged?

I am trying to understand whether I could use a supercap instead of a battery for applications which only required one jolt of power one time.

I mean enough power to heat the wire mentioned?

Regards

Last edited: Dec 22, 2012
14. ### poorfornow

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Dec 13, 2012
Ok guys . Perhaps you found my last question, somewhat of a noob question. I am sure it may be. You are the guys with valuable expertise and experience. It was a noob question, as I know very little about electronics.

NeverthelessI have a potential invention or application in mind for Memory Metal Alloys and I need to know how I could potentially make them fire ( as it were ) with a power supply that is portable and physically small.

I would like to know whether I can make a Memory Metal Alloy wire of around 2cm and 0.5mm thick, do what it has been programmed to do ( as it were) ?

In your opinion, could I use supercapacitors which was precharged to bring a wire of the above spec to 40 degrees centigrade?

Regards.

15. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,505
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Jan 21, 2010
I would doubt that a supercap would hold a usable charge for that long.

16. ### KMoffett

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Jan 21, 2009
Have you considered discussing your needs with an applications engineer at the wire's manufacturer?

Ken

17. ### poorfornow

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Dec 13, 2012
Hi Ken,

That is an idea I will consider. It is just that I am not sure how much of my ideas I can share as I was hoping that I might apply this to a potential invention.

Thanks for the input Ken