# capacitor and nichrome

#### rjjenkins

Joined Apr 16, 2011
214
Hello
I am trying to make a device to ignite flash paper using nichrome wire. This is easy in principle - you just pass a current through the nichrome, it heats up and the paper ignites. However I want to do this in a very small space (this is for a magic trick) which means that I have to use small button cells. The trouble is that they don't deliver enough current. I can easily get enough current from a single AA cell but that is too big.

I have an idea that I could use a capacitor so that the button cell charges the capacitor which then discharges through the nichrome so that for a brief period (a second or so) there is a high enough current. Does this sound feasible, given that for space reasons I would also have to use a smallish capacitor (say 100uF maximum)?

#### crutschow

Joined Mar 14, 2008
34,423
It's feasible but we have to know the amount of energy required to heat the nichrome wire to the desired temperature in order to calculate the size of the required capacitor.

Alternately you can just experiment with some different capacitors to see if it works.

#### joeyd999

Joined Jun 6, 2011
5,283
Alternately...
That would be 'alternatively'. Sorry for the criticism, it just stuck out at me.

#### rjjenkins

Joined Apr 16, 2011
214
It's feasible but we have to know the amount of energy required to heat the nichrome wire to the desired temperature in order to calculate the size of the required capacitor.

Alternately you can just experiment with some different capacitors to see if it works.
Thanks, let's see ...
the resistance is about 20 ohms, so with 1.5V, the current should in theory be 13A though I can't believe an AA battery can delivery that current. But it fires after a fraction of a second, much too short to measure, but I would guess somewhere between 1/10 and 1/100 of a second. So the energy is maybe about .25 coulomb?

Joined Dec 26, 2010
2,148
Thanks, let's see ...
the resistance is about 20 ohms, so with 1.5V, the current should in theory be 13A though I can't believe an AA battery can delivery that current. But it fires after a fraction of a second, much too short to measure, but I would guess somewhere between 1/10 and 1/100 of a second. So the energy is maybe about .25 coulomb?
With a resistance of 20Ω, a voltage of of 1.5V would produce a current of 1.5V/20Ω = 0.075A: How do you get 13A?

Alternatively, if the current needs to be 13A at 1.5V, the resistance would need to be 1.5V/13A = 0.115Ω.
That's not practicable, since even a fairly fresh AA cell probably has about 0.2Ω of internal resistance.

Charge is measured in coulombs, but energy is measured in joules.

The charge delivered by 1.5V into 20 ohms in 0.1 second would be 0.1*0.075A = 0.0075C or 7.5mC.

The energy would be 1.5V*7.5mC = 11.25mJ

#### crutschow

Joined Mar 14, 2008
34,423
To store 11.25mj at 1.5V requires 10,000μF.

#### rjjenkins

Joined Apr 16, 2011
214
Umm...idiocy on my part I'm afraid! Thanks for straightening me out.
Looks like it might not be possible with a small capacitor.

With a resistance of 20Ω, a voltage of of 1.5V would produce a current of 1.5V/20Ω = 0.075A: How do you get 13A?

Alternatively, if the current needs to be 13A at 1.5V, the resistance would need to be 1.5V/13A = 0.115Ω.
That's not practicable, since even a fairly fresh AA cell probably has about 0.2Ω of internal resistance.

Charge is measured in coulombs, but energy is measured in joules.

The charge delivered by 1.5V into 20 ohms in 0.1 second would be 0.1*0.075A = 0.0075C or 7.5mC.

The energy would be 1.5V*7.5mC = 11.25mJ

Joined Dec 26, 2010
2,148

#### crutschow

Joined Mar 14, 2008
34,423
There are also some small Lithium cells that may work.

#### rjjenkins

Joined Apr 16, 2011
214
I tried an A23 cell - no good
An AAA is fine though.

Joined Dec 26, 2010
2,148
An A23 is 12V: it will have more restricted current capacity than an N cell, although it might work with a reasonably sized capacitor. Since the energy stored in a capacitor goes as the square of the voltage [ E= 0.5*C*V$$^{2}$$ ] , you would need 64 times less or 160μF.

Whether your igniter would work as well with a brief pulse of a higher voltage is another matter.

http://en.wikipedia.org/wiki/A23_battery

#### CraigHB

Joined Aug 12, 2011
127
A cell like this coule handle it no problem and it's like 1/3 of a AAA cell.

#### tom66

Joined May 9, 2009
2,595
You could use a super cap, you can get them in multi-farad ratings. However, you must choose a capacitor with a low DCR, or you will not be able to sufficiently heat the nichrome. A 10mF supercap will be smaller than a button cell battery.

#### shortbus

Joined Sep 30, 2009
10,045
Instead of a nichrome wire, why couldn't you carefully break the glass of of a flashlight bulb of the correct voltage and amperage for your battery? If only turned on for a short amount of time per use the tungsten coil in the bulb should last quite a long time.

#### PaulEE

Joined Dec 23, 2011
474
Umm...idiocy on my part I'm afraid! Thanks for straightening me out.
Looks like it might not be possible with a small capacitor.
They make 1F 5.5V super capacitors that are roughly the size of 2132 3v button cells...that'll heat that nichrome up a bit.

EDIT: Tom66 beat me to it. In either case, they're small and affordable nowadays; fun to play with as well.

#### PaulEE

Joined Dec 23, 2011
474
Does this look suitable?
http://www.hobbytronics.co.uk/super-capacitor-0-2f
Could I charge it from a lithium cell?
Look at this:
http://search.digikey.com/us/en/products/EEC-S5R5V105/P6975-ND/125070

If you hook up a lithium cell, this capacitor, with nothing else but leads connected, would pull the maximum current that the battery could dish out until it is charged. C=Q/V and a button cell battery, 3V, 80 mAh, usually specs at 5mA current draw to achieve total mAh rating.

1F = Q/3; Q = 3 coulombs; 5 mA is 5/1000 of a coulomb per second. It would take 0.167 hours to charge fully, assuming. That was quick math, but you get the idea.

So, for your trick, I'd have the capacitor charged ahead of time. Once charged, it'll pour those charges through that nichrome as fast as it can. I'd also check the specs of the cap; they have internal resistance, also....

Edit: I assume you regulate current to it at 5 mA for that calculation above.

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#### Bernard

Joined Aug 7, 2008
5,784
If space is a problem, then it would be helpfull to know what that max space is. Ni-Cr, 0.1 mm, 1Ω/cm cold, ! A @ 3V, 16.8 mm wire = red hot.

#### rjjenkins

Joined Apr 16, 2011
214
The gauge nichrome I'm using is 34 swg (0.236 mm). The length needs to be about 2-3mm
The space restrictions are to do with the battery and/or capacitor and a small switch. The space is irregular but roughly a cylinder of diameter 15mm and depth 20mm

#### tom66

Joined May 9, 2009
2,595
The DCR is specified in the datasheet as being <200 ohms. That is far too high.

(Note DCR and ESR are effectively the same thing but DCR applies to super caps as they behave like a short circuit initially while charging and this can be tested using a multimeter... it's complicated...)