Mini electro-mechanical device

I’m building a little electronically operated release mechanism. I’ve been designing around mini-solenoids but they are unreliable. I had the idea that I could perhaps effect a release by shorting a piece of fuse wire, causing it to blow and release a mechanism. I only have 24v and whatever amperage I can get out of a moderately small capacitor to achieve this. The lightest guage “off the shelf” fuse wire I can find is designed for domestic applications and is about 6A or higher which is a bit much for the current capacity of my circuitry. I was hoping that could find something that would blow at 1A or preferably even less (200mA would be excellent but probably impossible). I had the idea of breaking apart a 500mA fuse and using the wire out of that, but it’s so fine you can barely see it, let alone work with it, and this only gives you an inch long piece of wire which is a recipe for fiddly frustration.

What could I obtain that would work as very light gauge fuse wire?

[sub]No I’m not building a bomb, I’m building water rockets[/sub]

Use a single copper strand out of a length of multi-strand wire.

Probably 0.1 mm or 0.12 mm diameter is about the smallest you’ll get that way. It’ll still take a few amps to blow it, but with a few thousand μF cap, you should be OK.

Try it and see - you’ve probably got all the stuff you need to try it out just lying around.

The filament from an incandescent light bulb might be ideal for this use. Be careful breaking the bub, as the shards are quite sharp.

Desmo, do you have any idea whether an electrolytic of the size you suggest is going to have sufficient internal resistance to be self limiting on current? Or do I need to put a resistor in series to keep the current within limits (and if so, what limits?) to prevent capacitor suicide?

Thanks for the idea, radar ralf, but I’m going to be going through dozens of these fusible links if I get this idea up and running and smashing a whole load of bulbs to get the filament out might get a bit impractical!

I tried it, and it didn’t work.

I found some hair thin copper wire. It got very hot with 3 A flowing though it, but didn’t blow.

Then I charged a 10,000 μF cap to 12 V, then discharged it through the wire. Nothing.

OK. In the capacitor, I had stored 1/2 C V[sup]2[/sup] = 1/2 x 10,000x10[sup]-6[/sup] x 12[sup]2[/sup] = .72 J.

The wire was about 0.1 mm in diameter, and about 25 mm long, giving a volume of about 2x10[sup]-10[/sup] m[sup]3[/sup].

Copper has a density of 9,000 kgm[sup]-3[/sup], so there was about 2x10[sup]-6[/sup] kg of copper.

Copper has a specific heat of about 0.4 kJkg[sup]-1[/sup]K[sup]-1[/sup], and melts at about 1080 [sup]o[/sup]C. So to raise its temperature by 1,000 K, I needed about 0.8 J.

Even assuming no heat losses, I didn’t have enough stored energy. Maybe 10 times that much would have worked, but then the capacitor size and cost is starting to get significant.

A 6V lantern battery may be able to do it, but I don’t have one handy, so I can’t test it.

I’ve been doing some experimentation of my own. Like you, I managed to find 0.1mm wire. I can blow it at about 20 amps, it’ll glow at about 4, and I can’t find a resistor to get amperage
in between that range that won’t blow faster than the wire to test out what the latter can cope with :slight_smile:

A 9v radio battery will blow it, but a tiny 12v alarm battery won’t. I could use one of the former but what I’d save on the weight of the solenoids I lose on battery, plus some probably. Though this method is probably more reliable than solenoids. Hmmm.

Another idea I had was 0.51mm solder. Thicker, but part of that is resin core, plus lower melting point and not as good a conductor as copper. Try that tomorrow maybe.

I’ll bet dollars to donuts you could blow any reasonably small wire to hell with a camera strobe circuit. You’d need to devise a switching circuit that could handle the 300v or so at the current you’d need, but the duty cycle would be low enough that I’m sure an SCR or triac that could handle the current/voltage wouldn’t be too terribly expensive. I’ve shorted strobe caps with a screwdriver to discharge them - the tip of the screwdriver melts and it sounds like a gunshot when they’re fully charged. And these circuits can run on as little as 3v.

You might look at the wires used to ignite model rocket engines. I haven’t done model rockets in years, but back in the day the igniters that come with the engines had a dab of goop on them to improve ignition but I use to make my own igniters using just fine wire. I had a spool of very fine nichrome wire scavenged from a lab, but I have no idea what size it was. I used to ignite it with a switch on a car battery because that was convenient, but the commerial launchers used a 9v battery and worked just as well.

How about steel wool filaments?

Ale got to it before me. Ever stuck a pile of steel wool across battery filaments? If not, give it a try.


Or just use cannon fuse with the strand of copper wire wrapped around it. The heat from the wire will ignite the fuse.

Fuse is strong until burned and easy to work with - cut to desired length. Easy to replace and cheap.

In the US, maybe, B & I. Not here. Rockin’ horse manure would be easier to obtain.

Try the steel wool. Sounds like it’s worth a shot. I haven’t got any to test myself.

(Heck, I don’t even have any Scotchbrite in the house. I made the discovery years ago that:

MIL visiting + abrasives + any kind of coated surface = bad idea.)

OK, so here’s what I’ve found. The strands in a ball of steel wool are not of a consistent size, even a single strand varies along its length. But if you get a fine one (about 0.7mm but getting below the accuracy of my calipers) you can usually blow it with 12V and a 4700uF cap. You can pretty much always blow it with 9400uF (two 4700uF caps in parallel which is all I have). But sometimes not. Lack of consistency is a bit of problem. Finding out that you’ve chosen a strand a bit too big to blow when the rocket is at apogee with no chute in sight is a bit on the late side!

And the tensile strength on the finer strands is getting down to a level where I’m not sure it will hold things as I need.

So more research needed, but looking promising, thanks all.

I just want to say that sounds kinky. Can I be the first to try it?

Cliché flashbulb lights over Ale´s head

How about a small Nichrome or copper coil, and a fish line or other plastic filament that passes through the coil, when the coil heats the line is cut.

[Willy E. Coyote]I´m such a genius[/Willy E. Coyote]

How about Nitinol (memory wire)? Cut a straight wire in half and hook the ends together; maybe put a little conductive goop in the middle to make sure you have a good circuit (ideally the goop should be a poor conductor so that it will heat up the hooks). If the capacitor can heat the wire enough the hooks should straighten out. As a bonus, it should be reusable.

(Disclaimer: I haven’t tried this – has anyone played with nitinol enough to know whether this is likely to work?)