quicksilver
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Electrical Ignitor Systems
Some time back hobby rocket folks used the old carbon resisters in low resistance (3 Ohm, etc) & @ 1/8 watt because it was noted that they really
POPPED when they burnt out. The technique was to buy 1/8 watt carbons and coat them with whatever pyrogen or energetic material one would want (often
NC lacquer, chlorate & Al) and feed just about anything (6 V battery, etc) with some amps through it to shoot w/ no delay.
Recently more and more people have been working with SMD resistors and leads from a computer ribbon cable. The SMD resistors are available in 1 ohm
& when they pop, they deliver a spit of flame (by themselves) due to their construction. Available in rolls of several thousand for often less
than a penny each, they are much cheaper than the old style carbon resistors, supplies of which are drying up.
Due to their construction, there is little chance of a static accident from a completed igniter / squib , whatever.
Experimentally I tried to see what level of heat was generated by this design and more than enough was there to shoot materials from BP to fulminate.
A serious problem existed when electrical ignition systems used non-resistance bridge-wire with both corrosion & misfire. Pyrotechnic systems
often used a chip of thin PC board with a 40 or smaller nichrome wire splash soldered to it & then to the leg wires themselves. The substitution
of a resister simplified the amount of soldering and put the flame spit where it was supposed to be.
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quicksilver
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I have many utilities I use to calculate resistance in designing a power supply (blasting box) for an ignitor. Figuring what level of current will
surpass a degree of resistance resistance wire (or resistor proper) - makes it easy to design an ignitor bridge-wire or the resistance in the
composition for a wireless ignitor (using carbon, graphite, or conductive lamp back).
One can always go with one ohm and most anything will shoot but at what wattage (because that will determine time in heating - or even melting, etc)?
If it's a thick nichrome wire, (rough example) it would be the same as a 5 watt resistor package; therefore, a LOT more amps!
Attachment: Ohms_Law_58770.rar (235kB)
This file has been downloaded 509 times
[Edited on 18-5-2010 by quicksilver]
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quicksilver
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USING BRIDGE-WIRE THAT'S TOO THIN FOR SOLDER?
Capture (right click) this little technique; I've used it for quite some time now & it works very well when making ignitors with VERY thin
nichrome wire.
Your sealing pyrogen can be simple NC lacquer, BP, and super fine aluminum OR you can utilize something much more energetic because the loop of wire
will expose a clean hot shot.
[Edited on 23-5-2010 by quicksilver]
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gnitseretni
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I use 48 guage nichrome wire. I don't solder or use pyrogen. I just strip the speaker wire to expose two leads and I then wrap the nichrome around one
lead a couple of times and then a couple of times around the other. I would then cut off the leads as close to the nichrome wire as possible.. but I
like the idea of folding the leads over, as shown in picture, better, so thanks for that 
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quicksilver
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I wish you drew a pic. I want this thread to expand w/ different ideas on this topic.... I hope folks feel free to add their own techniques.
I don't think there is one "best" way. I have a "needle" soldering system for repairing traces or placing replacement SMD's on a board. But my
eyesight is not the best so I have to find techniques that vary.
It's quite important to use resistance wire. Not only because that's where the heat will concentrate - focus (due to the actual resistance) but to
keep the material from corrosion if exposed to oxidizers (chlorate, etc).
The actual soldering of nichrome wire works best with ferric chloride as a flux. There is a brand name called "Ruby Fluid" that works VERY well. It's
ferric chloride and it really is great with both lead and lead-less solder (IF that's the technique some are going to use....). Like you, I prefer to
wrap and dunk in a NC lacquer composition.
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gnitseretni
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Ay, my virtual drawing skills suck. But is it neccesary? Look at step 2 in the pic you provided. Instead of going from one lead to the other in a
"figure eight" configuration, I just wrap the nichrome wire around one lead several times and then wrap several times around the other. I hope I
described that better?
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quicksilver
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Indeed; I've seen "one leg longer" style.
I once tested nichrome of various widths with a multimeter to see what the resistance really was. It takes about 2 cm to get close to a solid ohm
continually with stuff about 38ga. What is curious is that professional Nonel blank starters test at 2 ohms standard and the tip is tiny. But what was
done was a bulbous end to each side (being nichrome most likely). So while the "figure 8" technique is a good one, it's the longer wrap that gets a
solid resistance from the longer lead.
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quicksilver
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The semi-conductor bridge is another option for initiating energetic materials. It was originally designed for a "smart" det cap. However the
semi-conductor bridge may also be portable to a secondary-only detonator design. But more importantly the use of this concept could allow the
integration with a variety of safety features here to unavailable in such a simple design.
"Sandia National Laboratories’ semiconductor bridge, SCB, is
a maturing technology now in use in several new applications.
Those applications required explosive assemblies that were light
weight, small volume, low cost, and needed only small quantities
of electrical energy to function. Explosive assembly here refers to
the combination of the firing set (the current source for the firing
signal) and the explosive component."
Cost per production is so low with SMD materials that such a device would really be on par with double bridge-wire (anti-static) designs.
The possibilities were explored in the 1996 PEP Journal sometime after the design became available commercially.
Attachment: PEP-1996-5.rar (426kB)
This file has been downloaded 1639 times
[Edited on 9-7-2010 by quicksilver]
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Hennig Brand
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I have a method for making igniters that has worked well for me in the past. I was originally simply bridging the end of a copper pair with resistance
wire, but later found that using a rubber insert/plug/insulator was much more reliable. The rubber insert greatly helps prevent short circuits and
bridge wire damage as well as facilitating a secure marriage of the fuse head to the rest of the blasting cap assembly. I got the idea one day when I
was looking at a design for a commercial electric blasting cap which used a rubber insulator/insert in a similar way.
All I have available to me at the moment is 32 gauge nichrome wire, but this is just to demonstrate the concept anyway. I have a roll of 40 gauge on
the way.
The rubber insulator plug is made from a pencil eraser (cheap wooden pencils can be had for 50 cents to a dollar for a package of ten when bought in
economy packages, and I have seen them for as little as 25 cents per package occasionally).
Two small holes are drilled axially through the pencil eraser, and then the two ends of the lead wires which have had their ends stripped are pushed
through. A single strand of nichrome is used to form the bridge and is wrapped tightly around the ends of the lead wires (a couple of wraps per lead
wire conductor). I am looking for some Ruby fluid soldering flux, but for now I use HCl followed by a thorough rinsing in clean water before
soldering. Some solders work much better than others, I have found silver bearing solders to work satisfactorily. For this demonstration I used 60/40
electronics solder because it was all I had handy, but it is less than ideal (doesn't stick well).
After soldering the ends are trimmed close to the solder joints and while holding the rubber plug the lead wires are pulled until the bridge wire is
touching or close to touching the eraser.
Next a couple of wraps of electrical tape around the plug makes a nice little pocket at either end of the plug. The pocket with the bridge wire is
filled with pyrogen (usually good quality black powder and NC lacquer, but more energetic mixes could be used). The pocket at the end with the lead
wires coming out of it is usually filled with epoxy or often out of expediency is just filled with hot-melt glue.
   
[Edited on 9-12-2011 by Hennig Brand]
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Hennig Brand
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Is there any reason why nichrome wire with iron in it would be less suitable for use as electric blasting cap bridge wires? I ask this because I
recently ordered some Chromel C which has 24% iron by mass according to Wikipedia.
"Chromel C: Chromel C is an alloy containing 60% nickel, 16% chromium, and 24% iron. It is also commonly called Nichrome 60 and is used for heating
elements, resistance windings, and hot wire cutters."
Another common one is Chromel A which is reported as being better at resisting high temperature oxidation.
"Chromel A: Chromel A is an alloy containing 80% of nickel and 20% chromium (by weight). It is used for its excellent resistance to high-temperature
corrosion and oxidation. It is also commonly called Nichrome 80-20 and used for electric heating elements."
I assume both are perfectly fine for use as a bridge wire in an electric cap. I haven't looked into it that much but I assume the commercial people
have a preference.
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dann2
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Resistance to corrosion would be very important to manufacturers. The wire will be damaged or broken or/and dangerous sensitive substances may form
around the wire making the thing very dangerous. Depends on primary I guess.
Some primaries are very corrosive I would imagine. A thin Copper wire will do the job
so long as you are not going to store the device. Enamelled Copper wire may do for longer if you can protect the ends where a connection is made.
The wires ability to resist high temperature corrosion would have little bearing on how suitable it will be for detonators as it's more a figure of
merit for heating applications (heating elements or high temperature resistors).
Dann2
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Hennig Brand
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Thanks for the reply. I assumed that nichrome's ability to resist oxidation at room temperature would be related to its ability to resist oxidation at
higher temperatures. I am going to try and find some more information on the subject (what I found only discussed high temperatures). I expect that
the Chromel A has better low temperature oxidation resistance, as well as high temperature oxidation resistance, however I also expect that it makes
very little difference which type is used for most hobby igniter applications.
[Edited on 14-12-2011 by Hennig Brand]
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IrC
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Here is a useful page.
Richard Nakka's Experimental Rocketry
http://www.nakka-rocketry.net/igniter.html
"Science is the belief in the ignorance of the experts" Richard Feynman
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Texium
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