schatz
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Ignitor resistance
Hope I'm posting on the correct forum.
I am planning to use 0805 SMD resistors as bridge "wire" ignitors for ematches. The intended application is for theatric/film special effects, firing
distance about 25 meters max and pyro shell launching, distance 75 meters max.
For the shorter "effects" distances ( up to 25 meters max) the most usual supply voltage will be 12 volts.
For the longer range, voltage supply can be up to 48 volts.
My question concerns the bridge wire resistance to choose.
It would be best if I used one value for both applications, but it seems to me that I might need a suitable low resitance (2 ohms?) for the longer
shooting wire and perhaps a higher ( 10 ohms?) for the shorter.
Hope I have this one right!
Anyone been down this road before and care to chip in ?
Thanks.
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quicksilver
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We could expound on the chemical resistance of bridge-wireless composition using conductive lampblack, graphite, etc but in context to your question,
yes: I have experimented with resistance wire and SMD chips (as well as old fashioned carbon resistors). The basic element is that the longer the
distance, the thicker the wire lead (which will minimize resistance on the route). One method explored in this way was the use of inexpensive
commercial (not residential) extension cords. They have tough outer polymer shells and the 3 wires within are generally thick enough to keep
resistance from forming during that layout.. What's more they may be used over and over again. In the USA, places like Home Depot sell the orange-type
for about $17 per 100". They are an investment as they last a very long time (quite a few years in sun and ice) & they take enormous abuse.
The primary agenda is that resistance be focused at the point of ignition alone -- no where else. This is why "true" resistance wire or a mfg resistor
is often used: the guarantee of heat focus.
One ohm is not too small if the leads are appropriate. But depending on the blast-box, the level should not exceed15 ohms. The use of Cap-Discharge
designs do not improve resistance levels the CD design places more emphasis on the thickness of the lead wire-to-leg wire thickness ratio. It's
obvious not to use leg-wire type material for lead wires as they will offer up a great deal of resistance and can be injured easily, even by a foot
step or sharp rock.
The issue is more one of current supply rather than voltage. A sharp current spike will melt or pop a bridge-wire where a spike of high voltage will
function generally only with bridge-wireless designs (where the resistance is supplied by the pyrogen composition. But high voltage does have certain
advantages in lengthy runs. Therefore the basic criterion is one of blast-box design more so that exacting ohms in the bridge set-up.
Many types have been used throughout the decades. Some are generators/magnetos that push both moderate voltage and fairly strong current. But in the
last several decades the CD-box is popular in that it can be made small and inexpensively. Photo-flash capacitors are frequently used in both series
& parallel so as to keep a strong jolt. If this route is favored the disposable cameras should be collected so that you have roughly about 20
(10-series, 10 parallel; given that they will have 340V and 50-80uf each). If a battery unit is used the 12Vdc lead acid type should have about 10-30A
like a motorcycle battery as it can be re-charged easily. The circuit should also allow some capacitor charging so that the intermittent switch (fire
button) will have some voltage-carrier speed behind it. Some of the most effective designs have used an inverter (especially with EBW types) because
AC type current carries much more effectively in long distance than DC. This fact should not be overlooked. A simple automobile inverter has effect
energy levels, is designed with quality circuit breakers for long life, & has portability.
Regarding the question of using more than one level of resistance for multiple shots fired at the same time; it is always better to have consistent
resistance in the construction of a ignition-system. This allows the design configuration to be consistent within reason. If the shot is 30, 50, 100
meters away; there will be no difference if heavy enough lead wire (like commercial extension cord) is used. If the shots are 30 & 500M away or
situations similar in vast difference, there should be consideration for a wireless radio-type firing system.
EDIT:
Naturally, remember Ohm's Law when figuring out a blast box design or lead-wire topology and think of voltage as the "carrier" of current. While this
becomes more important in extremes of the principal - it has bearing on everything to some degree.
There are important reasons why we use AC-type current for power delivery over great distances.
One unit-type to remember is the generator (muscle powered). If a battery powered hand drill (without the battery in place) is fitted with a hand
crank in the chuck (& turned), power will be delivered BACKWARD into the contacts for the battery unit! This allows a quick and effective
hand-crack generator with sufficient current to be quite effective!!
[Edited on 3-6-2010 by quicksilver]
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schatz
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Ignitor resistance
Thank you for the information quicks.
Thus far I have good results with 2 ohm smd resistors, .1 Watt, size 0805. I have some .06 Watt and I might try these later.
I would imagine I might gain in terms of minimum fire current, but lose fire reliability because the fusing surface is so very small.
I have tried KClo3+ Sb2S3 in NC acetone solution as priming and results are good.
Somwhat wary of this mix, but I only mix very small amounts at a time and all the time under the acetone solution.
More tests later.
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quicksilver
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That appears to be appropriate; in that (depending upon the input current) a range of up to 10 ohms would concentrate the heat where you want it.
Nichrome wire has shown itself to be non-reactive to KCLO3 - however a resistor with leg wires MAY be tinned with lead & or copper. Depending upon
the time of exposure, this may be no big deal. But if such a ignitor is to be stored for a long period, further testing should be attempted to
determine physical sensitivity.
Often the issue was addressed by dipping the completed unit in plain NC lacquer (& dried) before being dipped in a pyrogen to protect the unit
from direct exposure.
Copper is the REAL killer when using any chlorate (or even perchlorate!). An ammonium perchlorate plant in Henderson Nevada blew up due to exposure to
a brass valve some years back. Even mild exposure can be a danger.
[Edited on 9-6-2010 by quicksilver]
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schatz
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Ignitor resistance
wow ! was the Nevada explosion due to A Perchlorate/ brass contact ?
Anyway I understand the erosion problem. I know a few local pyrotechnicians who, after buying loads of electric squibs from China, discovered a huge
percentage of which go open circuit in a few months.
I wonder if tinned copper wire would be more resistant to KClO3?
The tinning layer is usually lead and tin.
The idea of a preliminary coating with NC lacquer is a good one;
Provided of course that it does not form a barrier if it fails to ignite with the more sensitive .06 watt , very low area, SMD resistor.
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not_important
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Quote: Originally posted by quicksilver  |
... because AC type current carries much more effectively in long distance than DC. This fact should not be overlooked.
...
There are important reasons why we use AC-type current for power delivery over great distances.
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Actually this isn't quite true, for reasons that don't come into play for the situations being discussed here.
Conductors have resistance, this applies be it AC or DC current. But conductors also have AC impedence, they are transmission lines that offer more
resistance and higher losses to AC than DC. The inductance of the wire increases losses for AC, the current is crowded into the outer layer of the
wire in effect reducing the wire size, and capacitive coupling lets some current 'leak' before reaching the intended destination.
The reason that AC is used in power transmission is that it is easy to step voltage up and down with transformers, which are not terribly hard to
design for power line frequencies. Stepping the voltage up means less current and a given diameter of wire can carry the same amount of power with
lower losses. At the other end another transformer us used to drop the voltage back down to something reasonable to use in house mains.
At the time of the Current Wars there were not good ways to convert between DC and AC, especially in the DC to AC direction. Large amounts of power
convertion meant using motor-generator sets, and generators couldn't put out really high voltages. Going with AC throughout the power grid meant it
was fairly simple to carry bulk power at high voltages, reducing the amount of copper needed to transmit a given level of power at acceptable losses
while still being easy to supply lower voltages to consumers.
But modern semiconductor technology has made it easier and more efficient to interconvert DC and AC. For bulk power transmission across hundreds of
km, DC is significantly more efficient than AC, and is being used increasingly. Do a Web search on Ultra High Voltage DC power transmission
and see.
For applications such as being discussed here, the lower losses of higher voltages for a gfiven amount of power delivered is important. No one wants
to haul about big rolls of really heavy gauge wire, nor deal with its stiffness. Higher voltage, lower current, smaller lighter more flexible wires.
At the same time one of the most convenient portable sources of electric power is batteries, generally automotive batteries at 12 volts; low voltage =
high currents = fat wires. Going up to mains voltage means reducing the current for a given amount of delievered power by 10 or 20 fold. And one of
most accessable and economical ways to get that 12 volts to a higher voltage is a consumer grade inverter that puts out mains AC voltage.
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schatz
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ignitor resistance
True, Voltage/current relationship considerations is what my post is really about.
Possibly I'm fussing over practical trivialities:
The intended use is for both long distance cables for use in display pyrotechnics and the shorter ranges on film sets and similar sfx requirements.
My dilemma ( if indeed one exists) was over the choice of resistance value for both.
Common sense tells me that I should go for the minimum possible ( as per the respective current source), and perhaps more important, a lower power
dissipation rating.
In practice there might be limitations in the use of very small surfaces and areas as the fusing agent;
Not enough whoomp to set off composition and physical/chemical vulnerability.
Now that 2 rolls of microscopic, 2 ohm smd resistors have arrived from the ROC, I can get some answers.
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quicksilver
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| Quote: Originally posted by not_important |
Conductors have resistance, this applies be it AC or DC current. But conductors also have AC impedence, they are transmission lines that offer more
resistance and higher losses to AC than DC. The inductance of the wire increases losses for AC, the current is crowded into the outer layer of the
wire in effect reducing the wire size, and capacitive coupling lets some current 'leak' before reaching the intended destination.
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I jst got done with a book about Edison & Tesla & I don't want to drag this thread OT; but I find the whole "power company"decisions
interesting and worth discussing......but we'll have a better thread for that........Interesting stuff.
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schatz
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ignitor resistance
Just dont hand over AC/DC controversy history books to any elephant friends you might know.
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quicksilver
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Gary Purrington (of Firefox pyro) had a "standard" of making bridge-wire-less ignitors. In it - he made a "resistor" of sorts that was a pyrogen. It
consisted of:
Cellulose acetate solution / acetone (binder)
Graphite - 25%
Atomized Al - 5%
FerroTitanium - 5%
Conductive lampblack- manganese dioxide -1%
PbO2 - 9%
charcoal (air float) -5%
KCLO3 - 50%
This is perhaps one of the best pyrogens I have come across as you actually manufacture your own resistor level by alteration (addition / subtraction)
of the conductive elements. The ferro-titanium (5--100 mesh) can be substituted out for increase in Al but the other materials are mandatory for
effective conduction of electricity and resistance enough to gain heat.
I have used this technique for approx 8-10 years & it's always been effective. IF the need is for more than just match heat, the inclusion of lead
styphnate (moderately conductive & / or mercury fulminate (quite conductive) will provide either blasting flame spit or a true detonation. These
should be substituted for the ferro-titanium @ 5%.
It had been proposed often, that if the purpose is to make a detonator of unique shape, a combination of lead azide (50% to the binder) and NC lacquer
(binder element) should be dipped on TOP of the above pyrogen to the level of at least 250mg lead azide adjacent to a compressed pellet of PETN / ETN
(.8- 1gr).
In the pyrogen, the conductive lampblack is a straight conductor. The graphite, lead, & charcoal will provide resistance with single-micron sized
KCLO3. Manganese dioxide is utilized for stabilization. The above has proven itself over and over through time. One test that I preformed was to keep
completed ignitors wrapped in plastic and in cold surroundings for well over a year: they performed as new!
The value of this composition cannot be over-emphasized as it works quite well. It is consistent & predictable.
~*~ One of the most important aspects of this concept is that it is VERY resistant to static electrical discharge. A true amp of current needs to feed
it. Even some of the strongest static charges will not provide the needed current!
EDIT:
The cellulose acetate solution must be made VERY thin. One ounce of solid to one quart acetone. This would reduce to a level of approx .5gr to 100ml
on a very rough level. It HAS to be very thin - but it will work quite well. Cellulose acetate can be found in many medicinal tablets as a compression
binder, etc.
[Edited on 11-6-2010 by quicksilver]
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