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Author: Subject: Overloading a 1/4 watt resister
Deceitful_Frank
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[*] posted on 22-12-2006 at 14:45
Overloading a 1/4 watt resister


Forgive me if this is the wrong forum/section but...

Quick Question,

If I were to arrange for a fully charged 330V 230uF photoflash capacitor to be discharged across a 100 ohm 1/4 watt carbon film resister on the end of 100 metres of speaker wire with a total round trip resistance of say 20 ohms, Do you people KNOW if the current would overload the resister enough to cause it to "explode", or atleast heat up in an instant enough to initiate a primary explosive?

Ok I think I am reasonable proficient with ohm's law and such but please bear with me...

Total energy contained within the capacitor E=1/2QXV^2 or half the charge times the voltage squared. I make this about 12.5 Joules

OK so with that wiring setup and assuming the internal resistance of the capacitor is negligable, we would get 275V across the resister at the start of the discharge causing a current of 2.75 amps to flow and a power of 756 watts to need to be dissipated!

Now this is 3024 times the power rating of this resister so would it "blow"?

Do my numbers add up?

Any other thoughts, advice or constructive critisism welcome as always :D
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[*] posted on 22-12-2006 at 14:54


Such overcurrent may overheat your resistor so it may get somewhat damaged but it will not "blow". 1/4 watt limit is for continous power, short pulses can be far more powerful.

[Edited on 22-12-2006 by chromium]




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Deceitful_Frank
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[*] posted on 22-12-2006 at 15:10


Shit yes you are right, I didnt think of that!

Hell it might even hold out long enough for the capacitor to discharge to a level where it can dissipate the power for atleast the short term.

Suppose I could use a load of them in paralell as Ive got 100 on the way from eBay... or just go for suicide by 1250 joules of 330V capacitor bank!

Back to the drawing board then? :(
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Lambda
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[*] posted on 22-12-2006 at 15:31
SMD resistors as bridge wire elements of pyrotechnical initiators


Resistors also suffer from overvoltage flash over. Depending on which series the resistor belongs to, there are maximum voltages which may be applied. With 275 Volts, you will be overcooking most SMD and E-series resistors. Even resistors of the same series have different maximum voltages which are given in the specs according to there Ohmic value, regardless of the fact that these Voltages are pulsed and/or very short.

If you are looking for a way to make bridge-wire-free Detonators and Rocket Initiators, then by using SMD resistors may prove to be feasible.

Please check this link out:

SMD resistors as bridge wire elements of pyrotechnical initiators:
http://www.privatedata.com/byb/rocketry/Ignitor/pk_smd_cache...

Regards,

Lambda.
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[*] posted on 22-12-2006 at 23:01
ResisTOR


Quote:
Originally posted by Deceitful_Frank
Quick Question,

If I were to arrange for a fully charged 330V 230uF photoflash capacitor to be discharged across a 100 ohm 1/4 watt carbon film resister on the end of 100 metres of speaker wire with a total round trip resistance of say 20 ohms, Do you people KNOW if the current would overload the resister enough to cause it to "explode", or atleast heat up in an instant enough to initiate a primary explosive?


Unfortunately, this is not a "quick question". It will require a reasonable toolkit covering dynamic reaction (and assumptions, having been given no component values beyond the capacitor's value).

Quote:
Total energy contained within the capacitor E=1/2QXV^2 or half the charge times the voltage squared.


I don't like charge. I almost never use it.

The equation I know is E = 1/2*C*V^2. Since Q = V*C, C = Q/V and E = 1/2*(Q/V)*V^2 or 1/2*Q*V, not Q*V^2.

Given [easily] measurable or read quantities (voltage, capacitance), you need to do Q = V*C either way, so using charge is roundabout and useless here.

Quote:
I make this about 12.5 Joules


Thus I get 12.5J, which does happen to be correct then. I'm not sure what you were using for Q, but it can't be charge since my derived equation differs.

Knowing how much power reaches the bridge resistance (i.e., subtracting wiring and transmission losses), you can calculate the adiabatic temperature rise, considering the specific heat capacity of the stuff, that is, assuming it heats quickly enough (<1ms I would SWAG). If the discharge is significantly slower than this (which it is), you also have to consider heating of the surrounding material (conductivity, spec. heat, etc.).

Quote:
OK so with that wiring setup and assuming the internal resistance of the capacitor is negligable, we would get 275V across the resister at the start of the discharge causing a current of 2.75 amps to flow and a power of 756 watts to need to be dissipated!


(264V?) Something like that. But only for the first opening miliseconds (I would say microseconds, but that would be a stretch even for short circuit discharge of a fiftieth of the capacitance with a fraction of the ESR, ESL.), when current is rising (current at least should rise pretty quickly, as 2.5A isn't much in terms of pulsed current). Quickly, power starts dropping off as more energy is delivered. Remember, 756W in 1ms is 0.756J, the energy such a resistor burns in three seconds (at full ratings).

My estimation is that your 100m ~= 600ft (both ways) at 20 ohms is about 25AWG, which is 0.5mm dia. and about 2mm seperation (guessing about the dimensions of your "speaker cable" here). I get 70uH inductance, which has an LR constant of about 7ms. This limits the current rise in the first perhaps 10ms of the discharge.

Note also that the cable has capacitance. If it had only inductance, current would rise slowly, but it would rise uniformly along the entire length, apparently travelling instantaneously. This can't happen because capacitance along the cable sucks down voltage, which slows the risetime (in a first order look) and causes the wavefront to travel at perhaps 2/3 speed of light. It also causes the cable to look resistive, since the particular arrangement of infinnetessimal L and C works the same at all frequencies (but only acts resistive at one impedance, as contrary as that sounds). What you can do is figure the impedance of the cable (better to get some coax, which is usually 50 or 75 ohms), fit same resistance on the end, and drive the input with a nice fast beefy pulse (spark discharge from a capacitor, perhaps). The pulse travels along the cable, then hits the resistor and nothing remarkable happens -- the energy just plops into the resistor, promptly and without waste (admittedly, there WILL be loss of high frequencies on a *long* run...).

Quote:

Now this is 3024 times the power rating of this resister so would it "blow"?


Maybe. Some resistors can handle large spikes, but the average one might turn faulty (but not necessarily burn--that depends on the energy) given 10-100 times rated wattage. For sure, the whole shot will be over in perhaps 100ms, in which time most of 10J will have been delivered to the resistor, thus averaging 100W, sufficiently far in excess of 1/4W over a much longer time span.

Quote:
Do my numbers add up?


Yes, except for the equation as mentioned, and the neglect of inductance and matching, especially important on a long run for pulses, such as this.

Tim




Seven Transistor Labs LLC http://seventransistorlabs.com/
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[*] posted on 23-12-2006 at 04:22


you won't get much interesting at 300V


Try and get some 30 Kv caps and then you'll get some very interesting events!
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[*] posted on 23-12-2006 at 05:53


I can't help thinking that the easy answer is to use a torch bulb with the glass envelope removed.
Anyway, there is a simple way to do this. Calculate the total stored energy from E=QV or E=1/2 CV^2 pick whichever equation you want. 12.5 Joules- not a lot.
Guestimate the heat capaciy of the resistor. Interesting since I don't know its size and I don't know what it's made of.
IIRC typical 1/4W resistores are less than half an inch long and roughly a sixteenth of an inch in diameter. That gives a volume of roughly 25mm^3. Let's guess at a density of about 2 g/ml. That makes 50mg of stuff.
If it has the same heat capacity as water then each mg takes 4.2mJ of heat to raise the temperature by 1C. so the temperature rise would be 1C for 0.21J or about 60C for 12.5J
I don't see that setting anthing off. On the bright side, most things have a lower heat capacity than water so the temperature change will be higher. The closest to useful data I could find were heat capacities for a bunch of rocks which were roughly a quarter of the value for water. That turns a 60 C rise into 240C- much more like it.
With a couple of those photoflash caps you could certainly set off something sensitive. Probably worth a try, but check my maths first.
(NB a second photoflash cap is a lot cheaper tha 100M of high voltage cable rated for 30KV)

Having said all that, use a 10 ohm resistor and a car battery.

With 30 Ohms round trip resistance the current will be 0.4A so the resistor will dissipate roughly 1.6W and it will do so until something gives. It won't be the battery that gives up. With lower resistance leads you can do even better. IIRC the best value for the resistor is equal to the resistance of the leads (assuming the battery has negligible internal resistance) so 20 Ohms should be better (1.8W).
(NB a second photoflash cap is a lot cheaper tha 100M of high voltage cable rated for 30KV)

[Edited on 23-12-2006 by unionised]
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[*] posted on 23-12-2006 at 06:11


I have done a lot of experimentation w/ SMD resisters in a rocket club I belonged to awhile back and am rather familiar w/ them They are available via MOUSER Electronics. They were one of the best ways to accomplish a bridgewire-resister concept that we tried and are extremely inexpensive compaired to the old-fashoned type of carbon resisters. They come in rolls and are not too hard to work with if you have the correct type of equipment.



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[*] posted on 23-12-2006 at 11:08
No more wire hangers


It's a complete waste of energy to heat a carbon resistor to make it explode,
all you can hope for is a pop and a flash. Used as an improvised squib ignitor
seems excessive. Exploding Bridge Wires have very low DCR ( direct current
resistence ) and are fine pure metal wires. Using speaker wire except perhaps
a one meter length connecting a coaxial line to your detonator at the end will
defeat the purpose which is to reduce inductance to have a very rapid rise in
current.

.
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[*] posted on 23-12-2006 at 11:47


Considering mention of photoflash caps, an *exploding* bridgewire is obviously not intended here. He said primary, not secondary explosive.

Tim




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[*] posted on 23-12-2006 at 16:02


Well I still reckon you can't go past ordinary graphite powder mixed with nitrocellulose laquer. I use crushed up artists pencils and acetone + table-tennis balls.

The finished resistance of each element is less than 10 ohms(7-9typically), takes no soldering work and easily will initiate mercury fulminate, double salts, ap, hmtd and pb picrate.

I've only got a 10m cable, though it was the least bulky, finest one I could locate. It's something like 2 ohms for the 20m run, including connections. I can fire upto 5 of these caps. I'm guessin that if you scaled the resistance of the wire as though it were 100m long, you'd be looking at 20 ohms for the wire and 2 caps would bring it up to 38, or just 1 ohm more than the minimum expected value from 5 caps and 10m of wiring.

But the reasons I've chosen this method are firstly, I used to wirewrap nichrome wire onto lead wires, before soldering. This just took forever and was comparitively expensive. With the graphite method, it takes longer to strip the insulation from both ends of the wire than it takes to make the element!! As for resistors, although quite cheap, they're still (a) more expensive than a pencil and a table tennis ball + acetone, (b) slower to prepare than a dip into a conductive slurry and (c) harder to find in nice low values around these parts.
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[*] posted on 24-12-2006 at 02:23


or short out the little 1N4148 small signal silicon diodes, they get hot enough to melt the glass, you can coat it whatever you like and then laquer it.



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[*] posted on 2-1-2007 at 06:14


Looking at the data sheet for a 0207 resistor, it says weight = 210mg. At 0.8 J/g*K for most materials likely to be found, it will heat up 12.5J/0.16J/g*K = 78K. Not very hot. But wait:

Time constant here is 23ms, too short for the ceramic resistor body to heat up. The carbon film has a much lower heat capacity. So how much hotter does the film get?

Had to try it, and it *does* go BBBZZZZFLASH instantly. No real explosion but enough for a primary. This was a half-watt metal film btw but you get the point.
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[*] posted on 2-1-2007 at 07:14


Here is a little trick that I liked: One obtains a multi-strand speaker wire ( cutting it to length and forming a "U" shape) and strips that "U" section so that only a single strand of copper wire exists at the apex of the "U" bend. The "U" section has the multi-wire focusing into a single strand of wire just at that bend point. Then by using most any source of high amp battery power ( a 6v lanturn battery - or...) the central single starnd gets very hoy and a very fast connection. The pyrogen (or whatever at that central point is heated to a near white hot level due to the single strand of copper thread-wire and off you go. It's a simple way to apply a great amount of heat even from a 9 volt. Plus it can be accomplished in a very narrow space; unlike the use of a light-bulb. So if you have a narrow rocket motor to ignite, the thing will fit in place.
but getting back to SMD resistors, the type I've used was the "805" series SMD resisitor rated @ 14-15 ohms. Two strands of 0.1" pitch computer ribbon cable were tested with a 0805 rated @ 10 ohms. A 9v battery was used and the pyrogen was applied to dark organic protection surface only. Soldering was accomplished via those new pencil-type battery-pwered needle fine soldering irons for super-fine work on printed boards like video cards. It went well. I used NC laquer with a bit of KCLO3 and Sb2S3 (70/30) and it got a great flame. I tried a bit of infused BP in the NC laquer and the flame was less intense. but I was lighting rocket motors. I suppose you could just as well use anything.
I have posted the best wire-less setup previously (it's in the archives, along with a patent I found too). It's 95% lead styphnate 5% graphite in very thin cellulose acetate / acetone mix (has to be VERY well mixed......attention to detail!) and it will also pop with a 9v..but then you just get a pop. For my needs - I need a flame.




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Lambda
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[*] posted on 3-1-2007 at 09:36
Igniter for Hobby Rocket Motors and Pyrotechnical Displays


Copper wire has a very low Resistance, better would be to use a higher Resistance wire like Nichrome. Very thin Nichrome wire can be found in a low Wattage direct mains Voltage driven glass rod-shaped combined heater-stats for use as an aquarium water heater. Generally speaking, the lower the Wattage is of the heater element, the thinner would be the Nichrome wire from which the heater-coil is made up of. I have obtained in this manner, Nichrome wire which is much thinner than a human pubic hair. I am not shore about the exact Wattage of the Aquarium heater I had used, but it must have been not more than about 10 Watts, running directly via the 220 Volts (now 230 Volts) mains supply. After inquiry at an Aquarium supply shop nearby, they had 25 to 100 and even 300 Watt heater-stats in stock. Heater-stats with an even lower wattage had to be ordered. There was however, relatively little price difference between these heater-stats from 25 to 100 watts, and were sold for about 20 to 30 Euro. The temperature was adjustable within a certain region, and maybe elements with fixed temperature control are even cheaper. Maybe as little as 10 Euros for the manufacture of an excess of 5000 ignition devices made up of about 5mm Nichrome wire and 1-2mm bridge-wire length. Strip line PCB print can be used as lead-wire and bridge-wire junction block. By grinding about 0.5-1mm deeper in between the strip line Copper layer, the bridge-wire will nicely hover/space above the solid surface. By interleaving the nichrome-wire through all of the PCB holes at ones, they can simultaneously be soldered all in one go throughout the whole 10 * 15 cm PCB board. The Bridge wire PCB board is then cut up by means of a sheet metal cutter into 4 hole islands, of which two holes are still to be used to solder the lead wires to. In bundles, the bridge-wire heads are dipped into and cleand with Petroleum ether (Pentane, used to fill Zippo lighters and for paint jobs) to remove the acidic solder flux. The acidic flux is used in solder to dissolve oxides, and for free flowing means. Acetone works even better, for this PCB board does not have symbols printed on it which may inadvertently dissolve as is the case when using it on preprinted PCB's. It may however attack your lead wire isolation coating, but this can easily be tested beforehand. The bridge-wire junction blocks are then dipped into a very dilute Nitrocellulose solution, just enough to coat the bridge-wire with a microscopic thin layer of Nitrocellulose, and left to dry. You can then dip these junction blocks in the primary, or just paint it on the bridge-wire. Capillary effects will nicely soak up the suspension of the primary to form a uniform covering of the bridge-wire. After this has dried, a thicker Nitrocellulose solution can be used to cover the junction blocks against environmental effects like moisture, contamination and abrasion. By applying a dye of a certain color, the primary may be specified which has been used for this particular igniter and is kept in a personal logbook for means of reference.

Igniters made in this way, can be used to ignite Hobby Rocket Motors and Pyrotechnical Displays in a very reliable manner by the use of Capacitor Discharge Ignition (CDI) Units, Dynamo Blasting Machines, and even low Voltage Batteries.

Regards,

Lambda.
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Lambda
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[*] posted on 3-1-2007 at 10:11
The Carbon Resistor Igniter


Here are 4 web pages for offline viewing on "The Carbon Resistor Igniter"

The Carbon Resistor Igniter - By Bob Dahlquist - HTM.rar (15KB)
http://www.4shared.com/account/file.jsp?id=8284871&sId=5...

WHO DELETED THIS FILE ?:mad:

Here you have the online web page link:

The Carbon Resistor Igniter - By Bob Dahlquist (1998):
http://www.aeroconsystems.com/tips/dahlquist_resistor_ignite...

Enjoy !

Lambda.

[Edited on 3-1-2007 by Lambda]
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[*] posted on 27-5-2007 at 08:01


An igniter that I like to use is thin steel wool, much easier to get than a carbon resistor or nichrome wire and you only need a 9V battery. However, steel wool won't burn as hot and is less reliable.
Edit: actually, I it only takes 3V for fine steel wool to ignite.

You can also overload an electrolytic capacitor, explodes quite nicely. I don't know how much heat it produces though.

[Edited on 27-5-2007 by Vexing]




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[*] posted on 27-5-2007 at 11:22


Electrolytics explode from steam pressure, so they're probably not very useful as ignitors. A 10 ohm 1/8W resistor will glow red at 12V, and cost a fraction of a penny each when bought in bulk. Nichrome can also be bought on rolls.
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[*] posted on 27-5-2007 at 17:38


I used a small light bulb.

Xmas tree lights have too thin a filament but are suitable for lighting explosive gases.

Just crush the glass in a vice to avoid breaking the filament.
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[*] posted on 3-6-2007 at 15:47


I just tried the light bulbs. Another good way to destroy the glass (if you don't have a vice) is to heat with a blowtorch or even a candle and dip in in ice water. The glass will craze or break.



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[*] posted on 4-6-2007 at 07:31


My source for carbon resistors is fantastic. PM me if you want the source (international for the EU & US). Great prices for BOTH carbon and SMD in 1/4 and 1/8 watt. I bought a roll of SMD and still have some from 5 years back. I got the carbons for next to nothing as well.
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[*] posted on 11-6-2007 at 07:06


Resistance wire is good for cooking and gets really hot, the only problem I've found is thats a pain in the arse to solder, so making reliable ignitions is a bit a pain. You can somehow solder them with really big blobs of tin, but that looks ugly and is a bit awkward to get good.

If you have a good capacitor bank (of simple, but big) elco's you don't even need it, a small, thin (hair-size, diameter as small as possible) copper wire will evaporate, giving enough heat to even ignite quite shit black powder...
Normal Resistors will generally melt or create heat otherwise, go to your local shop, get an assortment of resistors up to 5ohm (they cost nothing!) and play until it works. If it doesn't get more elco's- they can be found in old TV's, but also bought for relatively cheap.

If you're using a disposable camera or a photo flasher as load-circuit you also can bung a bigger elco up to the circuit, thereby increasing the power...

Use a relay or something to switch them, and LOOK OUT with them if you're playing with high voltages though, capacitors are about as forgiving as most primary's, and could do substantial damage- they've taking bits out of screwdrivers, I wouldn't like to thing about what would happen if my 55J cap bank would discharge itself over my hands...




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[*] posted on 11-6-2007 at 10:13


Here are a few links that may be of interest:

SMD resistors as bridgewire elements of pyrotechnical initiators:
http://www.privatedata.com/byb/rocketry/Ignitor/pk_smd_cache...

Ignitors:
http://www.privatedata.com/byb/rocketry/Ignitor/index.htm

Ignitors.pdf (Electrical Current Requirements of Model Rocket Ignitors):
http://www.privatedata.com/byb/rocketry/Ignitor/igniters.pdf

There are also many Patents concerning this matter.

Regards,

Lambda.
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[*] posted on 11-6-2007 at 16:24


While I know the original subject is igniter-oriented, I'm curious to hear if anyone has made attempts at actual exploding bridgewire dets as a replacement for primary use. Any thoughts of this form of initiation?

[Edited on 11-6-2007 by Marsh]
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[*] posted on 11-6-2007 at 22:55


SWIM has, and it worked ok-ish, but couldn't get realiable enough to use it. The switching and the wiring are the main problems, if you have a nice digital occiloscope then you have good chance of tweaking the system till it works, but without such a thing its hard to optimise it.

Basically was the set-up a battery of 24 1KV 2.2uF caps (no elco's they're too slow, need these square guys), a 24V 50A car-relay, a coax cable to the wire itself, this last thing was an ever so thin copper wire, soldered over a bit of circuit board that was coated with copper on both sides. There was a little hole cut in the relay, so when the contacts were welded together they could be separated again with a screwdriver. This form of switching is not really optimal though, the best would be a triggered spark gap, but they are not that cheap to get, but if you're clever maybe you can make something with a 'normal' spark gap.

The trick is to get really high pulse in a really short time, so the wire does not evaporate immidiately, but stays in its form due to inertia, releasing energy in the form of a blast wave. A good aim is to get a 1KA rise in less than 500ns (IIRC), then you'll have a perfect working box. For monitoring this the occiloscope is really handy.
PETN was detonable with it, but the preferred set up was 100mg MHN with 300(ish) mg PETN, that was the most reliable. The fun thing of these experiments was that one is able to do them at home, take a suitcase full with old rags, place you EBW+secondary in there, close suitcase, fire.




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