Electrical Advice

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Postby markfh11q » Wed Dec 27, 2006 8:32 am

First off, this isn't for any of my designs. I've got a friend who just doesn't want to use a piezo for his ignition. He's got to have some high-voltage and over-advanced ignition system for his cannon, (I'm fine with the cheapo Chinese piezos myself :mrgreen:), so I thought I'd help him out.

I'm working on a simple circuit for ignition that I want to be sort of "plug and play". Therefore, I want to make the simplest and cheapest ignition unit possible that is also powerful and effective, and also able to be reproduced for other cannons with ease, (if it works, I think I can just built it from spare parts lying around if I want another one). I came up with a little something like this:

<center><img src="http://i10.photobucket.com/albums/a123/markfh11q/fcircuit.gif" border=1 bordercolor="black"></center>

I think the circuit should look something like this. My questions regard what type of resistor I should use. The capacitor will be out of a camera, so it will be marked as 120 microfarads with 330 volts. I also almost KNOW it's poled, but I don't see any pole markings. If it's poled, I know that means my resistor needs to be selected accordingly. So my question would be what type of resistor I needed to use.

Hope some of you people who've taken some physics or are in electrical engineering courses can answer this question for me, because I'm no good at anything past the basic concept, (yes, I've looked in the encyclopedia, no I couldn't find anything :evil:). Thanks in advance!

EDIT: Here's a picture of the capacitor I'm using. I'm guessing now that it is not polar.

<center><img src="http://i10.photobucket.com/albums/a123/markfh11q/capacitor.png"></center>
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Postby GalFisk » Wed Dec 27, 2006 9:02 am

A cap of 120 pF is not polarized, but it is also far too small. Sure yo don't mean uF (microfarad)? You want one with as many uF as possible, and a voltage rating of at least the output voltage from the battery. Some (older) power amplifiers have big caps.
You don't want a resistor between the transformer and cap, because you need a high current spike to get a good pulse. You'll probably want one in series with the safety switch though, to reduce wear (you can also use a lamp).
An ignition coil may work as the transformer, if the cap is big enough (not sure how big it needs to be).
edit:typo
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Postby TurboSuper » Wed Dec 27, 2006 9:19 am

I don't think 330 volts will be very good for a lamp. It will also slow your charge rate down quite a bit since standard lamps can be quite resistive.

I think the best way to tell when it's charged is to just listen to the whine of the cap as it charges up.

Someone posted a design for a camera-flash based ignition system a while back...im sure its around here someplace...
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Postby markfh11q » Wed Dec 27, 2006 9:27 am

Ooops. I did mean microfarads. I just read the thing wrong.

I thought a current had to be AC to work through a transformer? Are you saying that the burst from a capacitor will work directly hooked to the transformer? I guess that makes sense. So I can eliminate the resistor? Cool.

Turbosuper, I'm not running 330 volts through the lamp. Just a max of around 9 volts. Also, capacitors don't whine when they charge, that's the step-up system in the camera that charges the capacitor. Take a look at HowStuffWorks on camera flashes.
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Postby GalFisk » Wed Dec 27, 2006 10:05 am

You don't need AC, you just need the current to change. The higher the rate of change, the better.
I may have posted designs for camera flash based ignitons, the simplest modification is to just cut one of the main leads to the flash tube and connect the ignition coil in between. Triggering the flash will produce a spark from the coil.
And that cap is polarized, the negative lead should have some kind of marking (usually a stripe on the black plastic coating). If you connect it the wrong way around, it may short out. It's also not very practical for low voltage use, you can get a 3300 uF/16V cap not much larger than that one.
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Postby TurboSuper » Wed Dec 27, 2006 1:26 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by markfh11q
[br]Ooops. I did mean microfarads. I just read the thing wrong.

I thought a current had to be AC to work through a transformer? Are you saying that the burst from a capacitor will work directly hooked to the transformer? I guess that makes sense. So I can eliminate the resistor? Cool.

Turbosuper, I'm not running 330 volts through the lamp. Just a max of around 9 volts. Also, capacitors don't whine when they charge, that's the step-up system in the camera that charges the capacitor. Take a look at HowStuffWorks on camera flashes.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

My bad:|

Anywho, I still think you can use a resistor in parallel with a resistor and an LED to make a more efficient charging circuit. But hey, I guess a bulb is simpler.

Oh, and the discharge system should also have a resistor in series with it to avoid damaging the cap.
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Postby jimmy » Thu Dec 28, 2006 1:12 pm

1. You don't need the load resistor, the photocap is specifically designed to tolerate a dead short. Indeed, the load resistor will decrease dI/dt and decrease the output voltage of the transformer. In fact, photocaps are specifically designed to have low internal resistance ("equivalent series resistance", ESR) just for this reason.

2. It is your "safety/disharge circuit" that needs the resistor. Dumping the cap through the switch without any load will probably fry the switch's contacts pretty quickly. BTW, a 100W, 120V light bulb should work as the resistor for the "safe" circuit.

Go down to your local grocery store with a photoprocessing lab and ask for a couple of disposable camera bodies.

Rip the flash unit out of'm. That'll give you a 300V power supply, 1.5V source supply (the battery in the camera will still work), and a 300V 120~150MFD cap. Then do like others said, dump the cap through an automotive ignition coil. Or, really cheap out and just use the wimpy flashtube trigger voltage (5~10KV) for your sparker.

It really doesn't make much sense to try to build the high voltage supply from scratch when you can get a perfectly good one for free. A 120~150MFD cap at 300 volts has a heck of a lot of energy in it, ~5Joules. And, voltage is more important than capacitance when it comes to the energy stored in the cap and the ability to convert that energy into a high voltage spark; E<sub>cap</sub>=(1/2)(capacitance)(voltage)<sup>2</sup>.
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Postby markfh11q » Thu Dec 28, 2006 1:49 pm

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">
2. It is your "safety/disharge circuit" that needs the resistor. Dumping the cap through the switch without any load will probably fry the switch's contacts pretty quickly. BTW, a 100W, 120V light bulb should work as the resistor for the "safe" circuit.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Yeah, I was reading about the little "bleeder resistor" assemblies used for discharging capacitors. I'll just add the resistor to the safety circuit then.
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Postby SpudStuff » Thu Dec 28, 2006 3:24 pm

I could get you an 18 farad 1200v Cap if you have 52 dollars + shipping...
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