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Resistor Suggestion for Cap Bank

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ScotchTapeLord

Fri Nov 19 2010, 05:04AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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I already submitted to the point!

I just don't like the fact that you can't burn off 1/8 of a watt worth of electrical power using a 10 Megohm resistor rated for 1/8 watt...! It just feels... wrong. :(

Sir Fixalot

Sat Nov 20 2010, 05:19AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
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Okay, will something like this make everyone happy?

Steve Conner

Sat Nov 20 2010, 11:39AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I've had 1/8 and 1/4 watt resistors arc over at 500V and destroy my prototype circuits. The voltage ratings are real.

I use 100k, 2 watt metal oxide power resistors as voltage balancers/bleeders in tube amp power supplies.

For DRSSTCs and the like, where there's a really big cap bank that has to be discharged safely, I use 10k, 10 watt, either the cement block type or the gold aluminium housed ones.

ScotchTapeLord

Sat Nov 20 2010, 07:22PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Sir Fixalot,

Please note that I was corrected and that 1 Megohm resistors are not suitable for large electrolytic capacitors.
Steve McConner suggests a value as low as 10k.

Sir Fixalot

Sun Nov 21 2010, 02:17AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
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ScotchTapeLord wrote ...

Sir Fixalot,

Please note that I was corrected and that 1 Megohm resistors are not suitable for large electrolytic capacitors.
Steve McConner suggests a value as low as 10k.

So basically the resistors I originally asked about would be suitable?

I probably should have said earlier that I used the resistor equation on barry's coilgun site to predict that I would need a 36k resistor. I chose 10 watts arbitrarily just to be sure they could handle the discharge.

Would it be advantageous/Disadvantageous just to put the 8 in Parallel?

I just want to say that I really appreciate all the help you guys have given me so far.

ScotchTapeLord

Sun Nov 21 2010, 02:50AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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The power rating has nothing to do with the discharge. Discharging the capacitors puts less stress on the resistors.

Determine the wattage rating with (V^2)/R, where V is the voltage across the resistor and R is the resistance. As for the voltage, make sure the resistor is rated for V, too.

For a voltage of 350, you'll want a power rating of at least 3.4 watts according to the formula, so probably 4 or 5 watts depending on what's available.

As for series/parallel configuration, it depends on what you're looking for, a larger capacitance or a higher voltage.

Advantages of higher voltage include a slightly higher theoretical efficiency. You'll need more windings on your coil for a higher inductance using higher voltage.

Advantages of the higher capacitance configuration with all paralleled capacitors are that you don't need balancing resistors (but you should put one across all of them for safety reasons), that you can use a lower voltage charger, and that you can use lower-rated silicon for switching and freewheeling, etc. You'll need fewer windings of thicker wire for a lower inductance using the lower voltage.

Sir Fixalot

Sun Nov 21 2010, 07:08AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
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ScotchTapeLord wrote ...

The power rating has nothing to do with the discharge. Discharging the capacitors puts less stress on the resistors.

I meant discharge as in, the resistor could safely discharge one of my fully charged caps w/out overheating or popping or anything fun like that. Not as in Discharging the whole bank through the coil.

ScotchTapeLord wrote ...

For a voltage of 350, you'll want a power rating of at least 3.4 watts according to the formula, so probably 4 or 5 watts depending on what's available.

Ok. How 'bout these:

ScotchTapeLord wrote ...

As for series/parallel configuration, it depends on what you're looking for, a larger capacitance or a higher voltage.

Advantages of higher voltage include a slightly higher theoretical efficiency. You'll need more windings on your coil for a higher inductance using higher voltage.

Advantages of the higher capacitance configuration with all paralleled capacitors are that you don't need balancing resistors (but you should put one across all of them for safety reasons), that you can use a lower voltage charger, and that you can use lower-rated silicon for switching and freewheeling, etc. You'll need fewer windings of thicker wire for a lower inductance using the lower voltage.

Hmm....Interesting you mention that. In your opinion, which of the two do you think is more suitable for a coilgun?
Keeping that in mind do you believe this to be true(the 2nd post)?

Sir Fixalot

Tue Nov 23 2010, 01:17AM

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ScotchTapeLord

Tue Nov 23 2010, 01:58AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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You'll need to design your coil according to the voltage and capacitance to get the right waveform, and either configuration will ideally create the same amount of magnetic force. In a non-ideal world, the higher voltage at a lower current will create slightly less heat, which is why I said it would be more efficient. Though, this is negligible since coilguns are already grossly inefficient.

Saturation happens when you put too much energy into the coil. Energy of a capacitor = .5*C*V^2, where C is capacitance and V is voltage.

Eight of your 2900 uF in parallel would make a 350V at 23200 uF, so .5*(23200*10^(-6))*(350)^2 = 1421 Joules of energy.

With 2x4 in seriesxparallel you get a 700V equivalent capacitor with 5800 uF, so .5*(5800*10^(-6))*700^2 = 1421 Joules of energy.

It's not a coincidence... If you're going to saturate your projectile, it's going to happen no matter how you configure your capacitors, since the energy output is the same. The solution is not to lower the voltage but to either split your banks and use multiple coils or to use a larger projectile.

I think you should try the parallel bank. It'll be easier to charge and you won't have to worry about balancing the voltage with resistors and whatnot. You can buy lower-rated silicon and won't need as many windings on your coil, though you'll need thicker wire.

What size slug are you planning to use? You should consider multiple banks unless you want to propel steel pole sections. :)

Sir Fixalot

Tue Nov 23 2010, 02:48AM

Registered Member #2810 Joined: Sat Apr 17 2010, 07:17PM
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But isn't it better to have a very quick, high amplitude pulse for coilguns(Like what you would get from a higher voltage, lower capacitance) as opposed to a longer pulse(like a parallel, high capacitance bank) to reduce suck back? Or are you recommending 8P because it is much easier to build/wire?

If I do go with 2s4p, would the resistor I linked to a few posts back work well? Just wanna make absolutely sure.

BTW my Projectile is a stainless steel rod I tore out of an old printer and cut into sections. Each section is about 10mm in diameter, roughly 2.5 inches long, and probly around 40 grams(I'll have exact numbers soon).

I want to design my cap bank first and build the coil/projectile around what that, and I've generally heard that higher voltage/shorter pulse is preferable to lower voltage/longer pulse. Or do you think that 8P isn't enough capacitance to cause significant suck back?

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