Hey /diy/ how can wire this relay so that when the voltage

(the surge would be created by switching the power to the coil off)

I ALSO FORGOT TO PUT THE SWITCH BELOW THE REDIRECTION WIRE

>>906519
Is that relay even rated for 750 volts?

>>906524
its not

im asking how i could switch it on when the voltage across the coil went above

>>906525
ac or dc?

>>906527
I think its AC since the surge created by coil would be AC

>>906532
the capcitor banks are DC but the power surge is AC since its made by a coil's magnetic field

It sounds like you're trying to deal with the voltage spike from a collapsing magnetic field. If that is correct then you use a protection diode to deal with the magflux, not a relay. A relay would never be able to act fast enough to stop the surge. Do you still want to know how to trigger a 12v relay at 750vac?

>>906539
sure

and what protection diode should i use then?

i cant find one thats big enough

>>906540
Give me the specs you're looking for. Specifically the max voltage on the line and expected current of the collapsing field.

>>906541
i dont know the max voltage or the current but i know the coil's specs as well as the specs of the electricity before the surge

i dont know how to calculate it

if you can tell me how to calculate it and/or a link to a calculator

as well as a link to where i can buy diodes like this i can handle myself. but if you have time please help me figure this out


>voltage before surge is 700

> current is 550 before surge

>coil is 5.5 cm in diameter

>coil is 5.5 cm in length

>coil's hole is 2.5 cm in diameter

>>906543
i forgot the wire is 18 awg

>>906543
What is the inductance of the coil?

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>>906519

You don't need to clamp any AC. Inductors don't oscillate unless they're part of a tank circuit. They only try to keep current going when switched off, and only in one direction.

You just want a normal flyback diode to allow that current to circulate in the coil until it dissipates, preventing the voltage spike in the first place.

>>906549
links to this flyback diode?

>>906548
im getting the specs btw

>>906549

BTW, I'm assuming OP is attempting to build a coilgun, because that's realistically the only thing he could be building with ridiculously high currents and voltages, yet still get this far with a barely-functional knowledge of electricity.

>>906550

There's nothing special about a flyback diode.

It's called a flyback because it handles the "flyback" of the current in an inductor, not because it's of any special construction. The only things to take into consideration is whether or not it can handle the heat generated in operation, and whether it can switch conduction sates fast enough. Schottky diodes are often used for this purpose due to the latter consideration.

>>906550
Clamp diodes come in various sizes like all components. You pick one based on your requirements. Specifically current flowing through the inductor and voltage that the coil sees. Also, please include units on all your values. 700 and 550 are just numbers without units for context.

>>906548
i used this calculator tht gave me 1737.05 mH

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indsol.html

but in the 'radius' box it didnt account for the radius of the center hole of the coil

>>906553
700 volts and 500 amps

>>906551
yes that is what i am building

i have been trying to teach myself electricity for about 5 weeks now

>>906553
i need links to which flywheel you recommend

or at least a site where i can find a lot of variety of them

Try again. You can not push 500 amps at 700 volts though 18 awg wire. Did you mean mA?

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>>906558
no i meant amps

http://www.sengpielaudio.com/calculator-ohmslaw.htm

i used this calculator

what wire should i be using then? and how do i find the internal resistance of the capacitor bank?

i just didnt account for the resistance of the capacitors

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>>906559

>>906560
>>906559
this is the wrong capacitor i posted

FUCK ME

>>906561
http://www.ebay.com/itm/381344563950?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

this is it

>>906558
>Try again. You can not push 500 amps at 700 volts though 18 awg wire. Did you mean mA?

Again...he's building a coilgun. It's pulsed DC and you'd be seeing those peak values for less than a millisecond. Unless he's got a ridiculously large capacitor bank, there won't be enough energy in it to even heat the coil perceptibly.

>>906556

I have no idea how much energy you're dumping into the coils (because you make no mention of pulse duration or bank capacity), but I'd hazard a guess that, given the typical hobby budget for this kind of crap, you'd probably be fine with just about any diode rated for 1000V and 10A.

http://www.ebay.com/itm/10-pcs-Molded-Plastic-Case-1000V-10A-Rectifier-Diodes-10A10-MQ-/331579101990?hash=item4d33a79326:g:m7YAAOSwrklVeGYw

Might want to find something more local, but whatever.

im so fucking new at this shit

>>906563
i posted the capacitor bank

the bank will be two pairs of caps in parallel which are connected in series

>>906563 Agreed.

Forget the figures and the math. Just get a M101 or 10A10. They'll handle 1000v and more amps than your 18 awg wire can.

>>906568
what wire should i use then

i keep saying this but LINKS PLEASE

btw guys whats a really good book to start learning this shit properly

right now i have 'getting started in electricity' by Forrest M. Mims

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so just do this and turn the power to the diode on when power from the bank is off?

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>>906576

No.

The diode needs no active switching. Once the inductor's circuit is opened, it will immediately begin to build up a voltage across its terminals that is the reverse of what was applied to it. Since there's a diode in parallel to it with the polarity in the right orientation, the diode begins to conduct. The current in the coil, since it has somewhere to go, flows through the diode and inductor, without producing any high voltages. This continues until the magnetic field (which is where the inductor stores its energy) fully collapses, which, in your case, will take somewhere between hundreds of microsecond and a few milliseconds.

Basically, once you shut off the power to the inductor, the diode takes over. That's the point of having it.

>>906541
not OP, but I admire you helping others..

Somebody correct me if I'm wrong but I don't think you'd need a flyback diode for a coilgun. Flyback diodes are for when you actively switch the power to the coil off. If you're just draining the capacitor bank into the coil it doesn't need to be switched off (and thus won't generate a voltage spike) since it will stop when the capacitors are empty. If you're powering multiple stages off a single capacitor bank and turning them on and off electronically then you'd need them, but it seems simpler to just have separate capacitors for each stage and let them die out on their own.

>>907372
>Somebody correct me if I'm wrong but I don't think you'd need a flyback diode for a coilgun.

You technically could get away without one, IF you use capacitors that aren't polarized. However, electrolytic capacitors are far and away the type of choice for coilguns, since they generally store much more energy for a given unit of volume (not to mention unit of dollar).

An inductor connected to a capacitor (which is pretty much all a coilgun is) forms a tank circuit. When fed power momentarily, this circuit will oscillate, producing an AC wave that decays depending on how damped the circuit is. The negative peaks of this wave will damage polarized capacitors. The easiest way of preventing this oscillation in this case is simply to use a clamping diode.


By the way, something you're going to want to note when building one of these is the surge current. Even if there's enough thermal mass in the diode to prevent overheating, extreme current spikes (even if they're short) can damage them. Check the datasheet for whatever you buy to make sure it can handle the expected peak current.

stop replying to this moron, this is like the 3rd or 4th thread he's made about relays. Pretty sure he's trolling.

>>906583
Is it to protect the coil itself or protect other components from this EMF charge?

I ask this because I'd have thought that the current wouldn't be able to mobilise once the circuit had been broken anyway, so it can't damage anything else on the board can it?