Halloween Spark Wall power supply design and safety help

[parabuzzle]

Hey everyone,

I am doing a haunted house in my neighborhood this year and I found this really great prop online called a "spark wall" or spark fence...

Here is the "professional" prop: http://www.hauntedprops.com/category-s/357.htm
Here is a tutorial I found that says to use a 12v car battery charger (which is what the pro one appears to use anyway): http://www.hauntedillinois.com/sparks.php

It seems that there has to be a better (and cheaper) way to build the PSU for this using my junk bin of parts...

So here is what I have designed: http://note.io/16aHVFB (also attached)

Here are my thoughts about the design I'm working on..

 * I wanted to take the short load off of the rectifier, transformer, and mains... so I put the 24 farad cap in there to buffer the momentary shorting.
 * I put a momentary switch on the wand so that the wand is only charged when you want it to be and if fusing occurs to the fencing you can open the circuit by letting go of the button.
 * The fuse I have put inline will be a car type fuse and will be just above the current that I get when the shorting occurs... just as an extra safety.

My questions are:

 1) Is there a way to make this circuit "safer"
 2) What are an additional considerations that I should be worried about (even the obvious ones.... sometimes you don't see the forrest through the trees).

Thanks in advance for your comments.

-Mike

[homebrew]

oh dear ...

that circuit has a lot of problems ... let's see ...

If the capacitor is empty (probably after each sparc but certainly on power up) you might have a very large load current. The current depends of the equivalent series resistor (ESR) of the cap, but could easily go into the 10th-100th of amps. Possibly too much for the rectifier and transformer.

If the rectifier fails for any reason and AC is delivered to the cap, I don't want to be in that room... Especially not with that cap size!

The fuse is absolutely useless if it only blows on a dead short (that is exactly what you are after for the effect!?). Either it will blow all the time (good for safety, bad for the effect) or it will never blow at all (bad for safety).

The momentary switch is useless too, because it will instantly melt its contacts together. No safety at all!

The "load resistor" will die, because its rating of 10W is too low. After rectification you will have about 17V (1.414 * 12V) -> Nearly 30W power loss in the Resistor -> Resistor dies ...

Brainstorming of dangers:
Flying molten metal, UV-rays from the arcs, noise, fire-hazard, explosion hazard (cap), chemical hazards if that huge cap explodes/vents/melts, EMI interference with other devices, electric shock hazard if the transformer fails (because it has no fuse to protect it...)
If the fence is grounded, any contact of your "rod" to any other grounded part might overload the ground-system of your house (that sometimes happens with electric arc welding ...)

Ok, my opinion: Just don't do it! It's a really bad idea!

Play safe,
Pete
 

[homebrew]

Sorry, I forgot one thing ...

In your current schematic you have a direct connection between the center-tab of the transformer and the (-) port of your rectifier. That will short out the lower part of the winding every negative half-wave. Really bad for the transformer and rectifier ...

[parabuzzle]

Thank you for the notes so far.. So I'm way off I guess. I don't usually do this sort of work so, I thank you for speaking up. My problem is that the car charger thing sounds bad but are you saying its a better to use that? That's what people are selling anyway.

-Mike

[parabuzzle]

Also, I guess this is why software guys (me) shouldn't make hardware.. and software by hardware guys is (usually) horribly unusable

-Mike

[parabuzzle]

As far as the grounded center tap goes.. I read somewhere that its what you do with it.. I have also seen a lot of schematics that do the same.

-Mike

[Psi]

Add a 60W mains lighbulb in series with the 110V input to your transformer.

When the 24F cap is empty, (basically a short circuit), the lamp will illuminate.
As the cap charges up the lamp will get dimmer and dimmer. When it stops getting dimmer the cap is fully charged.
Quick and dirty calc says it will take ~2.4 minutes to get that capacitor to full charge.

Doing this limits your circuit from drawing more than 60W

60W is actually more than your transformer is rated for, but since its only for a short time (initial inrush current) it should be fine.
You can always change to a 40W or 25W if the transformer gets too hot.

You could even make a feature of the lamp.
A lamp getting brighter then dimming back down every time you make sparks may look cool in a haunted house.

Also, as others have said, the 10R resistors is wrong, make it 600R

[NiHaoMike]

Just use a 12V or so transformer and something that would make a good ballast inductor.

[David_AVD]

As far as the grounded center tap goes.. I read somewhere that its what you do with it.. I have also seen a lot of schematics that do the same.

The centre tap is used with a bridge (and 2 capacitors) when you want a split supply.  That's when you have a positive and a negative output with respect to a common terminal.

[Psi]

Just use a 12V or so transformer and something that would make a good ballast inductor.

yeah, that would work too

[amyk]

As far as the grounded center tap goes.. I read somewhere that its what you do with it.. I have also seen a lot of schematics that do the same.

The centre tap is used with a bridge (and 2 capacitors) when you want a split supply.  That's when you have a positive and a negative output with respect to a common terminal.

It's also used with 2 diodes for a supply that has less voltage drop.

[parabuzzle]

So I found a non center tapped 12VAC 2A transformer in the bin... so forget the center tap BS... (I've got more learning to do there, obviously)

As far as the inductor posts go...

Are you suggesting that I do away with the DC conversion? Just step down the mains to 12VAC, push it through an inductor, and wire it to the wand and fencing?

Or are you suggesting that I use the inductor to limit the current inline to the rectifier?

I was under the impression that using 12VDC would be better than AC in this application...

BTW, the rectifier I'm using is a 35A rectifier (Jameco Part no. 178431)

-Mike

[David_AVD]

So I found a non center tapped 12VAC 2A transformer in the bin... so forget the center tap BS... (I've got more learning to do there, obviously)

If you don't need the centre tap for the application, just leave it disconnected.  No need to seek out another transformer to get rid of it.   

[parabuzzle]

Oh I'm not getting rid of it.. Just putting it back in my bin-o-fun

[David_AVD]

Oh I'm not getting rid of it.. Just putting it back in my bin-o-fun

I meant no need to swap the transformer out just because you're not using the centre tap.  Leaving the tap unconnected (when not required) is not an issue in itself.

[parabuzzle]

I have downgraded the cap to a 3000mF cap I found at my local surplus store...

So, one more question here... Whats the best way of measuring the current level on the output side after the cap just before the momentary short that causes the spark? Also how do I limit that current so that I can actually put a fuse inline there and also how to prevent it from heating the 15 amp rated wire from heating up to failure...

-Mike

[Psi]

are you suggesting that I use the inductor to limit the current inline to the rectifier?

Yes, mains inductor or lightbulb in series will limit the mains current which can flow into your transformer which will prevent massive currents from damaging things.

Whats the best way of measuring the current level on the output side after the cap just before the momentary short that causes the spark?

Do you want to measure it or just know when the cap is charged?
If you only want to know when its charged look at one of my earlier posts about putting a mains lightbulb in series with the mains input. When it stops getting any dimmer it's fully charged.

how do I limit that current so that I can actually put a fuse inline there and also how to prevent it from heating the 15 amp rated wire from heating up to failure...

If you put the inductor or lamp in series you can work out the short circuit current on the secondary side and use a fuse to match. But the fuse must go before the capacitor. (It cant go after because there is no way to prevent the cap from blowing a fuse when you short it)

If you make the wire part of the wand you hold and use to strike the plate you will feel when it gets hot and you will know when it's time to let the wire cool down. (When you cant hold it any more )
Or you could add simple 60deg thermal switch attached to some of the wire. Maybe coil some of it around a metal object and have the switch bolted to that.  This way it will cut the power if the wire gets above ~60deg