How do the Chinese/ebay 5VDC to 7kV transformer work?

[raspberrypi]

They sell these on ebay for about 5 USD. They are loud and make a 2cm long spark at 5volts with big caps attached.

But how does this step up the volts if its DC?

I measured the resistance and got more confused: open/off the scale on the output side, 3.4M on the input side, and open/off the scale with the polarity reversed. Also it seems you have to discharge this if it doesn't spark. It seemed to make a little spark when I shorted it after I disconnected it, good thing I did. So that means it has a cap(s) in it?

[amyk]

It has an oscillator that drives a step-up transformer.

[raspberrypi]

So the output is AC? What kind of frequencies are we talking about? I know that any electronic within about a foot of it get destroyed. I found this out the hard way. I was clued in when all my LEDs lit up with a faint blue glow.

[capt bullshot]

I guess there's something inside like that:

http://www.bigclive.com/stunner.htm

[PartialDischarge]

It has an oscillator that drives a step-up transformer.

that and a multiplying ladder of diodes and capacitors at the output. Theres also a high ohmic value resistor at the output to discharge the  output. I did open one  , but mine gave much more than 7kv, probably 30-40kv based on arc length.

[james_s]

The output may either be AC, pulses of DC, or something resembling DC depending on the design, but it will always have an oscillator inside. I haven't opened up one of those cheap ones but I suspect it's a single transistor, frequency will probably be all over the place depending on supply voltage and load.

[raspberrypi]

I guess there's something inside like that:

http://www.bigclive.com/stunner.htm

The pictures don't work and it doesn't seem like he made a video on it.

[Red Squirrel]

Is there a transformer at all?  My initial guess would be pulsed DC.   I don't see much in your pic, is there more to it than just a few capacitors and wires?  I'm guessing that big one is a HV capacitor?  Looks pretty beefy!  Careful with that thing it will kill you if you touch the terminals when it's charged up at a high voltage.   Mains is one thing, normally the voltage is still too low to drive enough amps into you to be lethal (but it will if the conditions are "right" so still respect it of course), but when you hit the kv range things get ugly fast.

I was messing around with a self wound step up and a 555 timer and managed to get some decent arcs.  I think my transformer was too small to get anything crazy out of it or perhaps I was not driving it at a high enough frequency, but I was still able to make LEDs and CFLs light up wirelessly with it.   

[mtdoc]

You can see Julian ilett play with one of these that claims to put out 400 kV  ( ) at the 7:40 mark in this video:

[Buriedcode]

With the pulsed output ones (essentially a stungun/taser circuit) voltage output depends on the output spark gap.  Longer the gap, the initial voltage of the arc, since its effectively a current source coming from a transformer (the second step up transformer which has its primary switched across a high voltage cap).

[CCitizenTO]

You can see Julian ilett play with one of these that claims to put out 400 kV  ( ) at the 7:40 mark in this video:

Wow that stuff is kinda scary.

So how would you measure how much voltage it's putting out since I'm guessing when you get into the 400kV range odds are you can't just plug it into a Multimeter and get a reading (voltages that high probably would kill a multimeter right?). Would you keep putting the wires further apart to see when the sparking stops or something?

[helius]

You could build a 1000:1 divider probe: https://www.repairfaq.org/sam/hvprobe.htm
Using the pulse inverter with its leads too far apart may damage it.

[james_s]

There's no way that thing is 400kV. A lot of those generic Chinese devices have specs that are pure fantasy. I bet the actual voltage is much lower, and likely anything you can reasonably do to measure it will drag it down further.

[Buriedcode]

For the 5-7kV ones, I strongly suspect, as someone has already suggested, it'll be a flyback, royer, or self oscillating step up transformer, possibly followed by a voltage multiplier if the output is said to be 'DC'. You can get 5-7kV directly out of a transformer but will either be just high frequency AC (royer), or short pulses (flyback).

For that one in the video, as I said, its output voltage will be dependent on the spark gap.  If the output wires are far enough from each-other, then what limits its 'output voltage' is the breakdown of the internal insulation of the output transformer.  This is also the case for stun guns.  And yes, the figures provided are overblown and probably never tested, just marketing - although if the output transformers insulation is good enough, and the output electrodes far enough apart, I'm sure extremely high voltages are possible, but certainly never regulated.

Their only real purpose is to make big fat scary arcs to impress/scare people, to ignite gas/fuel, or to cause pain. The old 'proper' stun guns designed to temporarily paralyze used specific (albeit not very accurate) timing of pulses, with a controlled current and voltage limit (voltage was limited by an internal spark gap).  Modern ones are just nasty high voltage pulsers with short output pulses, but at very high power.

So ultimately, china just knocks these out for the novelty factor - who doesn't love high voltage arcs?  But in practical terms, powerful pulsed arcs like that can easily interfere with nearby electronics, can be very loud, produce ozone... burn/melt your arcing wires so it doesn't really have much practical use.  I'll probably still buy one though as they're so cheap!

[james_s]

For a while I was doing a lot of HV experiments, winding HV transformers and making big arcs and lots of corona. Preventing a transformer from flashing over is quite difficult, especially at high frequencies. I had difficulty getting beyond about 80kV, electricity would be spraying out into the air like pressurized water leaking out of fittings. It liked to track surfaces and once it flashed over it would leave carbon tracks that then flashed over very easily. Even submerged in oil there was a point where it would arc right through the oil. One of the more interesting experiments was using a flyback transformer from a CRT projection TV, I had measured it at a bit over 80kV and the wire was spraying corona out about 6" off the end. Several feet away a pile of assorted stuff in the corner of the room sparks started jumping between two metal objects that had no electrical connection to the stuff I was doing. I think it would be extremely challenging to make a reliable 400kV transformer in any sort of small scale device.

[PartialDischarge]

Even submerged in oil there was a point where it would arc right through the oil.

Not saying oil doesn't arc, but its quite difficult. It is possible that your oil had traces amount of water, even less than 0.1% content will decrease the strength dramatically. It's very important to heat this kind of oil before using it.
Of course I have no idea how "small" was your setup.

[james_s]

Oh there very well could have been trace amounts of moisture in the oil, and the oil did improve things immensely, but I still had it flash over when really pushing things. That is not to say that it's impossible, but beyond about 50kV it starts getting a LOT more challenging to keep the electricity in, and I can only imagine what it would require to contain 400kV.