I know I've had quite a few teardown posts recently, but I just had this device come in this evening and would like to write about it. I have a post on Escalante National Monument I'm starting to write for tomorrow.
Today I received a $2 electric stove lighter from Ebay after a long shipping wait. Like many cheap Ebay electronics, the description was a bit deceptive. This lighter doesn't actually ignite a flame, rather it produces a high voltage arc that can ignite gas stoves. Luckily for me I had no need for the flame part and was going to remove the fuel canister so this deception is a big help to me!
How the device is supposed to work is you hold it up to a gas stove and pull the trigger. A high voltage spark gap ionizes air, producing a thin plasma arc that ignites the flammable gas from the stove. I naturally bought it because it's a $2 high voltage source and I wanted to see what's inside.
High Voltage Lighter
Many actual barbecue lighters (with a self-sustained flame) use high voltage to ignite the gas. However, most of these lighters utilize a piezoelectric crystal, which is struck by a mechanical piston to produce an extremely high electric field that ionizes air between two electrodes (more on that here). This lighter uses an electronic high voltage converter to produce direct current arcs powered by a battery.
The lighter looks, well, like a lighter. A big white button on the back fires up the high voltage arc. The device is powered by a single AA battery. A black metal tube at the end of the lighter contains two sharp pins that produce an arc between them when the white switch is pressed.
Surprisingly the lighter is really easy to open. A single screw under the battery lets you snap open the plastic shell and see the insides.
Inside the lighter is a single circuit on a PCB connected to the battery bracket. I was pleasantly surprised to see that the lighter only used one battery, since that means that it can boost voltage up to arc-producing levels from only 1.5 volts - could be useful! Two wires, a thick HV output wire and a thin ground wire, lead out to the black tube where the arc is produced (the thin wire is clamped directly to the black tube to put it at the same potential as the battery negative terminal).
Nearest to the battery is the low voltage portion of the circuit. This uses an inductor (hidden under a mechanical metal switch), two transistors, three resistors and a capacitor to form an oscillator circuit, converting the low voltage input DC to a low voltage AC signal. Without an oscilloscope I can't get any more specific than that unfortunately.
The circuit wouldn't be too difficult to reverse engineer, if not for the HV converter (which I'll get to in a minute) and the fact that I'm putting off way too much coursework to write this tonight.
The far end of the PCB contains the HV converter. It's operation is a little bit of a mystery to me because, like many HV converters, the interesting part is wrapped up in an opaque cylinder that I don't want to destroy opening. AC input from the low voltage oscillator is fed into a black cylinder which I guarantee contains some sort of transformer. By putting two coils on top of each other with few turns on the input coil and lots of turns on the output coil, very high voltages can be obtained. This is then fed into the output.
The arc breaks and is re-established many times a second, resulting in a tiny but audible cracking reminiscent of a stun gun. This almost always indicates that the high voltage is produced by something called a CW Multiplier. CW multipliers use a "ladder" of high-voltage capacitors and diodes to double, triple, quadruple voltages. However, a lack of diodes on the high voltage side made me change my mind.
The actual source of the cracking is something a bit simpler. It seems that the voltage output of the transformer is the final voltage output that produces the arc. There must be a high-voltage diode inside the hidden transformer, converting the AC HV output to DC high voltage. This current then charges the blue 6kV capacitor, visible above.
As the capacitor charges, the electric field between the two output wires increases in strength. Eventually, the field becomes so strong that something called high voltage breakdown occurs, igniting a plasma arc between the wires. The plasma arc has only a few ohms of resistance, so current can freely flow across it. The arc is somewhat self-sustaining, so it can fully drain the capacitor before collapsed and breaking the circuit. Once the arc falls apart, the capacitor must recharge up to a high voltage before the arc can be re-established. This process happens several times a second, resulting in the cracking, restarting arc.
Based on the maximum length of the arc (about half a centimeter) and the capacitor rating, I believe the output voltage to be between 3 and 5 kilovolts (unfortunately I still can't find my gigaohm resistors to actually measure the voltage). This is very, very nice for a $2 converter that runs off a single 1.5 volt battery. Well, that or I'm crazy and waste money on useless dollar-store stuff on Ebay. I think the truth is somewhere in the middle.
The arcs are hard to image because they are quite small and disappear quickly. I used burst-shot on my phone to take a few pictures. Here's on with the lighter assembled:
The gas in that arc that is actually ionized is extremely high temperature, much hotter than a flame. However there isn't much of it, which is why you can't feel heat transfer off of the arc.
Something interesting happens if you bring the output wires further apart. I noticed that at about 5 mm of separation between the wires, about half of the "snaps" heard when the arc discharged weren't producing arcs between the wires. I noticed that arcs were occurring down on the PCB, between part of the high voltage part and the low voltage oscillator circuit. These arcs occur between two metallic PCB traces. The arcs are either going over the air on the other side of the PCB or through the PCB material itself. I was able to get a (blurry) picture of this occurring, in case you find it interesting:
The pale blue glow is a bridge of plasma forming between two PCB traces, one of which is at a high voltage. These arcs also discharge the blue capacitor.
Overall, I'm quite happy with this purchase. $2 for a high voltage converter like this is to me money well spent. Once I'm ready to potentially break it I want to try putting in more voltage than this thing is rated at (just adding more batteries basically) to see what the output looks like (this will require insulating the arcing PCB traces, with hot glue!). I also like that it's easy to take apart and I can tear it down without totally destroying it.
I recently made a post trashing a pair of absolutely terrible walkie-talkies, which were a good example of a horrible cheap Ebay electronic device. These lighters are a good example that some of the cheap, free-shipping Chinese electronics actually live up to their claims ... although I don't think most people care about mini arc devices.
Speaking of arcs, this sadly can't light paper on fire. It can obviously ignite gas stoves, but the capacitor-based arcs are too short lived to do anything but put tiny, tiny holes into paper. This interestingly means that the circuit isn't any sort of fire hazard, unless some HV devices.
Hopefully you found this interesting! Please tell me if you have any questions or comments.
Thanks for reading!
All images in this post are my own. You are welcome to use them with credit.