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Introduction

First off, I'd like to let you know I am a beginner at electronics so some of the stuff I say might be very wrong.

What I want to make is a 'spark generator' for lighting some gases in a chemistry project.

I have been having a bit of fun just shorting a mini capacitor bank and seeing the spark it produces. Now I want to include a remote switch so I can discharge them from a distance.

I have decided to use a transistor controlled by an Arduino Uno (Arduino is controlled by nrf24l01 module).


Circuit

schematic

simulate this circuit – Schematic created using CircuitLab

Sorry if my circuit drawing is bad, here is a written description if you prefer:

  • capacitors are 35v (I only charge them to 12v) 1000uF and there are 5 in parallel with eachother
  • arduino I/O pin connects to a 500 ohm resistor, which connects to base pin of transistor
  • transistor collector pin is connected to capacitor+
  • transistor emitter pin is connected to capacitor-
  • the arduino ground is also connected to capacitor- (HAVE NOT TESTED!) -

I have made separate cables for charging them up. Essentially I have charged capacitors hooked up to a transistor switch which is controlled by an arduino.


Here is what I need help with

Previously to making this circuit I had tested it without arduino ground connected to capacitor-. It did not work which is what led me to attach arduino ground to capacitor-. I am concerned that shorting the capacitor bank will actually just flush it all into the arduino ground and fry it. I was tempted to try it but I came here for reassurance from someone more knowledgeable. Thanks

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  • \$\begingroup\$ Welcome!! I just wanted to point out that there is a circuit editor on this site that you can access as you compose your question. \$\endgroup\$ Oct 31, 2018 at 1:20
  • \$\begingroup\$ It seems like every beginner puts NPNs on the high side. Not sure why that is, but you'd need your transistor to be on the low side if you want anything to happen at all in this circuit. \$\endgroup\$
    – Hearth
    Oct 31, 2018 at 2:38
  • \$\begingroup\$ This won't generate a spark. You will probably need to use some type of step up-transformer connected to a spark gap to create a spark. \$\endgroup\$
    – user57037
    Oct 31, 2018 at 5:39
  • \$\begingroup\$ Probably an ignition coil and spark plug from a small engine. \$\endgroup\$
    – user57037
    Oct 31, 2018 at 5:42
  • \$\begingroup\$ @Felthry Hi, could you explain why it has to be on low side? I tried looking it up but I still don't understand. From my understanding if transistor is on the low side, it consumes less power? I am unsure if that is what happens, but if so how does that work? Just as a wild guess, would there be a voltage drop across where it sparks (I might try putting a thin piece of wire across instead) and so there is less voltage on the transistor? But if that were the case wouldn't it be the same as if i put transistor on high side since it all has to flow in the same path? Sorry if none of this is right \$\endgroup\$
    – Buretto
    Oct 31, 2018 at 21:16

2 Answers 2

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I composed this answer before you posted your schematic. In what you drew the transistor will remain in the active mode and will not behave as a switch. This is what I thought you were describing:

schematic

simulate this circuit – Schematic created using CircuitLab

Unless the transistor blows up (which is very likely) the Arduino would be safe. On the worst case scenario you would be pumping around 20mA into the protection diodes of the Arduino micro controller, as the current will be limited by your resistor. Not advisable, but not too bad.

But this idea will not work. The reason you get the spark when you short the terminals by hand is because you are placing a very small impedance very quickly on a device that basically has no current limitation. This vaporizes the tiny region of metal that initiates contact which vaporizes and initiates the plasma that you can see as a spark as the rest of charge dissipates through it.

If you manage to get the transistor to switch quickly enough (unlikely, as several parasitics come into play that will reduce the rate of discharge) what you will get is all of this current heating up the transistor, possibly to the point of failure.

With this idea the only way I can see to generate a spark would be with a very thin wire (basically a low-value fuse) in series with your capacitor and transistor. The fuse will blow (if the transistor doesn't), just like the metal does on the point of contact.

To be able to get a spark across a fixed gap, you need to generate high voltages. Not high currents.

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  • \$\begingroup\$ Thanks for the answer Edgar. If I understand correctly, the transistor is kinda using up some of the energy and that's why it is heating up? Would I relay be a better choice? I also was thinking to try using thin wire to act as my igniter so I am glad you also mentioned it! \$\endgroup\$
    – Buretto
    Oct 31, 2018 at 1:57
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    \$\begingroup\$ @Bureto nope, a relay will be a much worse choice since it will produce the spark in it, which may lead to the relay contacts fused together. \$\endgroup\$
    – sx107
    Oct 31, 2018 at 3:51
  • \$\begingroup\$ This will also not produce a spark. In order to get a spark in air, you need to generate an electric field strength greater than the breakdown value for air. Generally speaking, this means you need to physically open a circuit that has current flowing in it, OR you need to apply a high voltage to the primary of a step-up transformer or auto-transformer. If you use a transistor to open a circuit, but there is no airgap, you will just have avalanche conduction (and possibly destruction) of your transistor. Read about automotive ignition. \$\endgroup\$
    – user57037
    Nov 3, 2018 at 5:09
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Just shorting the capacitors isn't the best solution for the spark generation; it is usually done by using a boost converter or some other high-voltage generator, like the marx generator. Look up the electric lighters schematics.

However, regarding the exact question you've asked: almost all of the current will flow through the transistor, arduino shouldn't care much. But I would highly recommend you to add an optocoupler between the transistor base and the arduino output pin.

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  • \$\begingroup\$ Hi sx, thanks for the info. In regards to high voltage generators, (eg. the electric lighter you talk about) would they ever be lethal to touch? I ordered this thing and I am waiting for my 18650 batteries to arrive. Just curious since I don't want to work with something dangerious I'm not familiar with. Also if you don't mind, what is the optocoupler for? I may just give it a go, and if it doesn't work I'll try the other thing. Thanks for the help :) \$\endgroup\$
    – Buretto
    Oct 31, 2018 at 2:07
  • \$\begingroup\$ That could give you a "surprise" but it's not likely to be fatal, it's not as powerful as a stun gun module like aliexpress.com/item/zap/1244823231.html \$\endgroup\$ Oct 31, 2018 at 2:39
  • \$\begingroup\$ @Bureto the optocoupler decouples the pin from the transistor base, so in case of a transistor failure no current will flow into the pin. Besides, there is always a capacitance coupling opportunity, so without the optocoupler the rapid spark voltage could make the pin voltage ripple, maybe a bit, maybe a lot, maybe it will kill the uC onboard the Arduino pcb. \$\endgroup\$
    – sx107
    Oct 31, 2018 at 3:43
  • \$\begingroup\$ @Bureto regarding the dangerousness of the lighters: there are safe circuits and there are dangerous ones. It depends on the output current limit; the Marx generator, for example, can easily kill you, while a more or less common 5v to 500v dc-dc converter most probably won't since the voltage will just drop and all what you will fill is a slight "ouch". Still, a car battery power inverter is basically a converter from 12v to 220v with high current output which is, as you may know, deadly, since it is pretty much the same as touching live wall power wire. \$\endgroup\$
    – sx107
    Oct 31, 2018 at 3:48
  • \$\begingroup\$ @Bureto in short: look specifically for a spark generator module, those are pretty safe, however I would not advice to put it on your tongue or any other parts of you body on purpose. Because why? \$\endgroup\$
    – sx107
    Oct 31, 2018 at 3:49

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