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I was tinkering with old camera circuits and I am amazed to find these can generate 400 V with 1.5 V battery. So how can I make such a circuit to generate and store high voltage?

I searched about these circuits in internet but results are people trying to avoid inductor backfire rather than store it.

I have 400 V 100 uF capacitors and inductor wire but I don't know how to discharge the inductor fast enough to create high voltage.

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  • \$\begingroup\$ But high voltage will destroy transistor ? I have inductors but they have 160mH or less \$\endgroup\$
    – Mordecai
    Aug 14, 2019 at 21:52
  • \$\begingroup\$ Re, "high voltage will destroy transistor." Depends on the transistor. Depends on how high the voltage is that you want to generate. Some transistors can withstand a thousand Volts. Other transistors can only withstand a few Volts. \$\endgroup\$ Aug 14, 2019 at 21:58
  • \$\begingroup\$ I have some high voltage transistors but they are quite expensive and hard to find .So I prefer other options if any available \$\endgroup\$
    – Mordecai
    Aug 14, 2019 at 22:16
  • \$\begingroup\$ something to read: electronics.stackexchange.com/questions/282053/… \$\endgroup\$
    – user136077
    Aug 14, 2019 at 22:43
  • \$\begingroup\$ Thanks , I looked it but I don't understand the part he draw the circuit . He say "The battery voltage was 4,5 V and the 5 Henry coil had 150 Ohm internal resistace => the mawimum available lesson depth was only 30 milliamperes" but why the current raise to 1.8a when the button pushed ? I thought internal resistance of the inductor is constant also voltage raises to 9v .Why ? \$\endgroup\$
    – Mordecai
    Aug 14, 2019 at 23:19

2 Answers 2

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It's known as inductive kick or flyback rather than backfire. Use a transistor to switch. But having inductor wire (magnet wire?) doesn't mean you have an inductor.

By the way, a circuit where storing inductor flyback is fundamental to its operation is a boost converter. They're everywhere. There's also an aptly named flyback converter which is similar.

> But high voltage will destroy transistor ? I have inductors but they have 160mH or less – Mordecai

Sure, but it doesn't sound like your understand what inductive kick actually is. When current is interrupted in an inductance, the inductor extracts the energy stored in its magnetic field by collapsing it. It uses this energy to turn the inductor into a source which tries to maintain that current level through the inductor. It will produce a sufficiently high voltage to force that level of current to flow through any obstacles in the easiest path (lowest impedance path) it can find. If the easiest path available is difficult, then that voltage will need to be very high. If that path is easy, then that voltage doesn't need to be so high.

That means you can control the level of the inductive kick rises to. Present the inductor with an path that is easy enough so the voltage required to maintain its current isn't so high it will blow transistors, but difficult enough that the voltage spike produced is as high as what you want. The simplest way is to just size your cap appropriately. Smaller capacitors will result in higher voltages since they will charge up faster for the same amount of charge (loosely current over time), thereby opposing the inductor's flyback voltage more causing the inductor's flyback voltage to increase to keep the current going.

Also, 160mH is quite a lot of inductance.

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  • \$\begingroup\$ Thanks for response , it really helped. So.if I understand right , inductor needs to be connected ground directly to collapse magnetic field and it will create high voltage ? But instead of transistors can I use relay or magnetic switch ?Also output voltage seems little unstable is there any way to get predictable values ? \$\endgroup\$
    – Mordecai
    Aug 14, 2019 at 22:11
  • \$\begingroup\$ An inductor doesn't need to be connected to anything to collapse the magnetic field. If you disconnect both ends of the inductor it will make an arc through the air between its terminals to keep that current going if the air is the easiest path. But if you keep one terminal connected to ground, then the voltage at the disconnected opposite terminal will be at a voltage relative to ground. \$\endgroup\$
    – DKNguyen
    Aug 14, 2019 at 22:21
  • \$\begingroup\$ You could use relays, but relays have their own issues such as welding and corrosion of the contacts. For a 1.5V source, a transistor is far and away your best option. Output voltage is tricky to figure out since it depends on a lot of things you can't measure. However, a relay could be a bit safer due to the inherent isolation between control and primary contacts if you are just starting out and don't really know what you're doing. But I don't think that's an issue if you're working off a 1.5V battery. There's only so much power behind a 1.5V battery that can harm you. \$\endgroup\$
    – DKNguyen
    Aug 14, 2019 at 22:22
  • \$\begingroup\$ So only one side of inductor connect to ground other side disconnected ? I won't use 1.5v , I'm thinking about 12-24 v power supply. Also can I store the voltage with diode and 400v capacitor ? \$\endgroup\$
    – Mordecai
    Aug 14, 2019 at 22:27
  • \$\begingroup\$ @Mordecai Yes, usually it is most convenient to leave one side of the inductor connected to whatever voltage you want the inductor spike to be relative to. A larger inductor with more current flowing through has more energy in the magnetic field to draw from when it collapses so can kick with a higher voltage with enough power behind to sustain high levels of current. This makes it hazardous (as opposed to a static shock which is high voltage but not enough power to sustain any real current). If your power supply has enough V and I to provide these conditions for a large inductor then caution \$\endgroup\$
    – DKNguyen
    Aug 14, 2019 at 22:31
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Search on "photo flash circuit", "photo flash capacitor charge", and variations.

And do be careful -- while I'm fairly certain that a typical small-camera photoflash circuit won't hurt you, at some point you'll pack enough energy into a capacitor that getting that voltage across your heart will do you serious harm, or kill you. I recommend that get into electronics with low power, low-voltage circuits first; once you're comfortable and understand the basics (including your role as a potential circuit element, and what it means to the continued functioning of your heart) then you can "step up" (as it were) to higher voltages.

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  • \$\begingroup\$ Actually I did look these circuits but I didn't manage the copy inverter and trigger parts.Also I know one high voltage pulse enough to stop heart , this why I'm using insulated gloves don't worry ^_^. I think I got what I asked for , I will ask my another question when I draw schematics .Thanks for everyone to helping \$\endgroup\$
    – Mordecai
    Aug 15, 2019 at 1:22

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