Can anyone explain the given circuit. I seem to have done it and its perfectly working. But can someone explain the principle/theory and working behind this circuit from when the switch is turned on.. And I also didn't connect the LED

If I bring a fluroscent light near it.. It glows

Pls explain that phenomena also..

enter image description here

  • 2
    \$\begingroup\$ This same circuit came up a day or 2 back, and the concensus was that there's a parasitic capacitance inherent in the 275 turn winding that allowed it to resonate. See also electronics.stackexchange.com/questions/209109/… and electronics.stackexchange.com/questions/145424/… \$\endgroup\$
    – user16324
    Jan 5, 2016 at 18:48
  • \$\begingroup\$ Yes, it's a Tesla Coil: a cylindrical quarter-wave resonator driven inductively by pulses, and high-voltage obtained via "resonant rise." For better operation you should connect your -9V terminal to an Earth connection, or to a wide piece of metal foil. Also try inserting an LED, or at least a diode to replace it, to provide a full AC path. Actually the 'ideal' Tesla coil (as Tesla himself pointed out) is a single resonator driven through the ground terminal, and not a transformer, see #2 and #3 in Tesla 1892 lantern slide: amasci.com/graphics/tes_coilsld.gif \$\endgroup\$
    – wbeaty
    Nov 26, 2016 at 8:50

2 Answers 2


That is not a Tesla Coil. It is a high frequency oscillator. A Tesla Coil depends on resonance, which means a tuned circuit with a capacitor and an inductor. In the case of a Tesla Coil, it is two matched LC circuits (one with the primary capacitor and primary coil, and one with the topload and secondary coil). The above circuit just oscillates at a very high frequency based on feedback from the secondary coil (the one with 275 turns). When the switch is thrown the transistor turns on due to the current flowing into the base through the 22K resistor. Current also flows through the primary coil (the one with 3 turns), through the transistor, and back to ground. When you get a "burst" of current in the primary coil, it induces current in the secondary coil. The lower end of the secondary coil (assuming both coils have the same priority) provides a signal that turns the transistor off. When the coils discharge the cycle repeats (the transistor is turned back on again and another "pulse" is sent through the primary, inducing current, etc). This oscillates at a very high frequency which creates an alternating current output on the secondary coil. With a turns ratio of 3:275, a 9v input would give you about 825v (roughly speaking) on the output, based on normal transformer operation.;thank you

  • \$\begingroup\$ Thank you very much.... But without changing Magnetic field how AC is produced and induction occurs? The source is DC battery... \$\endgroup\$ Jan 5, 2016 at 18:50
  • \$\begingroup\$ And also if a bring a fluroscent light bulb near it ... It glows pls explain that also \$\endgroup\$ Jan 5, 2016 at 18:56
  • \$\begingroup\$ Please don't edit the title of your post, especially when someone references it in their response. Anyway, to answer your question, when the transistor turns on the inductor (the primary coil) "charges", and when the transistor turns off the inductor "discharges". This charge/discharge generates an alternating current (see this image for more details: hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/indt.gif) \$\endgroup\$
    – DerStrom8
    Jan 5, 2016 at 19:14
  • \$\begingroup\$ Any high frequency electromagnetic waves will cause a fluorescent bulb to glow. Bring it near a poorly shielded power supply and it will probably glow then as well. The electromagnetic waves excite the gases in the tube, causing them to emit photons (light) \$\endgroup\$
    – DerStrom8
    Jan 5, 2016 at 19:17
  • \$\begingroup\$ Why the downvote? \$\endgroup\$
    – DerStrom8
    Nov 29, 2016 at 3:31

I think before the first cycle there is no current in primary so no flux in medium but during first cycle when transistor get on current flows through it and produce magnetic flux in medium which get induced in secondary coil then transistor get of first cycle complete. On / offf action of transistor produces conversation DC to AC


Not the answer you're looking for? Browse other questions tagged or ask your own question.