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I am using this circuit (please don't go by the design it is just a circuit block, this is not the whole circuit). I have connected a 12V DC at the center of the transformer and this 12V is also connected to a 555 timer IC. Here I am triggering the transistors alternatively and the transistor completes the circuit of the transformer. Thus transformer generates a square wave at the output.

The circuit is working fine but there is one problem i.e my 555 IC gets blow off in minutes when I switched ON my circuit. I thought it might be due to the Inductor spike at 12V line (as the only 12V line is the common line for both transformer and 555) so I have attached an electrolyte capacitor at the input of the 12V supply and my problem solves out.

But this generates a confusion in my mind that how the inductor can change the 12V line to another voltage.

My Input supply source --- 12V from SMPS. Maybe if I have used the battery for input supply line then I will never get to know this problem. (As battery is self-act as capacitor)

Correct me if I am wrong in any of the concepts. Please give your thoughts it will be very helpful to me in analyzing the inductor more closely.


When you flow current through an inductor you build up an electric field. When you suddenly stop flowing that current the field collapses which has the effect of trying to continue that current flow. The result of this is a voltage, that very well may be much higher than your 555 can stand.

  • \$\begingroup\$ Is that mean that higher voltage can also affect my input source!! And adding a capacitor at supply line is the correct way to do for such spikes?? \$\endgroup\$ – user152036 Oct 13 '17 at 4:14
  • \$\begingroup\$ Although that might help, a better solution would be to use diodes to the rails so that current has somewhere to go and protect the IC. Research 'diode snubber' circuits. \$\endgroup\$ – evildemonic Oct 13 '17 at 15:13
  • \$\begingroup\$ Can I place the two freewheeling diodes in parallel with both the winding. Diode Cathode at 12V and anode at another end(gnd by transistor ) \$\endgroup\$ – user152036 Oct 16 '17 at 5:22
  • \$\begingroup\$ @user152036 Yes, you need two freewheeling diodes (one per transistor) but should be connected in a way that cathode at collector and anode at GND (Place for each transistor). \$\endgroup\$ – Rohat Kılıç Oct 16 '17 at 7:35
  • \$\begingroup\$ But how in back voltage this circuit will discharge the inductor. In back voltage situation, the diode is in reverse biased. Correct me if I am taking it wrong!!! Maybe I did not get your answer. \$\endgroup\$ – user152036 Oct 16 '17 at 8:50

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