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I was following a tutorial online and found the following schematic:enter image description here

Here, there seems to be a base-emitter (and base-collector) short hence bypassing the transistor completely. I still constructed the circuit and to my surprise it does oscillate (it induces aa 10mA current in a secondary coil placed nearby). Why is this?

Edit: I forgot to mention that the secondary coil also has a potential difference of 3 volts. This current remains as long as the circuit I'd connected to a power source

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  • \$\begingroup\$ Where did the circuit come from? \$\endgroup\$ – Andy aka Sep 4 '15 at 17:37
  • \$\begingroup\$ @Andyaka This YouTube video: youtube.com/watch?v=YLu16Yd-c20 It is a Hartley oscillator \$\endgroup\$ – DividedByZero Sep 4 '15 at 17:54
  • \$\begingroup\$ It's not a hartley oscillator if the base is shorted to the emitter. \$\endgroup\$ – Andy aka Sep 4 '15 at 21:59
  • \$\begingroup\$ @Andyaka, yes of course but it is meant to be one. This exactly is the problem- it's not a Hartley oscillator but is behaving like one! \$\endgroup\$ – DividedByZero Sep 4 '15 at 22:05
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It looks like something was erased in the area containing the short. Perhaps it was another resistor to form a DC biasing circuit with R1.

Are you sure your secondary coil isn't picking up 50 Hz or 60 Hz interference from the mains?

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  • \$\begingroup\$ No, It was a capacitor that was used for voltages higher than 10 V. I am operating this on a DC supply so interference isn't possible. \$\endgroup\$ – DividedByZero Sep 4 '15 at 18:29
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    \$\begingroup\$ Why would you replace a capacitor for a short???? It should an open... that C2 that used to be in the schematic goes does as you decrease the voltage and it seems from the 3-6V range you get rid of it and the circuit ends up using the parasitic capacitance instead of C2. \$\endgroup\$ – Kvegaoro Sep 4 '15 at 19:35
  • \$\begingroup\$ @RandomUser What is the frequency of the oscillation? Mains interference is radiated by everything around you that's AC-powered. You don't have to plug your circuit into the wall to pick up interference. Also, does the oscillation last a long time, or does it die out? \$\endgroup\$ – Adam Haun Sep 4 '15 at 19:42
  • \$\begingroup\$ I haven't been able to measure the frequency but considering that the current is almost 10 milliamps with a voltage of 3 volts it is certainly impossible that this is mains interference. The oscillation lasts a long time \$\endgroup\$ – DividedByZero Sep 4 '15 at 19:48
  • \$\begingroup\$ Interesting. I thought the turn-on transient might produce temporary oscillation in the LC tank circuit, but it seems that's not the case. I'm out of ideas. Sorry! \$\endgroup\$ – Adam Haun Sep 4 '15 at 19:53

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