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Registered Member #3451
Joined: Sun Nov 28 2010, 11:13PM
Location: United States
Posts: 100
I recently bought a conventional iron-core transformer from Radio Shack so I could do some experiments. I was just playing around with a 9 volt battery, and I noticed that when I connected it to either one of the coils, some pretty decent sparks were made. I touched it while connected, and to my surprise, I received a small shock. The transformer's primary coil is designed for 120V, and the secondary for 6V. I don't quite know why this happened, but I'm guessing that the coil self-induced a larger current in itself.
Do you think this is the case? Note: Sparks were made on the connected coil itself, not the other one.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
What's happening there is the inductance of the primary or secondary are connected to a voltage, the inductance gives a significant spike of power when you disconnect it, because the magnetic field created by the battery collapses back into the wire, albeit all at once making a spark! Esentially, that's what flybacks and ignition coils are, except they use silicon to switch them off and on at very fast speeds, some like 20 khz, or 20,000 times a second.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
e = L (dI/dt) always applies for an ideal inductor but more appropriate as (dI/dt) = e / L in this case.
If you use a 1.5v alkaline battery (more current than a 9v) across the 6v winding the 120v winding can produce a high enough voltage to produce little sparks if you put a capacitor (e.g. 1uF, a few hundred volts) across the 6v winding or switch then you will probably get an even higher voltage from the 120v winding as the capacitor prevents the energy stored in the primary (E = 1/2 x L x I x I) 'jumping' across the switch contacts when opened.
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Tranformer Self-Inductance?
