Transformer + battery = School Thrills

Today in class, me and my friends were fooling around with this 120VAC primary, to a center-tapped 12VAC secondary transformer. I pulled out a 9 volt battery, and connected it to the secondary winding. My friend was holding onto the primary. I quickly connect and disconnect the 9 volt battery, and his hands fly back in pain. Apparently, we got almost 70 volt spikes

The battery quickly became super hot, and began to emit smoke. My question is, when the transformer steps up voltage (step up transformer); does it lower the current to make up for the higher voltage, or is the current the same?

Wouldn't this be limited by the ampere-hours of the battery? I've even seen it arc, how is this possibly at 9 volts? (note the arcing wasn't on the primary (where it is stepped up)).

This reminds me of the old capacitor throwing in school :wink:. I was thinking about driving the transformer with a 555 timer and make a mini stun gun.

This actually began as a debate with my teacher; that didn't beleive we could give him a huge jolt from a 9volt battery. Let me tell you that some Electronics Technicians are very naive :wink:

Thanks

windozeuser said:

Today in class, me and my friends were fooling around with this 120VAC primary, to a center-tapped 12VAC secondary transformer. I pulled out a 9 volt battery, and connected it to the secondary winding. My friend was holding onto the primary. I quickly connect and disconnect the 9 volt battery, and his hands fly back in pain. Apparently, we got almost 90 volt spikes

The battery quickly became super hot, and began to emit smoke. My question is, when the transformer steps up voltage (step up transformer); does it lower the current to make up for the higher voltage, or is the current the same?

Wouldn't this be limited by the ampere-hours of the battery? I've even seen it arc, how is this possibly at 9 volts? (note the arcing wasn't on the primary (where it is stepped up)).

This reminds me of the old capacitor throwing in school :wink:. I was thinking about driving the transformer with a 555 timer and make a mini stun gun.

Thanks

Click to expand...

Transformers keep the power the same (for 100% efficiency) so yer increase in voltage requires a decrease in amps.

The fact that you did a step-connection satisfied the trasnformers changing flux criteria.

The current (on the primary side, but equally the secondary) will be limited by the resistance, the inductance (since you are putting a step in) but more importantly the batteries chemical ability

Yes, in order to step up voltage the current must decrease (see the Law of Conservation of Energy). It's also a much more efficient way to distribute power. So if you use a 1:10 transformer you'll get approximately 90 volts out, at a much lower current. You can do the math to figure out exactly what it is. The heating of the battery was due to the huge amount of current that was drawn by the transformer. Recall they are just windings of copper which have very low resistance, again use ohm's law to compute the currents you were dealing with.

You don't need a transformer to make a high voltage pulse.
Connect a relay for a moment to a 9V battery. When the connection is cut then a high voltage is developed across its coil. If a transistor was connecting the relay to the battery then the high voltage would break it, unless you had a reversed diode across the coil to arrest the spike.