newbie question about transformer

[Jed841]

ok i want to build this circuit http://kipkay.com/wp-content/uploads/2010/12/snip-burn-galley_Page_088.jpg  BUT my transformer has an extra middle leg...... what is that for!? also when i hook  9v up to it, i get no voltage out the other side.......

[Mechatrommer]

9v what? dc or ac?

[williefleete]

its most probably a center tapping, if you have a multimeter test that pin to any of the others using the ohms range if it doesnt measure then its probably just for support when its on a PCB and open circuit

[osmosis321]

Transformers need AC to work, a battery won't do the trick.  It's the dv/dt, or the change in voltage, that transfers the power.  That's why DC won't work, it's a steady voltage.

The middle wire is called the center tap.  A center-tapped transformer is capable of push-pull operation and is commonly used in audio amplifiers.

[Jed841]

ahhh i was using dc.... THNX!!!

[jimmc]

NOOOO!!!

9v DC is the correct supply, with an AC supply the electrolytic capacitor will blow up!

The circuit works by charging the capacitor to 9v and then discharging it through the transformer when the button is pressed.
This produces the required dV/dt (or more correctly dI/dt) and gives a short high-voltage spike across the secondary of the transformer.

Jim

[Mechatrommer]

also when i hook  9v up to it, i get no voltage out the other side.......

NOOOO!!!

hook to "it" what? the transformer alone? or the complete circuit? whats the cap V rating? will be much fun to see an explosion!
merry christmas!

[Time]

This is pulse transformer basically.  Sounds like you answered your own question.  Charge the Cap to 9V and than dump the charge into the transformer getting a higher voltage pulse on the other side.  The relay is your switch.  Kind of rudimentary but it works.  To get the highest voltage possible youll want to take the ouput across the outside legs of the 2ndary.  It might be a fun learning experience, if you have a scope, to probe between the different legs and see what comes out.

  

[Fraser]

I think the poster needed to explain what the circuit actually is.....

Looking at the provided link, this appears to be a joke that you play on friends (?) A container with a metal lid has the unit built inside. The circuit is triggered by movement using a tilt switch. I suspect that this joke relies upon someone picking the container up and the movement causes the charge/discharge cycle across the transformer and shocks the person using the high voltage winding.
The deployment of such a device would require the use of a container with a conductive body and a separeate conductive lid insulated from the rest of the container. To work, the 'victim' would have to be touching both the container and it's lid.

[Mechatrommer]

T work, teh 'victim' would have to be touching both the container and it's lid.

and i suggest the poster will be the victim, for testing purpose.

[Zero999]

The capacitor will charge in the reverse direction even if DC power is used. The transformer is inductive and when the capacitor is connected to it, the capacitor will discharge but the current will keep flowing causing the voltage across the capacitor to go negative before discharging again, the osculation will exponentially decay. This is a classic LC tank circuit.