# A question about transformers

1. Dec 26, 2013

### Noirro

1. The problem statement, all variables and given/known data
Hey there! I have simple problem:
To start a spark in a car ignition system, voltages of 5-17kV are required. A transformer with winding ratio 1:100 is used with a 12V battery, so we get a 1200V voltage. How are higher voltages achieved in such systems?
Thanks

2. Dec 26, 2013

### Staff: Mentor

The transformer turns ratio gives the voltage ratio only for sinewaves, i.e., AC of appropriate frequency. The coil in a car is designed to operate differently. It is supplied with interrupted DC and the high voltage output is defined by the more general induction characteristic

$voltage = k. di/dt$

The faster the points interrupt the primary current, the greater is di/dt, hence a greater voltage pulse goes to the spark plug.

http://imageshack.us/scaled/landing/109/holly1756.gif [Broken]

Last edited by a moderator: May 6, 2017