Re:- ignition coil ebay item …………0043

Hi Neil… looks like an inductive relaxation oscillator to me.

Bob.

I haven't tried it but the circuit looks like a Hartley Oscillator:

The diode is said to be an ultra-fast type, either 1N4710 or UF4007. I suspect a 1N4007 would do. Not sure about the NPN transistor, said to be an N20 347Y, which is a new one on me. I suppose any NPN type with decent voltage ratings capable of switching 3-5 Amps would work.

Can anyone clever comment on the bias arrangements? As drawn the circuit looks to be putting a lot of DC current into the transistor's base.

Dave

PS Anyone else noticed the close relationship between an electronic oscillator and a steam engine.  The battery is the boiler.  The transistor's Collector and Emitter are the Piston and Cylinder.  The transistor's Base is the slide-valve. One end of the coil is the crank, while the other is the eccentric.  It drives the base 90° out of phase with the collector, causing the 'piston' to reciprocate.

Edited By SillyOldDuffer on 23/04/2017 10:23:15

As suspected…..

http://www.mouser.co.uk/Semiconductors/Discrete-Semiconductors/Transistors/MOSFET/_/N-ax1sf?keyword=N20&No=50

It a low level mosfet….gate volts max is 4.0v…….

Edited By John Rudd on 23/04/2017 11:12:08

Ah, ha – a MOSFET FQP19N20C , not an NPN transistor. That would explain the absence of magic smoke that's likely if the circuit is built as drawn by me with an NPN device.

To answer Neil's question about what to do with the 'deputy winding'. It should be left floating with the wire ends insulated. The windings purpose is to feedback the voltage needed to switch the MOSFET; therefore it's not needed if the primary is pulsed using points.

How efficient the coil would be operated by points is another question. We don't know the frequency of the oscillator but I'd guess tens of kHz. As such, the spark has energy pumped into it rapidly and very little is stored in the transformers core. This is in contrast to an old-fashioned car ignition with a hefty core in which the primary can store a lot of magnetic oomph. So I predict (from the comfort of my unreliable armchair) that the output produced from one of these transformers by a single input pulse will be pretty feeble.

For ignition purposes it may be better to build the oscillator and use the points to switch the oscillator on and off.

Dave

The diagram shows a "blocking oscillator". RCA used to publish an application note when early power mosfets appeared using a similar circuit to generate 400v to charge the cap of a capacitor discharge ignition system. I made one and it ran in my Triumph Herald for years very successfully.