Ignition Coil

After reading a therad which mentioned ignition coils I bought two as an experiment.

This one:

www.ebay.co.uk/itm/231829567632

When wired up to a DC supply produced a stream of sparks between the white wires, at about 8-10 per second off two AG13s and probably 40-50 run off a Lion battery. I set the gap about 1mm.

It could work well as a 'trembler coil' substitute if lasts more than a few minutes.

I also bought one of these, but to be honest I don't quite understand the description or the circuit diagram...

www.ebay.co.uk/itm/331982490043

Neil

P.S. if nothing else, when powered by a Lion battery it works as a sort of plasma-cutter for paper...

Re:- ignition coil ebay item ............0043

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

Bob.

Both the same .....

Self oscillating inverter......just dont grab the bit that bites...!!

Edited By John Rudd on 22/04/2017 21:39:56

Thanks,

The second one, though, presumably I should be able to ignore the 'feedback coli' (and leave it open circuit) and wire it up as an ordinary ignition coil?

Neil

You could....and you could also use the power transistor to switch the coil via a Hall device. Just make sure that the transistor remains in the off state until a spark is needed....

Edited By John Rudd on 23/04/2017 09:02:37

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

If the transistor is a high current power device, then depending on its hfe will require a reasonable base current to drive it into saturation or else it will just heat up if not fully conducting.... ( a 2N3055 has a very low hfe as an example   it could be a mosfet?

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

The diagram on eBay shows a 150 ohm resistor to limit the current.

I will guess the one with the thicker wires is the 'primary' and the medium wires is the 'deputy winding'.

As I assume I don't want the 'deputy winding' how should I terminate its wires to prevent problems?

(Just to be clear, I want to connect the primary to a set of points and a suitable voltage and generate timed sparks, not have a free running oscillator (it's a four stroke engine, not a taser).

Neil

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

I did wonder about that, the othwr one will suit as a ready made 'trembler coil'.

Neil

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.

Good job you said John - I would have bet the farm on it being a Hartley and lost the lot! In my defence they do look rather similar.

Ta,

Dave