Unknown SCR thing for car ignition - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: April 26, 2022, 03:47:16 am
content: In Electronics Australia magazine some time in the mid 70's there was a circuit published that was meant to be put in series between the points and coil negative of a car ignition circuit. The intended purpose was that it was supposed to disconnect the points capacitor at some short time after the points opened, allegedly making for a better spark. I'm quite doubtful it actually worked as intended but I'd like to find the original article if anyone remembers it. It was not a full construction article, just a reader submission of maybe a single column or two with a schematic and short description.This thing is DEFINITELYnota "dwell extender" circuit that goes inparallelwith the points and effectively turns them back on shortly after they have opened. This one goes inseriesbetween the points and coil negative. Attached is the schematic as best I remember it. If anyone does recognise it and can point me to which issue of Electronics Australia it was in that would be great.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: April 26, 2022, 03:59:57 am
content: I vaguely remember that also. Hehe. Around the time of the fuel crisis there was so much snake oil.I've since wondered if that problem trying to be solved was being caused by not using the right coil (resistive/non resistive) and burning out the points.Dunno. Will be interesting to see what peeps come up with.

author: bob91343
author_rank: Super Contributor
timestamp: April 26, 2022, 04:03:24 am
content: I used to own a couple of Capacitive Discharge Ignition Systems. They utilized an SCR to discharge a capacitor into the spark plug rather than the inductive spark. It worked but I don't know its effectiveness. I had it on two or three cars.

author: CaptDon
author_rank: Super Contributor
timestamp: April 26, 2022, 01:57:47 pm
content: They used the stored energy in a capacitor of around 200-300vdc and fired that into the conventional ignition coil. They did not fire a capacitor into the spark plug.

author: bob91343
author_rank: Super Contributor
timestamp: April 26, 2022, 04:32:24 pm
content: You are correct. Sorry, memory issues, will be 90 this summer.

author: Circlotron
author_rank: Super Contributor
timestamp: August 20, 2022, 01:58:12 pm
content: I finally found it!September 1973 Electronics Australia page 73.I'm quite sure it doesn't do what is claimed, to disconnect the points capacitor some microseconds after the points open and consequently enable a higher voltage to be produced by the coil. SCRs are very reluctant to turn off.

author: CaptDon
author_rank: Super Contributor
timestamp: August 22, 2022, 02:05:09 pm
content: I always thought the points capacitor remaining in circuit was important because it sets up a resonance with ring voltage across the plug and actually 're-strikes' the spark several times at the lower trailing ring voltages and this is possible because flame in the cylinder creates a condition of the 're-strikes' requiring less voltage to strike. You can discover several ways to fatten the spark and gain a few horsepower by swapping different values of the points condensor as well as the radio condensor and looking at the effects with a SunScope. Even moving the points condensor over to the coil and out of the distributor can have a good effect on power available to sustain the spark. 'Indexing' the spark plugs can also add a few ponies at wide open throttle and even improve idle. Those old Mark-Ten C.D.I. aftermarket ignition units really threw a hot spark!!! If you didn't have good plug wires those Mark-Ten units were known to punch right through the rotor to the shaft and short out all sparking and leave you stalled on the roadside. Dual-point kettering can be really hot if you like to tinker and have a good ignition scope!!

author: mikerj
author_rank: Super Contributor
timestamp: August 22, 2022, 06:22:50 pm
content: Quote from: CaptDon on August 22, 2022, 02:05:09 pmModern high energy systems don't have a capacitor, and the coil still "rings" but at a higher frequency.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 23, 2022, 11:30:41 am
content: Congratulations on your longevity.

author: Circlotron
author_rank: Super Contributor
timestamp: August 23, 2022, 11:45:10 am
content: After thinking about it a bit, here is my analysis of that circuit. When the points open the SCR stays latched on so the coil is going to pull the points capacitor positive about 200-300V in a 90 deg of a sine wave. Then as the coil primary starts to go down again because the spark has occurred, the SCR is reverse biased so the points capacitor remain charged at several hundred volts. Eventually the points close again and shorts the capacitor with a splat!, reducing the life of the points and probably the capacitor too. What's more, in a well designed conventional system the spark occurs very close to the peak of the capacitor voltage and so the capacitor energy is transferred to the spark as the capacitor voltage heads toward ground. In this circuit that energy is lost. Same with electronic systems that have no capacitor. This energy, about 10% of the total, gets wasted in a voltage clamp instead of contributing to the initial current spike through the spark gap.