Woops, the standard automotive relay will draw about 85 mA and this may cause the voltage on the capacitor to not get to a high enough point to pull in the relay. Try reducing the resistor to maybe 50 ohms. If your relay draws more than 85 mA the resistor may have to be reduced further.

You say you are seeing only 1 volt after the resistor. If relay terminals 87 and 87a are swapped it will keep the voltage low and not allow the relay to switch. Relay terminals 85 and 30 connect together to the + on the large capacitor just as you stated. <--(Just some random thoughts.)

I suspect that the 100 ohm resistor to too big. Try reducing that first. As this resistor goes down in value the pulse rate gets faster. The large capacitor may have to be increased to compensate.

It is tough to think through all the details of even a simple circuit like this on the first try, but at least there was no smoke.

SoTexRattler, I believe there is not enough voltage on the cap to get the relay to pull in, but after it pulls in it will stick “ON” until the voltage on the cap falls 3 or 4 volts. You are correct in stating the resistor I chose was a bad guess.

On edit: make sure the small 0.1 uF cap is rated for 200-300 volts. If not it will fail and short out.

 

A nice 1:100 transformer will take care of your problems. step up your 240Vac to 24,000 VAC. then the other half will be complaining about the burnt critter at the fence...

 

Well, the absolute best rig I built for turning 12vdc into 3/4" long fat blue sparks was using a surplus 4 prong RADIO VIBRATOR from an old vacuum tube car radio and an Autolite ign. coil!!! That thing ROCKED!!! (think TASER!) WAY too much for what Mexstan needs.

Of course, that Radio Vibrator thing was originally designed as a chopper for driving a center tapped power transformer and was just ideal for driving that ign coil. Too bad those things are so darned hard to find now...
I adjusted the capacitor value across the coil for the best sparks and it made it run cooler and drew less current.

I did have quite a bit of fun with that little rig... I had the "hottest" VW beetle in the whole highschool parking lot

Yep, I figured that relay may be a bigger load than intended. Also the pickup voltage and the dropout voltage is WAY different causing the automotive relay to stay latched in until the big cap has lost most of its energy into the ign. coil leaving very little pent-up magnetic field to "let go" when the relay opens up.

Ideally, a relay for this ckt would have a very close pickup/dropout voltage.. Automotive power relays typically don't.

I suggested 'Stan use small lightbulbs since finding the right resistor(at the required higher wattage) may be harder than simply mixing/matching 12v lamps to get the right current.

(((Whoops! I must have hit send before I was ready...
It scrunched all my letters together and tookout all the returns... )))

I guess I'm finished after all...
K.

 

Thanks guys for the interest and replies so far.
I got suspicious of the relay I was using and removed it entirely for checking. It is brand new (never used) and left over from something else I did some time ago. So I brought out the big guns; my 40 amp power supply and hooked that to terminals 86 and 85. I also connected my VOM to terminals 87 and 87a. Not sure exactly what terminal 30 does. I figured that if 40 amps does not energize that teeny coil that there had to be something wrong.
Guess what? I apparently have a dead short between terminals 85 and 86 because the breaker on the power supply immediately popped. So have come to the conclusion that either I have a bad relay or the schematic the bent valves gave me is wrong. Can anybody confirm the relay schematic? I am still puzzled why in the schematic it appears that 85 and 30 are connected together, but have separate terminals.
Will go out in the next day or two and buy a new relay - if I can find any shops open. It is a long weekend here and many places are mandated closed until Wednesday due to the oink-oink flu.
Will let you know what happens after I obtain another relay.

On edit: Found the relay schematic on the internet, so now understand terminal 30. Check out: http://www.installdr.com/TechDocs/999404.pdf

 

Sorry about my confusing relay schematic. Looking at the link you posted on automotive relays, it shows the coil of the relay, terminals 85 as negative and 86 as positive. This is opposite from my schematic and if the relay contains a suppression diode across the coil it will draw a large current. Most relays don’t care which polarity the coil receives but with a diode it is important to have 85 at ground.

Good luck and stay healthy.

 

Mexstan, I don't smell any 'ozone' yet... how goes it?

 

With so many shops shut down I have not yet been able to buy a new relay. Monday for sure I will have one.

General question:Why can't I use a normal auto cap used for ignition points (anybody remember them?) in lieu of the 0.1 uF cap?

 

There's no reason not to... It has the required capacitance AND the voltage rating for the job.

 

OK guys, here is the latest. Success - sort of. I finally managed to get a new relay. Everything now works, but only for 10 or 20 cycles and then it quits. When it stops working there is full voltage at the ignition coil. This implies that the relay sticks closed. Any ideas what could be holding it in? When I disconnect the 12 v for a short while, everything works for either two cycles or maybe 10 or so cycles before it quits again. I can hear and feel the relay cycling when it works.
I did notice that my 1 watt 100 ohm resistor was getting quite hot, but not smoking, so switched it for a 5 watt. No difference.
The large cap is only a 50 volt. Should that voltage be increased?

How can something so simple create so many problems?

 

Mexstan, could one of us ship you a factory unit to your house?

 

Hey tool, thank you for your very kind offer. The problem is the shipping. I can order a unit and have it shipped, but the shipping costs and occasional duty (up to 30%) is the killer and can often easily double the initial cost. Next time I am north of the border will buy a unit and bring it back with me.
Was not planning on heading north again as I only returned last month, but something has come up and I could be going up again in July.
Now if you want to bring a new unit down to me ............
Thanks again for the offer.

 

The 50 volt cap is just fine but having full voltage at the ignition coil tells me that the coil has failed. That is common for an old coil. It heats up a little and the primary winding opens.

When it stops do you measure about 8 volts on the coil primary? If so look around for a good coil.

 

Well I did always want to see Mexico....

 

I had a critter problem once. I plugged the end of my electric fence wire into the house 110v current. I cut a second piece of wire and stuck it into the ground and stuck the other end into the house outlet. It worked great and if you wear rubber soled shoes you can touch the fence and not get bit.

 

Im thinking a 12 gauge and a case of cerveza would be quicker and easier...