555 timer help

• posted

I am new to this newsgroup, I am typically a poster on rec. pyro or metalworking, but I have been working on a new project. I am trying to make an ignition coil for my new turbing engine, and I am quite terrible at reading electronic designs. I know that I will be needing a 555 timer, and an ignition coil. The input into the coil should be

12 volts. I have built a neef type vibrator switch, which made quite a nice spark when used as a relay to put the 12 volts into the ignition coil. But it was very unreliable, and I believe that a 555 timer should be the best route to go, but I am quite new to electronics and have become lost with all the complicated 555 timer circuits shown online. I just need something simple that will trigger a pulse of 12 volts to the ignition coil.

Can aybody help me out?

Thank you, Bob

• posted

Yes, try sci.electronics.basics.

• posted

Probably not. The 555 can't drive your ignition coil directly and survive - if you are using car ignition coil.

The 555 could apply 12V to the primary of your ignition coil and allow the current through the coil to build up to about 200mA or so, but you need to be able sink something closer to one ampere.

Once you've let the current through the ignition coil build up to an adequate level, you've got to turn off the current through the coil to generate your spark. The energy stored in the inductance of the primary coil forces the voltage across the coil up to around 200 to

300V which would blow up the output of a 555. (which can't take more than 18V).

The voltage across the secondary winding of your coil goes up to around 25kV (depending on the step-up ratio in your ignition coil and the self capacitance that has to be charged) which is what produces the spark over a suitable spark-gap.

You could use the 555 to drive a suitable high voltage, high current switching transistor, which could sink enough current and survive the voltage spike that comes with the spark, but that makes the circuit even more complicated.

You'd be much better off to buy a capacitor discharge transistor ignition unit from an auto parts store. It will not only work straight out of the box, but should also keep on working, and it will be cheaper than anything you can build yourself.

-- Bill Sloman, Nijmegen

• posted

Well, You need some kind of timing with the engine don't you?

In any case, I don't know what kind of coil you are working with?

for the moment, I'll assume you have a conventional type with a

2 post terminal for the + & - with a main high voltage lead?

The + lead is usually always connected to your + side of your battery. the (-) is connected to the trigger from the ignition circuit.. There are problems with these coils, when you connect the leads to the power source, lets say a 12 volt system here. The coil will start conducting to the point where it reaches max current of what the DC resistance is measured via an ohm meter. This is when the coil saturates and shouldn't be left in this state for long periods of time. How ever, when you remove the power from this, the coil will collapse with nothing holding it back. This basically is what you want which generates the high voltage spark you see. The stored energy is being release all at once instead of the time it took for the coil to saturate. So, what was done in the old days, you need something to capture the nice little excessive voltage that swings back on the post terminals also.. If you know you history about older ignition systems, they used condensers (capacitors).. a cap was connected across the points so when the points opened, the coil released it's energy slower on the input side so that arching wouldn't burn out the points.. Today, the priciples still apply.

Now I guess more information is needed here because you have options.

A single arc or a steady burst burn for a timed period ?

The 555 can be used as a one shot timer to give you a short pulse but I'm wondering if that is what you really want and not a steady burst of pulses for a short time ?

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