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NE555 - Ignition Coil Drive

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Suraj143

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Guys I made an ignition coil driver similar to the attached document.

Can somebody tell me

1) whats the purpose of the diode that I highlighted?If I not use it what will happen?

I added a 7812 to power the NE-555. Without EARTH the system after a few seconds the regulator burns.

2)Do I need to EARTH the circuits ground terminal?
 

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The diode is mainly used as protection for the MOSFET. If you don't use it, there is the risk of having back-emf flowing the wrong way through the transistor, shortening its lifespan.

7812s can only source around 1.5 amps, if I remember correctly. Ignition coils are going to want to draw 5 amps or more, which could easily burn out your regulator. Also, how are you supplying the 7812?

And no, you should not need to earth the circuit ground.
 
I added a 7812 to power the NE-555. Without EARTH the system after a few seconds the regulator burns.

Why?... If the car has a 12v battery then the 555 will be fine... If not, I take it this circuit isn't intended as a car ignition module as there seems no way of deriving the frequency pulse from the engine...
 
I dont think the red-circled diode does anything, except waste power.

The inverse shunt diode does only a little because the diode intrinsic inside the NFET conducts the bulk of the current as the NFET turns off. The internal diode is Schottky (forward drop ~0.3V while the external one is Si. (forward drop ~0.8V).

The IRF640's 200Vds is marginal in this circuit.

The circuit could be helped by increasing the size of the 1uF Tant. cap to about a 47uF electrolytic shunted by a 1E4 or 1E5 ceramic.

I would also add a car battery as the power supply, or at least put a a whopping 10,000 to 100,000uF between the terminal labeled Bat. and the source of the NFET (very short wires, observe polarity).

I agree that this will require more than a 2A supply...
 
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I did that way back in my youth. maybe 12 YO. Used a 555 timer and a 2n3055 transistor. Forget what I used for a battery. FETs may not have existed yet.
 
Thank you friends.I understood.

Here is the circuit I'm using.I'm powering my circuit from a transformer.That is why I use a regulator.

I use this circuit for cattle fence.

I tuned the RC network to 1Hz.But after loading the circuit, the frequency increases to 10Hz something like that.I just use a 10uF decoupling cap.

I removed the 7812 & added two 1N4007 diode in series to drop some supply voltage to NE555.

Now nothing to burn.But the frequency varies when loading the coil.
 

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You never answered my question, how were you powering the regulator? What voltage does the transformer supply, and at what current? Again, ignition coils really need 5 amps or more. This means that your transformer must be able to supply 5 amps or higher, and your rectifier diodes will have to be rated for the high current as well.

Two 1N4007s in series is only going to drop about 1.4 volts.

If you had the ignition coil connected before the regulator, as in the schematic, then you should be fine. In that schematic, the regulator is only used to supply the 555, not to power the ignition coil. If your regulators keep burning, check your wiring and connections. Make sure you have the pinout correct. Also don't forget the caps on the input and output of the 7812. They're often needed to ensure the output is stable.

In order to change the frequency you should change R2 to a potentiometer. That, based on the value of C2, will give you a range of frequencies you can go through by adjusting the potentiometer. Use a higher value for C2 to decrease the frequencies in the range, or use a lower value for C2 to increase the frequencies in the range.
 
The two diodes near the Mosfet do not do anything:
1) The coil is pulled to ground by the Mosfet and produces a positive voltage spike when the Mosfet turns off. Then the diode in series with the Mosfet delivers the positive voltage spike to the Mosfet the same as a piece of wire.
2) The same for the diode parallel to the Mosfet. It does nothing when the voltage spike is positive and is never negative.

The old spark coil was driven from breaker points, not a Mosfet. Breaker points allowed the voltage spike at the primary to induce a very high voltage at the output of the spark coil.
 
You never answered my question, how were you powering the regulator? What voltage does the transformer supply, and at what current? Again, ignition coils really need 5 amps or more. This means that your transformer must be able to supply 5 amps or higher, and your rectifier diodes will have to be rated for the high current as well.

Two 1N4007s in series is only going to drop about 1.4 volts.

If you had the ignition coil connected before the regulator, as in the schematic, then you should be fine. In that schematic, the regulator is only used to supply the 555, not to power the ignition coil. If your regulators keep burning, check your wiring and connections. Make sure you have the pinout correct. Also don't forget the caps on the input and output of the 7812. They're often needed to ensure the output is stable.

In order to change the frequency you should change R2 to a potentiometer. That, based on the value of C2, will give you a range of frequencies you can go through by adjusting the potentiometer. Use a higher value for C2 to decrease the frequencies in the range, or use a lower value for C2 to increase the frequencies in the range.

Oops I missed that.My transformer is 2A/12V.Please look the diagram in post number "7".NE555 powered via 7812 regulator.High current will supply before the regulator.

From the last diagram schematic I get 1cm arcs.The problem is after loading the circuit the frequency increases.

I think due to high current drawn by the coil will drop the supply voltage to 7812 regulator.So it will cause the frequency to change.
 
Electric cattle fences use a small hv power supply to charge a capacitor then trigger a SCR to dump the voltage across the coil. This type of system draws very little power.
 
I think due to high current drawn by the coil will drop the supply voltage to 7812 regulator. So it will cause the frequency to change.
Correct. You can mostly overcome that by supplying the 7812 input via a diode and connecting C3 at the diode cathode. C3 then provides a reservoir for the 7812 while the coil draws current from the bridge rectifier. But drawing 5A from a 2A-rated supply is not good practice.
 
Hello,
have to say that it is a poor design.
Someone wrote he did it with a 2N3055. We had nothing else in de '80...
You can now find a FET fully protected and designed for a car/bike coil.
IRBG14C40L. (Conrad.UK £1.79...)
The Fet needs one resistor to be driven by logic levels. That's all.
Just made a TCI ignition for bikes with it. Tested on several coils.
Wrote eveything on a french bike forum (sorry : my mothertongue is french).
Don't know if I can write the link to what I wrote. Can a moderator tell me?
 
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