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inductive kickback :(

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solidacid

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this is my first post on this forum so i hope im posting the right place.

i have build This and it works great, for a while...
when i use it the spark keeps getting shorter and shorter.

i think the 555 or the transistor is getting fried by the inductive kickback from the ignition coil. if i touch the transistor while it's on i can see and feel very small sparks.

if i change the 555 it goes back to full spark lenght, then the spark gets shorter and shorter.

can anyone tell me what i can do about this problem?
it's getting abit expensive with all those 555's i have to buy
 
There are diodes specifically designed for this that are called freewheel diodes that are used in alot of motor control electronics.
 
I've scoped a setup like that before and with a nice ignition coil was getting 200V kickback when I had the ignition coil saturated. Try reducing duty cycle on the waveform and use a high voltage FET instead of the BJT.
 
As mentioned above, use a higher voltage device than the 2N3055. Also, while a diode will save the 555, it will also limit the max spark you get out of the ignition coil. To protect the 555, you would add a supply decoupling circuit like this:
 

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solidacid said:
this is my first post on this forum so i hope im posting the right place.

i have build This and it works great, for a while...
when i use it the spark keeps getting shorter and shorter.

i think the 555 or the transistor is getting fried by the inductive kickback from the ignition coil. if i touch the transistor while it's on i can see and feel very small sparks.

if i change the 555 it goes back to full spark lenght, then the spark gets shorter and shorter.

can anyone tell me what i can do about this problem?
it's getting abit expensive with all those 555's i have to buy

hi,
If its a regular vehicle ignition coil, its usual to use a series ballast resistor.
I think you are expecting too much from the 555 driving via the 100R into a 2n3055.

The gain of a 2n3055 at about 4 to 5 amps could be as low as 15 to 20 at best.
So, say 5amps/20 = 250mA! to saturate the 2n3055, IIRC the 555 cannot source that level of current.

This means the 2n3055 will NOT saturate and could heat up and it ability to drive decreased, thats why the spark
could be diminishing, its cools off when you change the 555???

Connect a intermediate transistor between the 555 and the 2n3055 or use a power darlington transistor or use MOSFET.

For a standard ignition coil, they have a 2uF capacitor across the contacts, [ the 2n3055 in your circuit]
 
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Schotkky diodes make very good freewheeling diodes because of their uber fast switching speed.
 
Use a MOSFET and one with a high enough avalanche and voltage rating so it won't be destroyed by the back-EMF - no diode required. You also get the added advantages of a faster switching speed, not having to worry about the base drive current and the and a lower saturation voltage.
 
Most automotive ignition circuits use a capacitor (0.1 uF?) across the switch. Although you would think this might limit the voltage, in fact the right value resonates with the inductor and improves the energy transfer.
 
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