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Circuit Explanation Please

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I'mClueless

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Hi,

The illustrated circuit is using (3) 2N3055 switching transistors. I would like to know if that is due to the inability of the transistors to switch the current or is it to triple the transistors rated current? Is there a better way to switch the current?

**broken link removed**

Yep,
I'mClueless
 
No, it is because of the way you are driving the bases of the power transistors. The Hfe of the crappy 3055 is low, so it will take a good fraction of an Amp to drive three of them.

All the things wrong with this circuit:

Insufficient base drive.
No equalizing resistors in the emitters of the three power transistors.
Who says you need three power transistors?
No snubber diode across the coil.
No bypass capacitor at the coil positive terminal.
Useless 5V regulator (not needed, the 555 will run better on 12V)
Useless pot between the 555 and 3055's
Totally wrong way of installing the pot, anyway.
Might need ballast resistor in series with the coil.
What are the 1N914s supposed to do?
How big is the heatsink for the 3055s?
 
No snubber diode across the coil.

You don't want that in an ignition coil circuit. It allows the built-up current to leak away ( at turn-off ) and makes the spark weak, or kills it completely.
 
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You don't want that in an ignition coil circuit. It allows the built-up current to leak away and makes the spart weak, or kills it completely.

Then he sure as hell ain't gonna do it with 3055s! With a point-type ignition, there is a capacitor across the points which resonates the primary coil. The voltage swings to about +-250V, clamped/damped only by the reflected impedance of the spark gap in the plug. With a 3055 (actually any NPN transistor) hanging across the primary, the collector junction will be forward biased when the coil primary swings below ground, and the voltage will be clamped. This also kills the HV output. He will need an NPN with a much higher breakdown voltage than a 3055 for the positive swing, too!
 
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Actually, a 3055 works pretty well as an experimental device. Mine produced about 30kv for short periods of time. No capacitor is required in when using a transistor switch, because the switch cuts off the current sufficiently fast. I've measured 400V spikes on the primary, but the transistor seems to tolerate fast spikes even as they are well over the breakdown voltage. But using a higher rated transistor would be wise.

But, the 555 does not well tolerate spikes on the VCC rail, so supperssion should be used between the chip and the coil.

PS: Any regulated power supply will fry if powering this curcuit. Ask me how I know this!
 
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If you are going to use a 3055 in this circuit you should use a zener across the transistor collector to emitter, with a voltage rating slightly below the transistor voltage rating, to protect it from the spikes. The 3055 may tolerate spikes above its rating by breaking down in avalanche but it's also a good way to create a packaged short-circuit.
 
BrownOut,

So in your opinion is the circuit worth taking the time to build?

Currently I'm using an electronic igintion module from a V8 automobile engine. I mounted the magnetic impulse rotor on a DC motor shaft and vary the power to the motor to control the frequency. I want to replace the makeshift arrangement with an electronic driver.

I'm not experienced in electronics and at my age there's not enough time for me to become a electronical wizard like you guys, however I can copy circuits and solder the connections.

Yep,
I'mClueless
 
I guess the answer depends on the reason you want to build it. Is this just for cool effects and experimentation? If so, you can easily build one, although I would not build the one you posted. You need only a single driver mounted on an appropriate heat sinc, and probably a high gain transistor to provide sufficient base current, or else use a FET. Keep in mind these circuits are self-destructive if you neglect to use components to suppress the spilkes that appear on the supply rails. There are some better schematics available if you search. If I come across any, I'll post them.
 
BrownOut,

I'm using the ignition coil driver circuit for a high voltage source for a portable gas engine powered TIG welder. If you know of a better circuit then I would certainly appreciate it if you could point it out for me.

Yep,
I'mClueless
 
Why on earth would you need high voltage for TIG welding? Have you seen **broken link removed**
 
BrownOut,

I'm using the ignition coil driver circuit for a high voltage source for a portable gas engine powered TIG welder. If you know of a better circuit then I would certainly appreciate it if you could point it out for me.

Yep,
I'mClueless

From my for what it worth department. Some years back I built a circuit using an old automotive ignition coil like you have pictured. I went to Lowes Home Improvements and bought a cheap 300 watt incandescent lamp dimmer. I got the idea from an old Popular Electronics article where I noticed their circuit to drive the coil was about identical to a lamp dimmer. I replaced the lamp with the coil and it worked just fine. Then I placed aluminum foil like a bonnet on a clear large incandescent bulb and applied the coil secondary to it. I grounded the lamps (bulbs) filament leads. I photographed the little lightening storm inside the clear glass bulb.

Anyway, if you just want to drive the coil that would work. However, thinking back to your previous post and RF arc starting what I suggested would be limited to 60 Hz on the coil.

Ron
 
This circuit should work better than the one in the first post. I think it would be worth the time to build.
 

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So, you gonna build it or what? I neglected to give credit to the circuit's creator, so please see **broken link removed** Chemelec is a member here, or at least was.
 
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