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On 1/10/16 11:08 PM, tesla, Extreme Electronics wrote:
The diode has two functions.
1. To allow the inductive kick back from the coils to have a high
impedance ( like it would when the contacts open in a car ignition
circuit) this ensures the field collapses quickly generating the high
voltage.
2. To prevent the inductive kick (~400v -ve pulse) back flowing
back in to the 30v rated transistors releasing the magic smoke.
The kick, though, will be a positive voltage, and will reverse bias the
diode, so unless it's acting as a Zener, it won't protect the 3055s,
which are typically a 60V or 100V part.
I do have a problem with that circuit, a 1n4007 is a 1A diode. Most
ignition coils saturate at 5-10A, so either the on time is so short as
to limit the current to 1A (with reduced output) or the diode is
severely underrated.
OK, this circuit has a lot of problems, and it kind of looks like
something someone copied from somewhere else,but changed the parts
numbers without an appreciation for what is really going on.
Not that someone didn't actually build an electronic points
replacement/ignition using 3055s (these were popular projects in the 60s).
The issue isn't actually with the 1A rating: the conduction time is very
short in this sort of circuit, so the diode probably won't blow up from
that, nor from overvoltage (4007 is a 1000V part).
The real issue is that the BVceo (breakdown voltage of the transistor,
with it turned off) is only 60-100V, and typically ignition coils
produce about 400V when the current is turned off. Transistors *made*
for this application have well defined avalanche characteristics, and
typically have a BVceo of 350-400V.
Now, it is possible that a 2N3055 you get today, from some
manufacturers, might actually work ok. The data sheet says the "minimum"
BVceo, not what it actually is. The other thing is that breakdown in a
transistor doesn't kill it: it's all about the energy dissipation in the
die. With 4 transistors in parallel, that helps spread that energy
around, and the 3055 has a pretty big die. Of course, if the
transistors aren't well matched, the behavior can get interesting.
The other thing is that 3055s are a big slow device: they're not exactly
cutting off in nanoseconds. Since the di/dt isn't "infinite" like it is
with a switch, the inductive kick will be less, and might stay within
the transistor ratings. You'll note there's no capacitor across the
transistors (like there would be in a Kettering points ignition) - that
capacitor limits the di/dt, so the voltage across the points is limited
(and the points don't erode from arcing instantly).
The coil characteristics also have an effect. The specific L and R (and
the load) will affect the amount of kick you see (and therefore, the
transistor life).
Finally, the 2N3055 is a really low gain device. A hFE (large signal
current gain) of 5-10 isn't unusual at 5A collector current. You really
want a darlington or something or your pulse generator is going to have
to put out a lot of current, to get the transistor to turn on.
The upshot is that this is a circuit that "probably works", *if* you
happen to have the right lot of parts from the right manufacturer.
The Velleman kit uses a TIP162 (and a capacitor) with a couple zeners to
limit the collector/base voltage (another technique to clamp the voltage..
The TIP162 is a part designed for ignitions.. BVceo of 380V and a hFE of
200.
Derek
On 10/01/16 23:09, Ed wrote:
Answer is yes, at least in my experience, and a lot of current flows
through the diode. I forget the rules about attachments but I have a
file on the subject of ignition coils and ignition coil drivers
including schematics and pix and will send if big brother allows.
On 1/10/2016 1:49 PM, Reverend Fuzzy wrote: