Dumped:
I did something today that I think is a little unusual.
I did not connect anything the the transistor base connection.
And the Exciter fired up, but still overheats on 12 volts. Runs ok
on 5 or 6 volts.
Slayer had mentioned that once the system starts that the base
resistor could be removed and it would still work, but until today
I saw that that can happen. All by accident. I didn't know it could
even start without the base connected, as sometimes I tap an Av
plug to the transistor control or emitter to get it going if it does want
to start.
But it started all by itself, several times, with nothing on the base.
I recall the Doc mentioning something about that also. Makes me
wonder if a just diode can be made to do the same thing...
It sounds like what you're saying is that you removed the Startup
Resistor which connects the Base to Vcc in order to provide a small
forward bias to get the transistor conducting in order to initiate
oscillation.
Some transistors do have sufficient internal "leakage" from Collector
to Base and will start oscillation without an external startup resistor.
These transistors are nowadays rather unusual but back in the days
of Germanium transistors this sort of leakage was common.
There is variation in transistor characteristics even within a specific
type; ie the PN2222. Each transistor within a production run will have
its own individual specs which will differ from its neighbors slightly.
This is why in certain experimenter applications we find that some
PN2222s will work while other PN2222s will not; but all are actually
good transistors when placed in standard circuits.
The hang-up I'm having is with the transistor heat...
Small transistor heating is the result of excessive current flow; either
in the pulse magnitude or pulse duration, or both.
When pushed hard transistor heating is unavoidable. In those cases
it is customary to place a "heat sink" on the transistor to help dissipate
the heat buildup. With small transistors this may be as simple as a
piece of copper or aluminum carefully glued to the transistor body.
In years past there were special "slip-on" metallic heat sinks into
which the body of the transistor could be inserted.
To minimize heating of the transistor in power applications, such as
your Tesla Tower driver; adjustments must be made to the biasing
arrangement to minimize pulse duration and limit pulse amplitude.
Once oscillations start it is possible to transition the operation of the
transistor into solid Class C efficiency with the Self Bias RC combination
in the Emitter connection to Gnd.
The Capacitor in the Emitter connection will provide a low impedance
current path for the short drive pulses. The Resistor alongside the
Capacitor will tend to limit prolonged current flow to a low and safe
level in the event the Base Drive pulses are excessively wide.
When carefully "tuned" the Self Bias circuit in the Emitter to Gnd
connection will tend to function as Differentiator where it presents
a low impedance to short pulses and a higher impedance to long
pulses.
This should enable increasing the applied DC Voltage to the transistor
without producing destructive heating.
Disconnecting the Startup Resistor once oscillations begin may be
helpful too in reducing transistor heating.
An alternative would be using a different type of transistor designed
to carry heavy current without excessive heat buildup such as the
Low Vce (Sat), High Gain small transistors (6 Amperes/60 Volts)
referenced earlier.
I have used the PN2222 very briefly in an application where it carried
approximately 10 Amperes of current in pulses shorter than 10 uS.
As I altered circuit parameters the transistor was eventually killed
by a large flyback spike. It did run for several minutes though without
excessive heating before it was destroyed...
For there is nothing hidden that will not be disclosed, and nothing concealed that will not be known or brought out into the open.
Author
Topic: Woopy's kacher device... progress? (Read 38562 times)
