Re: solid state magnifier...accidentally...cool!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jan Florian Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>

Hi,

> > rise.  I have found that, for 12 V power source and typical ignition
> > coils, the voltage can go as high as three or four hundred volts.
> > Usually use a shunt capacitor across the transistor to control the
> > swing.  Value determined by careful "by guess and by gosh", usually of
> > the order of 1 ufd or more.
> The same idea used in classical induction coils. Without the capacitor
> most of the energy is dissipated in the opening sparks at the primary 
> interruptor.

But, that capacitor contributes significantly to heating the transistor,
because each time the transistor is turned fully on/to saturation, it also
shorts out the capacitor and gets all energy dumped into it. I once tested 
a flyback SMPS this way, it had a few 100W, and the heating was really
horrible and gets worse as the frequency goes up. 

It is better to use a RC snubber network, like:
 
from collector
---+
   | R1
   R
   R  maybe 200 Ohm
   R  R1*C <= T_turnoff_spike
   |
   +------+
   |      | R2
  ---     R
  ---     R   R2*C < (1/switch-freq)
   |      R
   |      |   R2 is high(er) wattage 
   +------+
   |
  GND


where it is the resistor R2 that heats up. Not the transistor.

Or:

supply rail / 12V?
-----+-----
     |              L coil pri
     |              |
     +---RRR---|<|--+
     |              |
     |              |
    ---  C          |
    ---              \|
     |                |---   
     |               /|
    ---             | 
    GND            ---
                   GND
  with a fast/ultrafast recovery diode like UF4007 or so


Not to forget the obligatory VDR/MOV accross the ignition coil primary...


 - Jan

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