Re: Re: [TCML] op amp wiring

["Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>]

Ed Phillips wrote:
> <div class="moz-text-flowed" style="font-family: -moz-fixed">    
> ALWAYS use a buffer amplifier between the NE555 and the base of a 
> power transistor in the primary of a flyback [or ignition] transformer.
Certainly. A 555 can only output about 200 mA. Another problem is a big 
waste of power in a driver using a resistor at the base of the power 
transistor.
Here are some of my flyback experiments:
http://www.coe.ufrj.br/~acmq/flyback/
The second driver uses an inductor to drive the transistor without waste 
of energy.
A thing that I see in several citations of flyback drivers is that they 
show resonances at a few tens of kHz. I can't observe any significant 
resonance in my flybacks. But there is something that looks as a 
"resonance", but isn't. The switching transistor takes some time to get 
switched off. If the driver switches it off and then on again without 
waiting for this minimum time, the transistor doesn't switch off, and 
the current trough the primary coil rises to twice the normal value. 
With the increased current, the transistor switches off at the next 
command from the driver, for some reason. When this happens, a sudden 
increase in the output voltage appears, looking as a sharp resonance. If 
the frequency is further increased, the maximum primary current 
decreases, and the output voltage decreases. The false resonance appears 
again when the transistor doesn't switch off for three periods, four 
periods, etc.
The (very fragile) oscillator with a bipolar transistor works with a 
mosfet too. The two resistors can have higher resistance, and something 
shall be added to avoid excessive negative voltage at the mosfet gate.

Antonio Carlos M. de Queiroz

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