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mhandsome
New Member

Aruba
1 Posts

Posted - Apr 23 2015 :  05:03:13 AM  Show Profile  Reply with Quote
I was wondering about this schematic. Just a mental exercise, you know. What caught my attention is the fact that both tantalum caps showed in - as I think - reversed polarity.
Look at the schematics: the base of the transistor never gets more than 1V above emitter voltage - when the transistor is opened; when the transistor is closed, the base voltage may go well below 0.
The other side of the cap is con

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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - May 07 2015 :  10:31:53 PM  Show Profile  Reply with Quote
Are you talking about the extremely simple 12VDC to 120VAC inverter? Yes, its capacitors had backwards polarity but their polarity has been fixed.

The collector side of each capacitor goes positive and to 0V. The base side of each capacitor goes positive about 1V and goes negative enough to cause avalanche breakdown.
The circuit works very poorly with low power.

Doors open and close, not transistors. Transistors turn on and turn off.
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yytbyebyet
New Member

1 Posts

Posted - May 19 2015 :  07:52:53 AM  Show Profile  Reply with Quote
the transistor never gets more than 1V above emitter voltage - when the transistor is opened; when the transistor is closed, the base voltage may go well below

raza
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - May 28 2015 :  07:54:38 AM  Show Profile  Reply with Quote
quote:
Originally posted by yytbyebyet

the transistor never gets more than 1V above emitter voltage - when the transistor is opened

Of course it does. The collector of each transistor swings to +24V when the transistor turns off.

quote:
when the transistor is closed, the base voltage may go well below

Why do you talk about "open and closed" instead of turned on or turned off? Is it turned off when it is "open" and is it turned on when it is "closed"?

When the base voltage is +0.7V or a little higher then the transistor turns on. The transformer causes charged coupling capacitor to try to cause the base voltage of the other transistor to go negative to -23V but the emitter-base has avalanche breakdown at about -7V or -8V which damages the transistor and capacitor. The horrible circuit should have diodes added in series with the bases and pull-down resistors to fix that like this:


Download Attachment: bad inverter corrected.PNG
6.96 KB


Edited by - audioguru on May 28 2015 07:55:58 AM
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