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Saturation questions.

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audioguru said:
Hi Freeskier,
You are very confusing because you don't attach the schematic that you are talking about.
You linked to an LED Flasher circuit but you seem to be talking about the flip flop circuit at the end of the page. Just copy then upload the circuit that you are talking about.

The charge on the capacitor stays for a while. When the transistor saturates and grounds the + end of the capacitor, then the - end of the capacitor is driven to a negative voltage which cuts-off the other transistor. Then the capacitor is slowly discharged by the 10k resistor then begins charging a little in reverse.
Thanks, I already understand the part why the PNP suddenly got itself turned off. :D

Now I'm building one, and actually, which is more important in influencing the LED blink rate? Is there a formula too for these? :)

edit: For the LED flasher, is it okay if I use CS9013 and CS9012 instead of the BC transistors? Found out in some hobby kits that they actually use these too. What do you think? :D
 
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audioguru

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More confusion.
We were talking about the capacitors in the flip flop circuit that have their - end go below ground to a negative voltage. The flip flop circuit doesn't have a PNP transistor.

The capacitor charges in an exponential manner in the circuit. If you know its voltage when it starts charging and its voltage when it ends charging and its value, and the charging resistor's value then they can be used to calculate the blinking rate.
I couldn't find a datasheet for a CS transistor. Maybe they are Oriental 2SC9012 and 2SC9013.
 

Roff

Well-Known Member
freeskier89 said:
Sorry about the confusion guys. I was talking about the astable monovibrator or "flip flop" at the bottom of the page. Thanks audioguru for your information, that clarifies it a bit for me. I still do not seem to fully understand how a negative charge relative to ground is developed on the negative lead of the capacitor, but that I will look into that very soon. Its about time I get rid of my ignorance towards analog stuff. :) Thanks again!
Another way of saying what AudioGuru said: The voltage across a capacitor can't change instantaneously. When the collector voltage drops abruptly, the other end of the capacitor will follow. This causes the base of the other transistor to go below GND, reverse-biasing the base-emitter diode so that it stops conducting, turning this transistor off. The base voltage of this transistor then starts to rise due to the current through the resistor from base to +V, which is charging (or discharging, if that makes more sense) the capacitor in question. When the base voltage reaches about 0.6V, the transistor starts to turn on, beginning the other half of the cycle.
 
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