about Electrolytic capacitor on multivibrator

Thread Starter

xljin2014

Joined Nov 11, 2014
42
hi,friends,
here is a typical multi vibrator.
Screenshot_20210922_165811_com.huawei.photos_edit_392341954874692.jpg

I would start with how this circuit works:

device group R2,BG2 and R3,BG1 are not the same exactly, when power supply added, one of BG1 and BG2 is on. suppose BG1 is on, then voltage of collector of BG1will go down , and the electrolytic capacitor couples this low voltage to the base of BG2, so BG2 is off.

the white part of the Electrolytic is positive,and the black part is negative.

after BG1 is on and BG2 is off, it is said a current comes from power supply---R2--C1--collector of BG1--emittor of BG1. which raise voltage of base of BG2.

my question now is since the negative side( right) of electrolytic capacitor(C1) is rising, it may be higher than positive side(nearly 0 V). I think that is not correct.that is my question. this is the first cycle of the circuit action. maybe later will be ok?

thx for reading. help me.
 

DickCappels

Joined Aug 21, 2008
7,878
The base voltage will go negative to about -4V (the reverse Emitter-Base breakdown voltage for the BDG6, so that is where the - end of the resistor goes when the other transistor turns on. The capacitor will then charge up to +0.6V (the approximate voltage at which appreciable collector current will flow) which will result in the capacitor have "reverse" voltage across is - the negative plate will be more positive than the positive plate. This condition will only persist for a very small percentage of the time.

Connecting the positive plates to the collectors is the preferred arrangement when using NPN transitors, though if you are really particular, you can use a non-polarized capacitor (or two electolytics back-to-back. I have not seen an electorlytic capacitor fail in this circuit in the 50+ years I have been using this circuit.
 

MrChips

Joined Oct 2, 2009
24,423
Electrolytic capacitors are used reliably in low voltage applications, for example, for interstage signal coupling in audio amplifiers.
 
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