# how this multivibrator works?

Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
my friends,

below is circuit of a multi vibrator.

the two pics below are helpful to understand how the circuit works. the solid line means discharge, the dot line means encharge.

I hope you can explain how the multi vibrator circuit works.

thx.

Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
This link pretty well covers the circuit you posted. Keep in mind that the instant power is applied one transistor will conduct before the other since parts can not be perfectly matched.

Ron
what I don't understand is that when transistor1 is on, the collector voltage goes to zero, so does voltage of left side of C1, and right side of it will go up to 0.7V to open transistor2. so the point comes: the negative side voltage is higher than positive side of a electrolytic .capacitor.

#### AnalogKid

Joined Aug 1, 2013
9,493
That is correct, which is why an electrolytic cap would not be allowed in a high-reliability design. However, the reverse voltage is only 0.5 V (Q2 Vbe - Q1 Vcesat), which is only 2% of the operating voltage range of a 25 V cap. And, the cap is reverse biased for a short length of time.

ak

Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
That is correct, which is why an electrolytic cap would not be allowed in a high-reliability design. However, the reverse voltage is only 0.5 V (Q2 Vbe - Q1 Vcesat), which is only 2% of the operating voltage range of a 25 V cap. And, the cap is reverse biased for a short length of time.

ak
thx. I will study bit of electrolytic cap when reversed biased.

#### DickCappels

Joined Aug 21, 2008
7,978
We just had this discussion a couple of days ago. Must be related to an electronics class somehow.

Yes, the capacitor is reverse biased for a small fraction of the total cycle. It does no harm to the capacitor.

#### AnalogKid

Joined Aug 1, 2013
9,493
Note that an electrolytic capacitor's tolerance for reverse bias is dependent on the type of capacitor (aluminum, tantalum, hybrid, etc.) and *very* dependent on the capacitor's rated working voltage. As above, a 25 V cap would barely be affected by a 0.5 V reverse bias. However, a 6 V cap would feel it much more, and it would decrease long-term reliability.

ak

Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
Note that an electrolytic capacitor's tolerance for reverse bias is dependent on the type of capacitor (aluminum, tantalum, hybrid, etc.) and *very* dependent on the capacitor's rated working voltage. As above, a 25 V cap would barely be affected by a 0.5 V reverse bias. However, a 6 V cap would feel it much more, and it would decrease long-term reliability.

ak
I want to know why, maybe it's a silly question (chemical) and should learn by myself.

#### DickCappels

Joined Aug 21, 2008
7,978
Some types of capacitors like tantalum act more like diodes -revers bias them and then look at the sweep second hand on your watch to see low long it takes to die. My guess is it is because of their low equivalent series resistance, but I don't really know. Other types of electorlitics are a little more forgiving.

Edit: After xljin2014's comment below: When I mentioned r"everse bias" I intended to refer to biasing the electrolytic "backwards", not the diode.

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Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
Some types of capacitors like tantalum act more like diodes -revers bias them and then look at the sweep second hand on your watch to see low long it takes to die. My guess is it is because of their low equivalent series resistance, but I don't really know. Other types of electorlitics are a little more forgiving.
I thought it will cause large current if reverse biased.

#### DickCappels

Joined Aug 21, 2008
7,978
I thought it will cause large current if reverse biased.
You have it right. I guess I was not careful enough in my reply. When I wrote "revers bias them and then look..." I thinking about the capacitor being biased backwards, but the text was still on "diode". I hope my mistake did not cause too concern. Thank you for pointing it out.

Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
You have it right. I guess I was not careful enough in my reply. When I wrote "revers bias them and then look..." I thinking about the capacitor being biased backwards, but the text was still on "diode". I hope my mistake did not cause too concern. Thank you for pointing it out.
you mean it's not a diode? sorry my wood head.

#### DickCappels

Joined Aug 21, 2008
7,978
Strictly speaking, a diode is a device with two terminals, but let's not over-think this. Electrolytic capacitors conduct in one direction all the time, and in the other direction after charging up it stops drawing current.

For a tiny part of each cycle the capacitors in your circuit in post #1 are biased the wrong way, but it is for a very short time but it seems to not have any ill effects.

Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
Strictly speaking, a diode is a device with two terminals, but let's not over-think this. Electrolytic capacitors conduct in one direction all the time, and in the other direction after charging up it stops drawing current.

For a tiny part of each cycle the capacitors in your circuit in post #1 are biased the wrong way, but it is for a very short time but it seems to not have any ill effects.
you mean it's ok to reverse bias?

#### DickCappels

Joined Aug 21, 2008
7,978
you mean it's ok to reverse bias?
Yes in this circuit. The capacitor is reverse biased for only a tiny bit of time -the time between when the voltage on the base crosses 0 VDC and the time it takes to get to +VBE (About 0.7V). This might be safe because the average voltage is still of the proper polarity. Maybe somebody with a greater depth of understanding will comment.

I have run this circuit for years and neither have I experienced nor have I heard of an electrolytic capacitor being damaged by this kind of operation.

The simulated circuit (one base resistor had to be made larger than the other so the oscillation would start - a "problem" with perfect components):

The voltage seen across C1. Below 0 volts is when the capacitor is reverse biased:

Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
Yes in this circuit. The capacitor is reverse biased for only a tiny bit of time -the time between when the voltage on the base crosses 0 VDC and the time it takes to get to +VBE (About 0.7V). This might be safe because the average voltage is still of the proper polarity. Maybe somebody with a greater depth of understanding will comment.

I have run this circuit for years and neither have I experienced nor have I heard of an electrolytic capacitor being damaged by this kind of operation.

The simulated circuit (one base resistor had to be made larger than the other so the oscillation would start - a "problem" with perfect components):
View attachment 250718

The voltage seen across C1. Below 0 volts is when the capacitor is reverse biased:
View attachment 250719
ok. thank you very much.

#### MrSalts

Joined Apr 2, 2020
719
I want to know why, maybe it's a silly question (chemical) and should learn by myself.
It is electrochemical. Look up "reduction potentials" and half-cell reactions. The reverse reaction of any reaction on the reduction potential list is, by definition, an oxidation reaction. The capacitor is designed to drive an oxidation reaction which will keep the tantalum or aluminum as aluminum oxide on the surface of the foil.

If you reverse the potential on the cap, you could reduce the metal oxide to a metallic state. That can cause conductivity between layers in the two plates and a short - which would quickly melt the thin foil - which could then cause leakage of electrolyte across the foil and conduction through the electrolyte.

Thread Starter

#### xljin2014

Joined Nov 11, 2014
73
It is electrochemical. Look up "reduction potentials" and half-cell reactions. The reverse reaction of any reaction on the reduction potential list is, by definition, an oxidation reaction. The capacitor is designed to drive an oxidation reaction which will keep the tantalum or aluminum as aluminum oxide on the surface of the foil.

If you reverse the potential on the cap, you could reduce the metal oxide to a metallic state. That can cause conductivity between layers in the two plates and a short - which would quickly melt the thin foil - which could then cause leakage of electrolyte across the foil and conduction through the electrolyte.
do you mean it's not good to reverse bias an Electrolytic capacitor?

#### crutschow

Joined Mar 14, 2008
28,198
do you mean it's not good to reverse bias an Electrolytic capacitor?
Yes, it's not good.

#### Danko

Joined Nov 22, 2017
1,184
do you mean it's not good to reverse bias an Electrolytic capacitor?
In every particular case you can find solution, where reverse bias will eliminated.
For example:

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