VCC Coupling capacitors Reverse Polarity

[danny davis]

They are electrolytic capacitors, they get put in backwards by accident

When One of the electrolytic capacitors is backwards, it pulls down the VCC voltage, why is that?

The VCC's are either 12 volts or 5 volts

If there is a electrolytic capacitor put in backwards by accident it will pull the VCC down lower

I just don't know why it pulls down the voltage when a polarized capacitor is put in backwards

When a Polarized capacitor is reversed in a circuit it acts like a short or open? or how does it operate when its put in reverse polarity or backwards polarity?

What is this called? is this "reverse biasing" the electrolytic or polarized capacitor?

 

[rahdirs]

@danny davis:

• Most electrolytic capacitors are polarized and require one of the electrodes to be positive relative to the other; they may catastrophically fail if voltage is reversed. This is because a reverse-bias voltage above 1 to 1.5 V will destroy the center layer of dielectric material by electro-chemical reduction.

• Following the loss of the dielectric material, the capacitor will short circuit, and with sufficient short circuit current, the electrolyte will rapidly heat up and either leak or cause the capacitor to burst.

• An electrolytic can withstand a reverse bias for a short period, but will conduct significant current and not act as a very good capacitor. Most will survive with no reverse DC bias or with only AC voltage, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time.

When connected in reverse fashion,it will conduct a significant current & tends to pull down Vcc.

 

[danny davis]

Thanks for the info

When connected in reverse fashion,it will conduct a significant current & tends to pull down Vcc.

Why does it conduct a significant current and tends to pull down the VCC? any reasons why?

When Connecting a Polarized or Electrolytic capacitor in REVERSE what is this called?

Is a Electrolytic is rated at 35volts or 50 volts , how much voltage can the capacitor take in REVERSE?

 

[rahdirs]

When Connecting a Polarized or Electrolytic capacitor in REVERSE what is this called?

If you're intentionally connecting a capacitor in REVERSE you might call it Reverse biasing a capacitor.

If an Electrolytic is rated at 35volts or 50 volts , how much voltage can the capacitor take in REVERSE?

• For some very good tantalum capacitors,the existence of a threshold voltage above which tantalum capacitors installed backwards would fail within seconds, and below which they would withstand hundreds and thousands of hours enduring relatively high leakage currents (in the milliampere range) but without hard failures is probably between 15% and 25% of the rated voltages.

• I take no guarantee but some good tantalum capacitors could withstand 10-15% reverse bias voltage of rated forward voltage at room temperature.The voltage may change to 3% of rated forward voltage at 100
  degrees C.

• Some capacitors could withstand reverse voltage up to 25% of rated voltage with very little degradation occurring below 15% of rated voltage. Healing could occur after 5 minutes of the application polarity being corrected.But these ratings may depend on the batch of capacitors & may vary from manufacturer.

• Why it conducts significant current in reverse bias is it will destroy the center layer of dielectric material by electro-chemical reduction.Following the loss of the dielectric material, the capacitor will short circuit.

 

[ted]

I would like to add here a special warning regarding tantalum capacitors:

They may burn violently, and cause fire hazard, and also emit toxic fumes, if they are failing either due to over-voltage or reversal. That happens very easily, if the power supply has sufficient current capacity!

Therefore: If you "misuse" aluminum electrolytic capacitors, they usually "just" leak or vent -and sometimes burst, causing a sticky mess. Very seldom they explore or behave otherwise violently (except maybe very big capacitors, and I mean physically so big, that they have screw terminals). I don't have much experience with those huge can sizes, but have seen many smaller capacitors "giving up".

However, a tantalum capacitor is often much more dangerous, if used beyond their specifications - were it then reversal of polarity, too high voltage, or too large AC current. I have seen many times burned circuit boards and other secondary damage as a result!

-ted

p.s. Also a quick summary of already earlier explanations, why polarized capacitors short-circuit when reversed: When the isolating oxide is damaged due to conditions it is not intended for, such as reversed polarity, it breaks down. The result is either high leakage current, or so low resistance, that we call it short-circuit. Depending on materials used and energy available, the consequences can vary from zero Vcc to destroyed circuit board and even to a fire. Thus it is not a good idea to reverse or overload those components.

 

[godfreyl]

Very seldom they explore or behave otherwise violently (except maybe very big capacitors, and I mean physically so big, that they have screw terminals).

I've had a small electrolytic cap explode violently in an ATX power supply. IIRC, it was something like 47uF, 50V.

 

[danny davis]

My Manager said that when you put a Polarized Capacitor in Backwards it acts like a Resistor, that's why the VCC rail voltage sinks and gets pulled down

Why does it act like a Resistor when a polarized capacitor is in reverse polarity?

 

[rahdirs]

@danny davis: There is a very good paper titled "Reverse Bias Behavior of Surface Mount Solid Tantalum Capacitors" by Alexander Teverovsky, Ph.D.QSS Group, Inc./NASA
Considering the fact that capacitors that were tested were military grade,no where did he find that reverse bias a capacitor would make it act like a capacitor.

At room temperature and reverse voltages below 2 V, similar to the forward bias conditions, the reverse leakage currents in all groups of capacitors decreased with time according to the power law with the exponent, n, varying from 0.8 to 1.1. At higher reverse voltages the leakage current increases with time following a relatively short period of current decrease.

 

[danny davis]

So the Capacitor act like a ? when reverse voltage is applied to it?

When the Polarized Capacitor is Reversed voltage it will pull down the Voltage

The Positive Terminal on a polarized capacitor is a HIGHER voltage than the Negative Terminal on a polarized capacitor

When The Negative Terminal on a polarized capacitor has a HIGH voltage than the Positive Terminal What happens? the polarized capacitor act like a ?