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Reverse Voltage on Polarized or Electrolytic capacitors , what happens?

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dougy83

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So the Plates aren't polarized it's only the chemicals that are polarized?
Wasn't this addressed in your other thread (AC/DC caps)? Reverse biasing an electrolytic cap causes its oxide layer to dissolve and allows current to pass. For non-polarised caps the oxide layer is on both foils. The oxide layer that is destroyed by voltage in one direction can be restored when the voltage is reversed (http://en.wikipedia.org/wiki/Capacitor)
I do see Varistors in series with a bank of electrolytic caps in power supplies, why do they use these Varistors? it limits the current at a certain voltage right? These Varistors will Open if there is a SHORT or drawing to much current from the power supply right?
They are more likely to be NTC/PTC devices or polyswitches that are in series with the supply; varistors are not used in that mode. PTC/polyswitches will have an increase in their resistance as their temperature increases, which means that they can be used in place of fuses to protect/disconnect against overcurrent. An NTC device will decrease its resistance as its temperature is increased, which makes it suitable for limiting the inrush current (current when first powered up).

If you do see a varistor (MOV), they will be connected across (in parallel with) the circuit to be protected. If their characteristic voltage is exceeded, they will conduct (similar [but different] to a zener diode; see http://en.wikipedia.org/wiki/Varistor)

I also see a 1 ohm resistor on the return path going to ground, I'm guessing these 1 ohm resistors are used a fuses if there is a DIRECT short from VCC to ground the 1 ohm resistor will open on the return path going to ground
That may be the case, but it may have other uses also (inrush limiting, current sensing, snubbing, etc.); it depends on where it is and what's around it.
 

KeepItSimpleStupid

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no full kb avalable

Varistor used parallel to caps.
Inrush current limiter in series.
1 ohm = current limiter, possible fuse if metal oxide
 
1 ohm resistors can be current limiting how? it's just 1 ohm resistor without any op amp connected to them

I thought current sensing was when an op amp was connected to a milliohm resistor

A Varistor is a Zenor diode and resistor inside of it? If the DC voltage is Higher or raises up the Varistor will conduct? I'm not sure how it work and when does it work and how does it work

Why are metal oxide resistor only used as fuses?
 

KeepItSimpleStupid

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1 ohm resistors can be current limiting how? it's just 1 ohm resistor without any op amp connected to them

*OHms law

I thought current sensing was when an op amp was connected to a milliohm resistor

*No. resistors, hall effect, current transformers

A Varistor is a Zenor diode and resistor inside of it? If the DC voltage is Higher or raises up the Varistor will conduct? I'm not sure how it work and when does it work and how does it work

*Think of it as a high power zener diode.

Why are metal oxide resistor only used as fuses?

*Not always uses as fuses. They usually fail open.
 

dougy83

Well-Known Member
Why do I get the feeling you didn't read any of the wikipedia links above?
 
My work uses Varistor in Series to protect the power supply from the turn on in rush current, the Varistors are High resistors at first and when the capacitors charge up the varistors lowers its resistance down
 

kubeek

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Varistors are overvoltage protection, see wikipedia. On the other side PTC thermistors are usually used in series to limit inrush current. Your boss seems to know everything very well :D
 
He was saying that the Varistors are in series to Time Delay the current so it doesn't Dump a lot of current into the power supply

The Varistors are in series after the Filter caps

The Varistors are High Resistance until the filter caps are all charged fully, and the Varistors changes its resistance from High resistance to low resistance based on the "Temperature"

Is this true what he explained?
 
Yes I did look up the datasheets and they are varistors for sure that we use

But after reading the wiki , you're right , they are using the wrong part for the job it seems
 

dougy83

Well-Known Member
Varistors are overvoltage protection, see wikipedia. On the other side PTC thermistors are usually used in series to limit inrush current. Your boss seems to know everything very well :D
NTC should be used for inrush limiting. PTC should be used for overcurrent protection.
 

KeepItSimpleStupid

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It's typical to see MOV's ACROSS the capacitors, but not in series. I had to use them in y power amp to protect a circuit where I was monitoring the volatge across the capacitors.
 
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