What's the difference between an AC capacitor and a DC capacitor?

[danny davis]

What's the difference between an AC capacitor and a DC capacitor?

I know that DC capacitors use chemicals that are only One Uni Directional not bi directional

At work they use AC capacitors in DC circuits, but I don't understand why they use AC capacitors

Also I have noticed when using AC capacitors they solder a Diode on the Terminals of the AC capacitor, does this turn a AC capacitor into a DC capacitor?

 

[keith1200rs]

I think you probably mean polarised and non-polarised capacitors. A polarised capacitor must always have a voltage across it which is the correct way round (positive terminal higher than the negative terminal). You can still use them for AC, and they are frequently used for AC - you just have to be careful of the voltage across them.

Polarised capacitors used to be essential for large values (typically 1uF or more) but now you can get non-polarised capacitors to 100uF or more.

A diode across a capacitor is usually to ensure it discharges quickly when the power is removed.

Keith

 

[albert22]

The most common of polarized capacitors is the electrolytic capacitor. Often used in filtering DC power supplies where will be connected to a pulsating DC voltage. Which never inverts its polarity. This capacitors explode when connected backwards.
Some non polarized capacitors are specified different for AC and DC. For example you may see 100VAC or 100VDC. This does not mean that they are intended to be used only at DC or AC. It is the maximum voltage rating spec.

At work they use AC capacitors in DC circuits, but I don't understand why they use AC capacitors

I would ask the guys at your work I wonder why they do that.

A diode across a capacitor is usually to ensure it discharges quickly when the power is removed.

The standard practice is placing a "bleeder" resistor in parallel to discharge the capacitor. Some power supplies have a diode in reverse at the output for protection. And this diode may be in effect connected to the output capacitor.
I dont see how a diode could help discharge the capacitor. Please explain.

 

[danny davis]

They Use AC capacitors in a DC circuit to filter out Battery Ripple?

I'm not sure how an AC capacitor filters out Battery Ripple

 

[albert22]

Perhaps you can give more details of the circuit and the capacitors that they are using. Why do you call them AC capacitors ?
A photo will help to identify the type. Value, part number or any other info ?

 

[Ogu Reginald]

I support the guy who suggested that what you are talking about is polarised and unploarised capacitors.
Polarised capacitors have negative and positive terminals. They don't usually handle high voltages and their values are always high. When it comes to circuits involving high frequency they are not used because they don't handle much frequency.
Unpolarised capacitors unlike the polarised type don't have positive or negative.Most of them can handle high frequency and their values are less relative to the polarised capacitors.
Connecting a diode between the terminals of the capacitor helps in safe discharge when power is removed.
Also note that the voltage produced by the battery has no ripple in it.

 

[pebe]

Connecting a diode between the terminals of the capacitor helps in safe discharge when power is removed.

A diode is never connected for this purpose - but a resistor is. A cap doesn't change polarity when it is being discharged, so if it conducted when the cap was being discharged then it would always conduct.

A diode in parallel with the cap is to prevent it being reverse biased.

 

[danny davis]

So it's called Reverse Biasing a Polarized Capacitor when you reverse the voltage on it? I though biasing was for semiconductors not capacitors

 

[pebe]

Call it reverse voltage, if you like.

 

[FvM]

"reverse voltage" is in fact the usual technical term that can be found in capacitor literature and manufacturer specifications.

Applying reverse voltage can be also called "reverse biasing", not only for semiconductors. Literally, the meaning of the word "bias" suggests that there's an additional signal voltage (e.g. AC or varying DC) superimposed to the bias voltage (DC).

 

[danny davis]

So when "Reverse Voltage" on a Polarized capacitor, how does the polarized capacitor act? It's not a capacitor anymore, so what is when its reversed voltage?

The polarized capacitor is polarized reversed? so what happens, it's not a capacitor anymore, it doesn't behave like a capacitor anymore , so what is it doing?

 

[FvM]

The nature of electrolytic capacitors involves several non-idealities like considerable leakage current, relative high ESR, very low tolerance to reversed voltage.

Most engineers are satisfied to know that an electrolytic capacitor will be potentially damaged when a reverse voltage is applied. The exact damage process might be interesting in case of accident, e.g "silent" massive short, excessive leakage current with high power dissipation and potential explosion, etc. The topic will be probably interesting for safety regulations or third party insurance experts.

Some technical details in the appended manufacturer's application note.

 

[danny davis]

When a Electrolytic capacitor is REVERSE Voltage, it will draw more current and pull down the Voltage, why is that? what is causing it to do this and why?

When An Electrolytic capacitor is REVERSE voltage, it act like a low resistance, or the ESR is low?

 

[FvM]

Simply assume that the defective capacitor shorts the supply voltage. It's pretty sure that this can happen, I don't see that much additional explanation is needed why and how.

 

[danny davis]

If the capacitor is shorted it will make the VCC zero volts or close to zero

When a polarized capacitor is REVERSE VOLTAGE, it draws more current and pulls down the voltage between 2 volts to 5 volts

The polarized capacitor is not shorted , its REVERSE voltage and it doesn't act like a capacitor anymore

The Positive terminal needs to have more voltage than the Negative terminal

REVERSING the voltage, reverses the polarized and potential difference between the positive and negative plates cause will cause more current to draw and drops the voltage down 2 to 5 volts

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I just don't know why it's causing it to draw more current and causing the voltage to be pulled down

When Reverse voltage on a polarized capacitor the ESR is very low?

The Potential difference is Reversed which puts a HIGH voltage on the Negative terminal and ground on the Positive terminal , This cause what to happen and why?

 

[FvM]

Besides acting as massive short, the defective capacitor might show a leakage resistance between 0 and inifinity. You need to consider the power supply characteristics to predict what happens in this case. But generally speaking, the defective capacitor doesn't behave much different from a defective IC drawing excessive supply current.

This happens every day and really doesn't need much explanation.

P.S.: Excessive leakage current hasn't to do with high or low ESR.

 

[pebe]

I thought the reply from FvM was more than adequate. But this may help.

The junction between the anode and the anodic film (the dielectric) can sustain a voltage between the two (ie. it acts like a high resistance) when the polarity at the anode is positive relative the anodic film, but cannot when that polarity is reversed. So if the polarity is reversed, electrons will travel freely from the anode foil through the anodic film and through the electrolyte to the cathode foil.

That current is limited by the resistance of the electrolyte, i.e. the ESR of the capacitor, which is the physical resistance of the electrolyte.

Is that clear enough?

 

[Ogu Reginald]

A diode is never connected for this purpose - but a resistor is. A cap doesn't change polarity when it is being discharged, so if it conducted when the cap was being discharged then it would always conduct.

A diode in parallel with the cap is to prevent it being reverse biased.

Thank you for the correction, now I understand what you mean.