• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Reverse Voltage on Polarized or Electrolytic capacitors , what happens?

Status
Not open for further replies.

kubeek

Well-Known Member
Most Helpful Member
Still there is a huge difference in doucupling a processor in a calculator and decupling a power hungry FPGA runing near its limits. The datasheets sometimes say what decoupling should be used. When not specified or without any requirements, you just use what you have in stock or what is cheapest.
 

kubeek

Well-Known Member
Most Helpful Member
Decoupling for big logic chips is usually done in three or more stages, first the really large supply caps near the power input say one or two 470uF caps, then closer to each chip a few smaller ones like 47uF to serve for localized decoupling, and then each power supply pin should have one cap somewhere in 100nF-1uF range.
The first two groups will probably be polarized caps beacuse you can´t really get such large values in other dielectric types. The small caps will usually be ceramic.

Edit: wow, 666th post :D
 
ok thanks for your information

So for a general rule of thumb

For decoupling Digital VCC to ground
You use 100nf to 1uf range
Use either Ceramic or metalized film caps?

For power supplies use Electrolytic caps

Localized Decoupling around a Microprocessor use tantalum polarized capacitors?
 
Do you know any other general rule of thumbs like this when using different capacitor types for circuits?

The other reason why they use electrolytic capacitors in power supplies is because of the HIGH capacitance value and the HIGH voltage rating
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Good enough for "rule of thumb". Second "rule of thumb" is to follow the manufacturer's advice.

Tantalums are expensive, BUT they have a high capacitance to Volume Ratio.

FINAL bypassing really done by "trial and error". The PCB layout guy isn't an artist. He has some rules to follow. Sometimes there are recommended layouts by the manufacturer. Sometimes they will even show you a 2 and 4-layer suggested layout patterns.

Bypassing is influenced by layout.
 
Not Decoupling and Bypassing right will cause what kind of problems?
1.) Crosstalk
2.) Superimposed waveforms
3.) Noises
4.) AC signals bleeding mixing into DC signals
5.) oscillations
6.) HUM
 
My Manager said that film and ceramic capacitors do the same thing, but tantalum are for digital timing circuits, tantalum is good for timing

Ceramic capacitors are for digital circuits that are for high frequencies signals
Film capacitors are for digital circuits that have low frequency signals
 

crutschow

Well-Known Member
Most Helpful Member
My Manager said that film and ceramic capacitors do the same thing, but tantalum are for digital timing circuits, tantalum is good for timing

Ceramic capacitors are for digital circuits that are for high frequencies signals
Film capacitors are for digital circuits that have low frequency signals
Sounds like you manager has not done a lot of design. His recommendations are a little off. :rolleyes:

Film capacitors are generally the most stable with good high frequency performance. They are used where good stability is required such as filters and timing circuits.

Ceramic capacitors are smaller than film but less stable with temperature. Also have very good high frequency performance. They are commonly used for local decoupling of high speed analog and digital active components.

Tantalums are usually only used when you need a large capacitance, such as decoupling and DC blocking, where the physical size of film or ceramic caps is too large. Generally they are inferior in performance and stability as compared to film or ceramic caps. Tantalums would only be used for timing purposes if you needed a large capacitance for a long time constant.
 

crutschow

Well-Known Member
Most Helpful Member
What is localized decoupling mean?
When you decouple you always use .1uf right? For digital and analog signals
Localized means connect the capacitor as close to the device's power and ground pins as possible.

0.1μF is a typical value used for local decoupling but it can vary from that if you like. Sometimes they use 0.01μF caps in high frequency circuits, since they have slightly lower parasitic inductance.
 
Thanks for your info

Do you know why electrolytics and Tantalums are polarized or why they have to be polarized? any reason why they are polarized or have to be?

I know electrolytics have liquid electro-chemicals and Tanalums have solid electro-chemicals
 

crutschow

Well-Known Member
Most Helpful Member
The chemical dielectric film used in electrolytics is inherently polarized. It's not desirable, just a fact of the chemistry of the films.

Non-polarized electrolytics are actually two back-to-back electrolytics in one case.
 
The chemical dielectric film used in electrolytics is inherently polarized. It's not desirable, just a fact of the chemistry of the films.
Does the chemicals make for higher capacitance in a smaller?without the chemicals there is no way to get really high capacitance right?
 

crutschow

Well-Known Member
Most Helpful Member
Does the chemicals make for higher capacitance in a smaller?without the chemicals there is no way to get really high capacitance right?
Yes, the chemical dielectric layer is much thinner than any physical dielectric layer possible in a film or ceramic type capacitor. This allows for more capacitance in a given volume.
 
So the Plates aren't polarized it's only the chemicals that are polarized?



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?

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
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top