Input de-coupling capacitors

Thread Starter

engr_david_ee

Joined Mar 10, 2023
166
I have heard a discussion on power distribution board to an electronics project. They have been mentioning super-capacitors to be use as de-coupling capacitors at the input for voltage regulators or DC-Dc converters. I am wondering about these supper capacitors. How they differ then normal ceramic SMD capacitors that we use on the PCBs together with tantalum and aluminum electrolyte capacitors.

Regarding supper-capacitors, are they expansive and what are the pros and cons using supper-capacitors in power rails.
 

Papabravo

Joined Feb 24, 2006
21,225
I've never heard of this before, and I consider it a dubious proposition. At the same time, I would be interested in learning the substance of the discussion you are referring to. Can you provide a summary or a link?
 

MrChips

Joined Oct 2, 2009
30,790
When we talk about super-capacitors, we usually refer to values of 1F and higher. These are more likely to be used as batteries or reservoir capacitors in power storage applications.

Decoupling capacitors are used for attenuating or eliminating high frequency noise and as such need to have very low ESR.
Decoupling capacitor values are usually in the range 1nF - 10μF. It is not unusual to see two or three different values wired in parallel, for example, 10nF//100nF//1μF.
 

Thread Starter

engr_david_ee

Joined Mar 10, 2023
166
The one which I know has been used in an application is 470 mF which is a bigger capacitor. I have not seen it. I don't have part number.
 

nsaspook

Joined Aug 27, 2009
13,254
I've never heard of this before, and I consider it a dubious proposition. At the same time, I would be interested in learning the substance of the discussion you are referring to. Can you provide a summary or a link?
I can't find very much on the subject other than a few saying it's not a good idea to drop short circuits (the super-capacitors is near a dead short for a while when empty of energy) a random places on a PCB board.
What? Is my first response again to the OP.
 
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Papabravo

Joined Feb 24, 2006
21,225
In the days of power hungry TTL chips we seldom needed more than a couple of microfarads to supply current demands while switching. We now have CMOS chips switching much faster than TTL ever did so holding up the power supply rails does not seem to require anything like 470 millifarads.
 

nsaspook

Joined Aug 27, 2009
13,254
In the days of power hungry TTL chips we seldom needed more than a couple of microfarads to supply current demands while switching. We now have CMOS chips switching much faster than TTL ever did so holding up the power supply rails does not seem to require anything like 470 millifarads.
Exactly. We had logic power supply lines that looked like welding cable to the backplane. There was a need for pretty massive filter capacitors (beer can sized electrolytics) in the old linear power supplies to reduce ripple.

Sea story. I once flew all the way from San diego, CA to the base in Diego Garcia in the Indian ocean, just to replace one of the massive power supplies on a critical computer system for a ship.
 

Papabravo

Joined Feb 24, 2006
21,225
Exactly. We had logic power supply lines that looked like welding cable to the backplane. There was a need for pretty massive filter capacitors (beer can sized electrolytics) in the old linear power supplies to reduce ripple.

Sea story. I once flew all the way from San diego, CA to the base in Diego Garcia in the Indian ocean, just to replace one of the massive power supplies on a critical computer system for a ship.
We had a technician, "Fast Eddie" who welded a Craftsman screwdriver to the terminal of one of those big "blue can" capacitors. It popped its pressure relief valve and spit steam for a couple of minutes, and nobody was hurt thankfully. Director of Engineering confiscated his screwdrivers, but he kept his job.
 

nsaspook

Joined Aug 27, 2009
13,254
We had a technician, "Fast Eddie" who welded a Craftsman screwdriver to the terminal of one of those big "blue can" capacitors. It popped its pressure relief valve and spit steam for a couple of minutes, and nobody was hurt thankfully. Director of Engineering confiscated his screwdrivers, but he kept his job.
1709137405861.png

We still see them in the old magnet power supplies.
 

Thread Starter

engr_david_ee

Joined Mar 10, 2023
166
I read that "Super-Capacitors: Supercapacitors are a type of an electrochemical energy storage systems which have great power density and specific capacitance. These systems have the ability to efficiently release energy with a high density over a relatively short time."
Does it means that charging/dis-charging is fast ? What are the drawbacks of using super-capacitors in the PCB ? How big they are physically and what about prices.
 
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