Continue to Site

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.

  • 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.

what value of capacitor should be used in high current supplies?

Status
Not open for further replies.
what is the relationship of current and Capacitor in a power supply... e.g; we simply use a 1000uf capacitor with bridge rectifier in a power supply of approximately 1.5A at the out put that can be got using an LM317 regulator.... what should be the value for a high current supplies e.g; up to 20A....
1) how much value capacitor can be used for that.......
2)is there any formula or relationship...
3)Will the increase of capacitance drop the voltages at the output terminalso of the capacitor???
i am talking about simple bridge rectifiers power supplies.
please help.
 
Last edited:
Hi Qaisar Azeemi,

a power output of 20A using the reactance of a capacitor is just an illusion.

For 20A output current the capacitive resistance of the cap would be 11Ohm for a mains voltage of 220VAC.

Using a capacitor with two air insulated plates the capacitor would require the size of a cathedral.

Even using supercaps you'll have to consider their maximum allowed voltage of 2.5V per cap.

A bank of supercaps able to provide 20A would easily fill your living room.

Many transformerless power supplies don't provide more than 50mA!

They still are a safety hazard with life on all components.

Touch them and you might get electrocuted.

Boncuk
 
Last edited:
Hi Qaisar Azeemi,

a power output of 20A using the reactance of a capacitor is just an illusion.

For 20A output current the capacitive resistance of the cap would be 11Ohm for a mains voltage of 220VAC.

Using a capacitor with two air insulated plates the capacitor would require the size of a cathedral.

Even using supercaps you'll have to consider their maximum allowed voltage of 2.5V per cap.

A bank of supercaps able to provide 20A would easily fill your living room.

Many transformerless power supplies don't provide more than 50mA!

They still are a safety hazard with life on all components.

Touch them and you might get electrocuted.

Boncuk
I think you miss understood the OP.
He is talking about a PS fillter cap.
Not a reactance cap.
 
Please reread the original thread. He wants a transformerless power supply providing 20A!

The intentions are not quite clear!
 
Last edited:
Please reread the original thread. He wants a transformerless power supply providing 20A!

The intentions are not quite clear!

I read it again but thats not what I am getting out of it.


Language barrier?

Maybe the OP can clarify.
 
I read it again but thats not what I am getting out of it.


Language barrier?

Maybe the OP can clarify.

Possibly language barrier.

Sorry for the confusion.

Boncuk
 
A 20A power supply using a transformer/bridge/cap design is not really feasible. It's possible, but you will need a fork lift to carry the transformer and the cap will have to be HUGE, not to mention the heatsink for the regulator.

These type of high current supplies are almost always built using some kind of offline switcher. A 60 Hz transformer design is just not practical.

As an example: if you use a 60 Hz transformer fed to a bridge and capacitor, and you want the ripple voltage to be not more than about 3V at 20A current:

The filter capacitor would have to be about 60,000 uF.
 
Last edited:
A 20A power supply using a transformer/bridge/cap design is not really feasible. It's possible, but you will need a fork lift to carry the transformer and the cap will have to be HUGE, not to mention the heatsink for the regulator.
Really just what are you basing that statement on?
 
Really just what are you basing that statement on?
Nothing really. Just those 33 years I spent designing power supplies and power converters.

Seriously: do you think filter capacitors whose value is 60,000uF are feasible? Power transformers that you could use as boat anchors?

30 years ago, there actually were 60 Hz power supplies of that current/power, I know what size the parts were. That's exactly why offline switching converters (and all the components necessary to build them) were forced to be developed.... necessity = invention. The switching technology has advanced so far you have offline converters (ie, PC power supplies) selling for $10 that supply 10 - 15A outputs.

It is hard for me to sit by and watch somebody trying to do that with a 60 Hz transformer and diode bridge without saying something.... but it's a free world.
 
Last edited:
have a look here. i think it will explain what you're looking for....
it's mostly about power supplies for audio power amps, but the principles for any analog supply are pretty simple.

**broken link removed**
 
Last edited:
I hear you but realy you don't need a fork lift for a 20 amp supply.

https://www.amazon.com/Pyramid-PS21KX-Amp-Power-Supply/dp/B0002JTD2A

16 pounds not quite a boat anchor but close.
However, you can not specify the size or weightof the transformer for the simple reason the OP did not specify the power:

"what should be the value for a high current supplies e.g; up to 20A...."

He said 20A, he did not say voltage so the VA specs of the supply are undefined.

However, the current does define the losses in the rectifiers and ballpark power losses if this is a linear design, which it almost surely is.

The power transformer is probably a boat anchor, battleship or cruiser is inknown, but this is a non feasible endeavor IMHO, especially for a novice. When you look at the total size of the whole unit you'll see why I say that.
 
Last edited:
A 20A power supply using a transformer/bridge/cap design is not really feasible. It's possible, but you will need a fork lift to carry the transformer and the cap will have to be HUGE, not to mention the heatsink for the regulator.

These type of high current supplies are almost always built using some kind of offline switcher. A 60 Hz transformer design is just not practical.

As an example: if you use a 60 Hz transformer fed to a bridge and capacitor, and you want the ripple voltage to be not more than about 3V at 20A current:

The filter capacitor would have to be about 60,000 uF.

I am using a 30V, 20A transformer whose size is about 8"x6"x4" approximately with a waight of 3kg to 4kg approx: . so i think it is easy to handle for one person :) no lift is required.. for bridge the available bridge in our market is 35A so it is also most suitable to use for the 20A power supply...

my question was that . .... How much value of a Capacitor that i should use to smooth the DC?

so a rule of thumb is that i got from a friend that if you are using 1000uf capacitor with 1A power supply ; you should add 1000uf with each additional amp. so the rough estimate is 20,000uf capacitor for that purpose. thus i can connect 6, 3300uf (50V) capacitors in parallel to get an equal of 19800uf capacitance..............

overall size of powersupply will not much greater.

am i right in this concept??

and my second question is that what will happened if i use small value capacitance ... for example only 3300uf for whole purpose??? will it explode out or just not smooth the ripples at the rectified output of bridge circuit?
 
so a rule of thumb is that i got from a friend that if you are using 1000uf capacitor with 1A power supply ; you should add 1000uf with each additional amp.

That is something I learned years ago but as 1 uF per mA. So yes, 1,000 uF per amp. The link I provided in the second post of this thread covers the actual formula in some detail if you have an interest. Yes, you can parallel filter capacitors and no, nothing will explode if you do not provide enough filtering, the supply will just have higher ripple on the DC level. Did you take the time to read the link I posted?

Ron
 
I am using a 30V, 20A transformer whose size is about 8"x6"x4" approximately with a waight of 3kg to 4kg approx: . so i think it is easy to handle for one person :) no lift is required..
If you are building some kind of regulator, there will be a massive heatsink required since there will be at least 4-5V of drop and 20A of current ( at least 100W dissipated in the regulator device). That will require a large heatsink and probably a fan. The transformer is not the only size/weight consideration.

I am using a 30V, 20A transformer whose size is about 8"x6"x4" approximately with a waight of 3kg to 4kg approx: . so i think it is easy to handle for one person :) no lift is required.. for bridge the available bridge in our market is 35A so it is also most suitable to use for the 20A power supply...
Actually, that may not be adequate. For a diode bridge, the RMS current flowing in it is typically about 1.8 times the DC load current, about 35A average in this case. I think you need a 50A rated bridge rectifier which mounts to a heatsink since it will dissipate about 70 Watts in use.

so a rule of thumb is that i got from a friend that if you are using 1000uf capacitor with 1A power supply ; you should add 1000uf with each additional amp. so the rough estimate is 20,000uf capacitor for that purpose. thus i can connect 6, 3300uf (50V) capacitors in parallel to get an equal of 19800uf capacitance..............
It depends on how much ripple is allowed. You have not stated the regulated output voltage for the design. Allowable ripple is set by the dropout voltage. In other words: if you want a 25V output, you need at least 28V DC input. You have to know the load regulation of the transformer under load to know the minimum voltage it will provide. Then calculate allowed ripple to prevent dropout from happening. calculate cap using I = C dV/dt where dt is 8.3 ms since ripple is 120 Hz. The dV is allowed ripple voltage.

There are many factors which must be known to do this design. You have not stated most of them.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top